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Metabolism of Nitrogenous Compound
Mpenda F.N
1
Introduction
• This topic describes the amino acids that are
important in human nutrition.
• It covers the digestion and absorbtion of proteins
in the gut.
• The amino acid degradation pathways in the tissues,
and the urea cycle.
• The amino acid synthesis pathways
• There are separate topic dealing with nucleotide
and 'one carbon' metabolism and with porphyrin on
the integration of metabolism.
2
Introduction…
• Human proteins have very different lifetimes.
• Total body protein is about 11kg, but about 25% of
this is collagen, which is metabolically inert.
• A typical muscle protein might survive for three
weeks, but many liver enzymes turn over in a
couple of days.
• Some regulatory enzymes have half-lives measured
in hours or minutes.
• The majority of the amino acids released during
protein degradation are promptly re-incorporated
3
into fresh proteins.
Introduction…
• Net protein synthesis accounts for less than one
third of the dietary amino acid intake, even in
rapidly growing children consuming a minimal diet.
• Most of the ingested protein is ultimately oxidised
to provide energy, and the surplus nitrogen is
excreted, a little as ammonia but mostly as urea.
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Protein degradation
• Soluble intracellular proteins are tagged for
destruction by attaching ubiquitin, a low molecular
weight protein marker.
• They are then degraded in proteasomes to short
peptides.
• A very few of these are displayed on the cell surface
by the MHC [major histocompatibility] complex as
part of the immune system,
• But most of them are further metabolised to free
amino acids.
• Some proteins are degraded by an alternative
system within the lysosomes.
5
Protein degradation
• Dietary proteins are initially denatured by the
stomach acid, in conjunction with limited
proteolysis by pepsin.
• In young mammals gastric rennin partially
hydrolyses and precipitates milk casein and
increases gastric residence time.
• Gastric acid also kills most ingested bacteria,
rendering the upper part of the gut almost sterile.
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Protein degradation
• Protein digestion is largely completed in the small
intestine at a slightly alkaline pH.
• The pancreatic proteases trypsin, chymotrypsin
and elastase divide the proteins into short
peptides.
• These are attacked from both ends by
aminopeptidase and carboxypeptidase, and the
fragments are finished off by dipeptidases secreted
from the gut wall.
7
Amino acid catabolism
• In animals, amino acids undergo oxidative
degradation in three different metabolic
circumstances:
During the normal synthesis and degradation of cellular
proteins.
When a diet is rich in protein and the ingested amino
acids exceed the body’s needs for protein synthesis.
During starvation or in uncontrolled diabetes mellitus,
when carbohydrates are either unavailable or not
properly utilized, cellular proteins are used as fuel.
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Amino acid catabolism
• The liver is the principal site of amino acid
metabolism.
• Other tissues, such as the kidney, the small intestine,
muscles, and adipose tissue, take part.
• The first step in the breakdown of amino acids is
the separation of the amino group from the carbon
skeleton, usually by a transamination reaction.
• The carbon skeletons resulting from the
deaminated amino acids are used to form either
glucose or fats, or they are converted to a metabolic
intermediate that can be oxidized by the citric acid
cycle.
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Amino acid catabolism
• Under all these metabolic conditions, amino acids
lose their amino groups to form α-keto acids, the
“carbon skeletons” of amino acids.
• The α-keto acids undergo oxidation to CO2 and H2O
• More importantly provide three- and four-carbon
units that can be converted by gluconeogenesis into
glucose, the fuel for brain, skeletal muscle, and
other tissues
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Amino acid catabolism
• Therefore, the processes of amino acid degradation
converge on the central catabolic pathways.
• The carbon skeletons of most amino acids finding
their way to the citric acid cycle.
• In some cases the reaction pathways of amino acid
breakdown closely parallel steps in the catabolism
of fatty acids.
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Amino acid catabolism
• The pathways for amino acid degradation
include a key step in which the α-amino group is
separated from the carbon skeleton and shunted
into the pathways of amino group metabolism
• We will deal first with amino group metabolism
and nitrogen excretion.
• We will then deal with the fate of the carbon
skeletons derived from the amino acids; along the
way we see how the pathways are interconnected.
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Metabolic Fates of Amino Groups
• Excess ammonia generated in other (extrahepatic)
tissues travels to the liver for conversion to the
excretory form.
• Glutamate and glutamine play especially critical
roles in nitrogen metabolism, acting as a kind of
general collection point for amino groups.
• In the cytosol of hepatocytes, amino groups from
most amino acids are transferred to α-ketoglutarate
to form glutamate, which enters mitochondria and
gives up its amino group to form NH4.
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Metabolic Fates of Amino Groups
• Excess ammonia generated in most other tissues is
converted to the amide nitrogen of glutamine,
which passes to the liver, then into liver
mitochondria.
• Glutamine or glutamate or both are present in
higher concentrations than other amino acids in
most tissues.
• In skeletal muscle, excess amino groups are
generally
transferred to pyruvate to form alanine, another
important molecule in the transport of amino
groups to
the liver.
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Central role of glutamate
• Glutamate also occupies a special position in amino acid
breakdown
• Most of the nitrogen from dietary protein is ultimately
excreted from the body via the glutamate pool.
• Glutamate is special because it is chemically related to 2oxoglutarate (= α-keto glutatarate) which is a key
intermediate in the citric acid (Krebs) cycle.
• Glutamate can be reversibly converted into 2-oxoglutarate
by transaminases or by glutamate dehydrogenase.
• In addition, glutamate can be reversibly converted into
glutamine, an important nitrogen carrier, and the most
common free amino acid in human blood plasma.
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Central role of glutamate
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Central role of glutamate
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Central role of glutamate
• Because of the participation of 2-oxoglutarate in
numerous transaminations, glutamate is a
prominent intermediate in nitrogen elimination as
well as in anabolic pathways.
• Glutamate, formed in the course of nitrogen
elimination, is either oxidatively deaminated by
liver glutamate dehydrogenase forming ammonia,
or converted to glutamine by glutamine synthetase
and transported to kidney tubule cells.
• There the glutamine is sequentially deamidated by
glutaminase and deaminated by kidney glutamate
dehydrogenase.
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Central role of glutamate
20
Central role of glutamate
• The ammonia produced in the latter two reactions
is excreted as NH4+ in the urine, where it helps
maintain urine pH in the normal range of pH 4 to pH
8.
• The extensive production of ammonia by peripheral
tissue or hepatic glutamate dehydrogenase is not
feasible because of the highly toxic effects of
circulating ammonia.
• Normal serum ammonium concentrations are in the
range of 20–40μM, and an increase in circulating
ammonia to about 400μM causes alkalosis and
neurotoxicity.
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Biosynthesis of urea
Urea biosynthesis occurs in four stages:




Transamination
oxidative deamination of glutamate
ammonia transport
reactions of the urea cycle
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Transamination reactions
• The first step in the catabolism of most L-amino
acids, once they have reached the liver, is removal
of the α-amino groups, promoted by enzymes
called aminotransferases or transaminases.
• In these transamination reactions, the -amino
group is transferred to the -carbon atom of alphaketoglutarate, leaving behind the corresponding keto acid analog of the amino acid
• The effect of transamination reactions is to collect
the amino groups from many different amino acids
in the form of L-glutamate
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Transamination reactions
• Transamination is an exchange of functional groups
between any amino acid (except lysine, proline, and
threonine) and an α-keto acid.
• The amino group is usually transferred to the keto
carbon atom of pyruvate, oxaloacetate, or αketoglutarate, converting the α-keto acid to alanine,
aspartate, or glutamate, respectively.
• Transamination reactions are catalyzed by specific
transaminases (also called aminotransferases),
which require pyridoxal phosphate as a coenzyme.
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Transamination reactions
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Transamination
reactions
•Inmanyaminotransferase
reactions, α -ketoglutarate is
the amino group acceptor.
• All aminotransferases have
pyridoxal phosphate (PLP) as
cofactor.
•The reaction is readily
reversible.
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The role of Pyridoxal Phosphate in the Transfer of
-Amino Groups to alpha-Ketoglutarate
• All aminotransferases have the same prosthetic
group and the same reaction mechanism.
• The prosthetic group is pyridoxal phosphate (PLP),
the coenzyme form of pyridoxine, or vitamin B6.
• Also we saw pyridoxal phosphate in as a coenzyme
in the glycogen phosphorylase reaction.
• Its primary role in cells is in the metabolism of
molecules with amino groups.
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The role of Pyridoxal Phosphate in the Transfer of
α-Amino Groups to α-Ketoglutarate
• Pyridoxal phosphate functions as an intermediate
carrier of amino groups at the active site of
aminotransferases.
• It undergoes reversible transformations between its
aldehyde form, pyridoxal phosphate, which can
accept an amino group, and its aminated form,
pyridoxamine phosphate, which can donate its
amino group to an α-keto acid.
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L-GLUTAMATE DEHYDROGENASE OCCUPIES A CENTRAL
POSITION IN NITROGEN METABOLISM
• Transfer of amino nitrogen to α-ketoglutarate forms
L-glutamate.
• Release of this nitrogen as ammonia is then
catalyzed by hepatic L-glutamate dehydrogenase
• (GDH), which can use either NAD+ or NADP+
• Conversion of α-amino nitrogen to ammonia
by the concerted action of glutamate
aminotransferase and GDH is often termed
“transdeamination”
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L-GLUTAMATE DEHYDROGENASE OCCUPIES A CENTRAL
POSITION IN NITROGEN METABOLISM
• The L-glutamate
dehydrogenase reaction.
• NAD(P)+ means that either
NAD+ or NADP+ can serve
as co-substrate.
• The reaction is reversible
but favors
• glutamate formation.
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L-GLUTAMATE DEHYDROGENASE OCCUPIES A CENTRAL
POSITION IN NITROGEN METABOLISM
• Glutamate dehydrogenase operates at an important
intersection of carbon and nitrogen metabolism.
• An allosteric enzyme with six identical subunits, its
activity is influenced by a complicated array of
allosteric modulators.
• The best-studied of these are the positive
modulator ADP and the negative modulator GTP
• Mutations that alter the allosteric binding site for
GTP or otherwise cause permanent activation of
glutamate dehydrogenase lead to a human genetic
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disorder called hyperinsulinism-hyperammonemia
L-GLUTAMATE DEHYDROGENASE OCCUPIES A CENTRAL
POSITION IN NITROGEN METABOLISM
• Serum aminotransferases such as aspartate
aminotransferase, AST (also called serum
glutamate-oxaloacetate transaminase, SGOT) and
alanine transaminase, ALT (also called serum
glutamate-pyruvate transaminase (SGPT) have been
used as clinical markers of tissue damage,
• with increasing serum levels indicating an increased
extent of damage.
• As indicated earlier, ALT has an important function
in the delivery of skeletal muscle carbon and
nitrogen (in the form of alanine) to the32 liver in a
Glutamine Transports Ammonia in the
Bloodstream
• Ammonia is quite toxic to animal ,and the levels
present in blood are regulated.
• In many tissues, including the brain, some
processes such as nucleotide degradation generate
free ammonia.
• In most animals much of the free ammonia is
converted to a nontoxic compound before export
from the extrahepatic tissues into the blood and
transport to the liver or kidneys.
• For this transport function, glutamate,33critical to
Glutamine Transports Ammonia in the
Bloodstream
34
Glutamine Transports Ammonia in the
Bloodstream
• The glutamine is sequentially deamidated by
glutaminase and deaminated by kidney glutamate
dehydrogenase.
• The ammonia produced in the kidney reactions is
excreted as NH4+ in the urine.
• This helps maintain urine pH in the normal range of
pH 4 to pH 8.
• The extensive production of ammonia by peripheral
tissue or hepatic glutamate dehydrogenase is not
feasible because of the highly toxic effects of
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circulating ammonia.
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Glucose-alanine cycle
• Alanine also plays a special role in transporting
amino groups to the liver in a nontoxic form, via a
pathway called the glucose-alanine cycle
• In muscle and certain other tissues that degrade
amino acids for fuel, amino groups are collected in
the form of glutamate by transamination.
• Glutamate can transfer its -amino group to pyruvate,
a readily available product of muscle glycolysis, by
the action of alanine aminotransferase.
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Glucose-alanine cycle
• The alanine so formed passes into the blood
and travels to the liver.
• In the cytosol of hepatocytes alanine
aminotransferase transfers the amino group
• from alanine to alpha-ketoglutarate, forming
pyruvate and glutamate
37
Glucose-alanine cycle
38
Urea cycle
• About 80% of the excreted nitrogen is in the form of
urea is produced exclusively in the liver, in a series
of reactions that are distributed between the
mitochondrial matrix and the cytosol.
• The series of reactions that form urea is known as
the Urea Cycle or the Krebs-Henseleit Cycle.
• In the urea cycle, amino groups of urea are donated
by carbamoyl phosphate and aspartate, while the
carbon atom of urea is contributed by bicarbonate.
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Urea cycle
• The essential features of the urea cycle reactions
and their metabolic regulation are as follows:
 Arginine from the diet or from protein breakdown is
cleaved by the cytosolic enzyme arginase, generating
urea and ornithine.
In subsequent reactions of the urea cycle a new urea
residue is built on the ornithine, regenerating arginine
and perpetuating the cycle.
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Arginine
41
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Urea cycle
• The net reaction for urea synthesis shows
consumption of 4 "high energy" phosphoanhydride
bonds, contributed by ATP.
• Two of these are used by for synthesis of carbamoyl
phosphate from bicarbonate and ammonia.
• The ammonia is itself ultimately derived from
various amino acids by the combined action of
transaminase enzymes and glutamate
dehydrogenase.
• Carbamoyl phosphate synthesis occurs in the
mitochondrial matrix, and is catalyzed43by carbamoyl
Urea cycle
• The carbamoyl phosphate produced is then
consumed in the synthesis of citrulline from
ornithine.
• This reaction is catalyzed by ornithine
carbamoyltransferase.
• The citrulline is shuttled out of the mitochondrion
and into the cytosol, where the rest of the urea
cycle takes place.
• Another amino acid-derived amino group is
incorporated into the intermediate Aspartate is
joined via its α-amino group to citrulline
in
the
44
Urea cycle
• The next reaction is the elimination of fumarate
from argininosuccinate, yielding arginine.
• This step is catalyzed by argininosuccinase.
• Finally, urea is produced by arginase, acting on
arginine and water as substrates.
• Urea is secreted into the bloodstream, from which it
is ultimately eliminated by the kidneys for excretion.
• The ornithine produced in this last step is shuttled
into the mitochondrial matrix, completing the cycle
45
Regulation of the Urea Cycle
• The urea cycle operates only to eliminate excess
nitrogen.
• On high-protein diets the carbon skeletons of the
amino acids are oxidized for energy or stored as fat
and glycogen, but the amino nitrogen must be
excreted.
• Enzymes of the urea cycle are controlled at the
gene level.
• long-term changes in the quantity of dietary protein,
changes of 20-fold or greater in the concentration
of cycle enzymes are observed.
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Regulation of the Urea Cycle
•
•
•
•
•
When dietary proteins increase significantly, enzyme concentrations rise.
On return to a balanced diet, enzyme levels decline.
Under conditions of starvation, enzyme levels rise as proteins are degraded and amino acid carbon skeletons are used
to provide energy, thus increasing the quantity of nitrogen that must be excreted.
Short-term regulation of the cycle occurs principally at CPS-I, which is inactive in the absence of its obligate activator Nacetylglutamate.
The steady-state concentration of N-acetylglutamate is set by the concentration of its components acetyl-CoA and
glutamate and by arginine, which is a positive allosteric effector of N-acetylglutamate synthase
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.
Read on molecular basis of
ammonia intoxication
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Krebs bicycle
• Because the fumarate produced in the
argininosuccinase reaction is also an intermediate
of the citric acid cycle.
• each cycle can operate independently and
communication between them depends on the
transport of key intermediates between the
mitochondrion and cytosol.
• The fumarate generated in cytosolic arginine
synthesis can therefore be converted to malate in
the cytosol, and these intermediates can be further
metabolized in the cytosol or transported into
49
Urea Cycle Disorders (UCDs)
• A complete lack of any one of the enzymes of the
urea cycle will result in death shortly after birth.
• However, deficiencies in each of the enzymes of the
urea cycle, including N-acetylglutamate synthase,
have been identified.
• These disorders are referred to as urea cycle
disorders or UCDs.
• Take your time read on UCDs
50
Krebs bicycle
• Aspartate formed in mitochondria by
transamination between oxaloacetate and
glutamate can be transported to the cytosol, where
it serves as nitrogen donor in the urea cycle
reaction catalyzed by argininosuccinate synthetase.
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Catabolism of the Carbon Skeletons
of Amino Acids
• The 20 catabolic pathways converge to form only six
major products, all of which enter the citric acid
cycle.
• From here the carbon skeletons are diverted to
gluconeogenesis or ketogenesis or are completely
oxidized to CO2 and H2O.
• All or part of the carbon skeletons of seven amino
acids are ultimately broken down to acetyl-CoA.
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Catabolism of the Carbon Skeletons
of Amino Acids
• Five amino acids are converted to alphaketoglutarate, four to succinyl-CoA, two to fumarate,
and two to oxaloacetate.
• Parts or all of six amino acids are converted to
pyruvate, which can be converted to either acetylCoA or oxaloacetate.
• Note that some amino acids appear more than once,
reflecting different fates for different parts of their
carbon skeletons.
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Some Amino Acids Are Converted to Glucose,
Others to Ketone Bodies
• The seven amino acids that are degraded entirely or
in part to acetoacetyl-CoA and/or acetyl-CoA—
phenylalanine,
tyrosine,
isoleucine,
leucine,
tryptophan,
threonine,
lysine
• Can yield ketone bodies in the liver
57
Some Amino Acids Are Converted to Glucose,
Others to Ketone Bodies
• Where acetoacetyl-CoA is converted to
acetoacetate and then to acetone and hydroxybutyrate .
• These are the ketogenic amino acids
• Their ability to form ketone bodies is
particularly evident in uncontrolled diabetes
mellitus, in which the liver produces large
amounts of ketone bodies from both fatty
acids and the ketogenic amino acids.
58
Some Amino Acids Are Converted to Glucose,
Others to Ketone Bodies
• The amino acids that are degraded to pyruvate, ketoglutarate, succinyl-CoA, fumarate, and/or
oxaloacetate can be converted to glucose and
glycogen.
• They are the glucogenic amino acids.
• However, division between ketogenic and
glucogenic amino acids is not clear.
• For examplel, five amino acids—tryptophan,
phenylalanine, tyrosine, threonine, and isoleucine—
are both ketogenic and glucogenic
59
• Leucine is an exclusively ketogenic amino
acid that
Enzyme Cofactors Playing Important Roles in
Amino Acid Catabolism
• We have already considered one important class:
transamination reactions requiring pyridoxal
phosphate.
• Another common type of reaction in amino acid
catabolism is one-carbon transfers, which usually
involve one of three cofactors:
biotin,
tetrahydrofolate,
or S-adenosylmethionine
 Read on how these cofactors are synthesized
60
Amino Acids Degraded to Pyruvate
• The carbon skeletons of six amino acids are
converted in whole or in part to pyruvate.
• The pyruvate can then be converted to either
acetyl-CoA (a ketone body precurs) or oxaloacetate
(a precursor for gluconeogenesis).
• Thus amino acids catabolized to pyruvate are both
ketogenic and glucogenic
• Tryptophan, cysteine, serine, glycine, and threonine
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Amino Acids Degraded to Acetyl-CoA
• Portions of the carbon skeletons of seven amino
acids—Tryptophan, lysine, phenylalanine, tyrosine,
leucine,isoleucine, and threonine—yield acetylCoA and/or acetoacetyl-CoA, the latter being
converted to acetyl-coA.
• Tryptophan breakdown is the most complex of all
the pathways of amino acid catabolism in animal
tissue
63
Amino Acids Degraded to Acetyl-CoA
• Portions of tryptophan (four of its carbons) yield
acetyl-CoA via acetoacetyl-CoA.
• Some of the intermediates in tryptophan
catabolism are precursors for the synthesis of other
biomolecules , including nicotinate, precursor of
NAD and NADP in animals; serotonin.
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65
Amino Acids Degraded to Acetyl-CoA
• The breakdown of phenylalanine is noteworthy .
• Genetic defects in the enzymes of this pathway lead
to several inheritable human diseases.
• Phenylalanine and its oxidation product tyrosine are
degraded into two fragments, both of which can
enter the citric acid cycle:
Four of the nine carbon atoms yield free
acetoacetate, which is converted to acetoacetylCoA.
Second four-carbon fragment is recovered as
fumarate.
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Amino Acids Degraded to Acetyl-CoA
• Phenylalanine, after its hydroxylation to tyrosine, is
also the precursor of dopamine, a neurotransmitter,
and of norepinephrine and epinephrine, hormones
secreted by the adrenal medulla’.
• Melanin, the black pigment of skin and hair, is also
derived from tyrosine.
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•
•
•
•
Phenylalanine catabolism is genetically
defective in some people
(Phenylketonuria: PKU)
Phenylketonuria (PKU) is an autosomal recessive
disorder resulting from defects in the metabolism of
phenylalanine.
PKU is a hyperphenylalaninemia with multifactorial
causes.
There are several hyperphenylalaninemias that are
not PKU and are called non-PKU
hyperphenylalaninemias (HPAs).
Hyperphenylalaninemia is defined as a plasma
phenylalanine concentration >120μM.
69
•
•
•
•
Phenylalanine catabolism is genetically
defective in some people
(Phenylketonuria: PKU)
PKU is caused by mutation in the phenylalanine
hydroxylase gene (gene symbol = PAH).
The HPAs are disorders of phenylalanine
hydroxylation.
The reaction catalyzed by PAH involves
tetrahydrobiopterin (BH4) as a co-factor, the HPAs
can result from defects in any of the several genes
required for synthesis and recycling of BH4.
Removal of excess phenylalanine normally proceeds
via the tyrosine biosynthesis reaction and then via
70
•
•
•
•
Phenylalanine catabolism is genetically
defective in some people
(Phenylketonuria: PKU)
However, defects of other factors in phenylalanine
hydroxylation system may have similar effect as
phenylalanine hydroxylase defect.
Also, hyperphenylalaninemia results in
abnormalities in the metabolism of many
compounds derived from the aromatic amino acids.
If left untreated, these metabolic abnormalities
cause postnatal brain damage and severe mental
retardation.
PKU is the most common inborn error in amino acid
71
Phenylalanine catabolism is genetically
defective in some people
(Phenylketonuria: PKU)
• The mental retardation is caused by the
accumulation of phenylalanine, which becomes a
major donor of amino groups in aminotransferase
activity and depletes neural tissue of 2-oxoglutarate
(α-ketoglutarate).
• This absence of 2-oxoglutarate in the brain shuts
down the TCA cycle and the associated production
of aerobic energy, which is essential to normal brain
development.
• The product of phenylalanine transamination,
72
phenylpyruvic acid, is reduced to phenylacetate
and
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Amino Acids Converted to αKetoglutarate
• The carbon skeletons of proline, glutamate,
glutamine, arginine, and histidine enter the citric
acid cycle as α-ketoglutarate.
• Proline, glutamate, and glutamine have fivecarbon skeletons.
• The cyclic structure of proline is opened by
oxidation to produce glutamate.
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•
Amino Acids Converted to SuccinylCoA
The carbon skeletons of methionine, isoleucine,
threonine, and valine are degraded by pathways
that yield succinyl-CoA, an intermediate of the citric
acid cycle.
• Methionine donates its methyl group to one of
several possible acceptors through Sadenosylmethionine, and three of its four remaining
carbon atoms are converted to the propionate of
propionyl-CoA, a precursor of succinyl-CoA.
• Isoleucine undergoes transamination, followed by
oxidative decarboxylation of the resulting α-keto
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acid.
Amino Acids Converted to SuccinylCoA
• Valine undergoes transamination and
decarboxylation, then a series of oxidation
reactions that convert the remaining four carbons
to propionyl-CoA.
• Some parts of the valine and isoleucine degradative
pathways closely parallel steps infatty acid
degradation.
• In human tissues, threonine is also converted in
two steps to propionyl-CoA. This is the primary
pathway for threonine degradation in humans.
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Branched-Chain Amino Acids Are Not
Degraded in the Liver
• Although much of the catabolism of amino acids
takes place in the liver, the three amino acids with
branched side chains (leucine, isoleucine, and
valine) are oxidized as fuels primarily in muscle,
adipose, kidney, and brain tissue.
• These extrahepatic tissues contain an
aminotransferase, absent in liver, that acts on all
three branched-chain amino acids to produce the
corresponding-keto acids.
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Non-essential amino acid biosynthesis
Non-essential amino acid
• Alanine, Asparagine, Aspartate, Cysteine, Glutamate,
Glutamine, Glycine, Proline, Serine, Tyrosine.
Essential amino acid
• Arginine, Histidine, Isoleucine, Leucine, Lysine,
Methionine, Phenylalanine, Threonine, Tyrptophan,
Valine.
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Non-essential amino acid biosynthesis
• The amino acids arginine, methionine and
phenylalanine are considered essential for reasons
not directly related to lack of synthesis.
• Arginine is synthesized by mammalian cells but at a
rate that is insufficient to meet the growth needs of
the body and the majority that is synthesized is
cleaved to form urea.
• Methionine is required in large amounts to produce
cysteine if the latter amino acid is not adequately
supplied in the diet.
• Similarly, phenylalanine is needed in large
amounts
82
Glutamate and Aspartate
•Glutamate is synthesized from its' widely distributed α-keto acid
precursor by a simple one-step transamination reaction catalyzed by
glutamate dehydrogenase (GDH).
•As it has been discussed, the glutamate dehydrogenase reaction plays a
central role in overall nitrogen homeostasis
83
Glutamate and Aspartate
Like glutamate, aspartate is synthesized by a simple
one-step transamination reaction catalyzed by
aspartate aminotransferase, AST (formerly referred to
as serum glutamate-oxalate transaminase, SGOT).
84
Glutamate and Aspartate
Also, aspartate can be derived from asparagine
through the action of asparaginase.
The importance of aspartate as a precursor of
ornithine for the urea cycle is described in the urea
cycle.
85
Alanine and the Glucose-Alanine
Cycle
• Alanine is second only to glutamine in prominence
as a circulating amino acid.
• It serves a unique role in the transfer of nitrogen
from peripheral tissue to the liver.
• Alanine is transferred to the circulation by many
tissues, but mainly by muscle, in which alanine is
formed from pyruvate at a rate proportional to
intracellular pyruvate levels.
• Liver accumulates plasma alanine, reverses the
transamination that occurs in muscle, and
proportionately increases urea production.
86
Alanine and the Glucose-Alanine Cycle
There are two main pathways to production of muscle
alanine:
directly from protein degradation, and
via the transamination of pyruvate by alanine
transaminase, ALT (also referred to as serum
glutamate-pyruvate transaminase, SGPT).
87
•
•
•
•
•
Cysteine Biosynthesis: Role of
Methionine
The sulfur for cysteine
synthesis comes from the
essential amino acid methionine.
A condensation of ATP and methionine, catalyzed
by methionine adenosyltransferase (MAT), yields Sadenosylmethionine (SAM or AdoMet).
In the production of SAM all phosphates of an ATP
are lost: one as Pi and two as PPi.
It is adenosine which is transferred to methionine.
MAT is also called S-adenosylmethionine synthetase.
88
Cysteine Biosynthesis: Role of
Methionine
• The synthesis of cysteine represents an extremely
important and clinically relevant biochemical
pathway.
• Several vitamins are required for this metabolic
pathway to proceed emphasizing the nutritional
impact.
• Folate, pyridoxal phosphate (PLP, B6), and B12 are
all necessary for cysteine synthesis.
• The enzyme methionine synthase requires both
folate and B12 for activity.
• Deficiency in either of these vitamins89contributes
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Homocysteinemia / Homocystinemia
• Homocysteinemias (homocystinemias) represent a
family of inherited disorders resulting from defects
in several of the genes involved in the conversion of
methionine to cysteine.
• As the name implies, these disorders result in
elevated levels of homocysteine and homocystine in
the urine, where the elevated urine output of the
metabolite is referred to as homocysteinurina.
• Homocystine is a disulfide-bonded homodimer of
two homocysteines.
• This is similar to the formation of cystine from two
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cysteines.
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B Vitamins and Homocysteine
• Several studies have shown consistent and strong
relationships between low concentrations of blood
folate, vitamin B12 and vitamin B6, and high
concentrations of homocysteine which, in turn,
have been linked with heart disease, stroke and
other vascular outcomes.
• The breakdown of homocysteine to cysteine
requires the vitamin B6 dependent enzyme,
cystathionine beta synthase
• Re-methylation to methionine requires a vitamin
B12 dependent enzyme, with folate as a cofactor.
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• The most common cause of homocysteine
B Vitamins and Homocysteine
• Active folate, known as 5-MTHF or 5methyltetrahydrofolate, works in concert with
vitamin B12 as a methyl-group donor in the
conversion of homocysteine back to methionine.
• Normally, about 50% of homocysteine is
remethylated; the remaining homocysteine is
transsulfurated to cysteine, which requires vitamin
B6 as a co-factor.
• This pathway yields cysteine, which is then used by
the body to make glutathione, a powerful
antioxidant that protects cellular components
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B Vitamins and Homocysteine
• Vitamin B2 (riboflavin) and magnesium are also
involved in homocysteine metabolism.
• Thus a person needs several different B-vitamins to
help keep homocysteine levels low and allow for it
to be properly transformed into helpful antioxidants
like glutathione.
• Without B6, B12, B2, folate, and magnesium,
dangerous levels of homocysteine may build up in
the body.
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Homocysteine and Heart Disease
• Lowering high homocysteine is thought to be
important because of its strong and consistent
relationship with heart disease.
• High homocysteine has been linked with occurrence
and mortality from heart disease - as well as with
indicators of heart disease risk, including thickening
of the carotid arteries and venous thrombosis.
• In 1968, a Harvard researcher observed that
children with a genetic defect that caused them to
have sharply elevated homocysteine levels suffered
severe atherosclerotic occlusion and vascular
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disorders similar to what is seen in middle-aged
How Elevated Homocysteine
Leads to Vascular Damage
• If unhealthy levels of homocysteine accumulate in
the blood, the delicate lining of an artery
(endothelium) can be damaged.
• Homocysteine can both initiate and potentiate
atherosclerosis.
• For example, homocysteine-induced injury to the
arterial wall is one of the factors that can initiate
the process of atherosclerosis, leading to
endothelial dysfunction and eventually to heart
attacks and strokes
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Tyrosine Biosynthesis
• Tyrosine is produced in cells by hydroxylating the
essential amino acid phenylalanine.
• This relationship is much like that between cysteine
and methionine.
• Half of the daily requirement for phenylalanine is
for the production of tyrosine;
• If the diet is rich in tyrosine itself, the requirements
for phenylalanine can be reduced by about 50%.
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Tyrosine Biosynthesis
• Phenylalanine hydroxylase (PAH) is a mixedfunction oxygenase: one atom of oxygen is
incorporated into water and the other into the
hydroxyl of tyrosine.
• The reductant is the tetrahydrofolate-related
cofactor tetrahydrobiopterin, which is maintained
in the reduced state by the NADH-dependent
enzyme dihydropteridine reductase (DHPR).
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Tyrosine Biosynthesis
• Missing or deficient phenylalanine hydroxylase
results in hyperphenylalaninemia.
• Hyperphenylalaninemia is defined as a plasma
phenylalanine concentration greater than 2mg/dL
(120μM).
• The most widely recognized hyperphenylalaninemia
(and most severe) is the genetic disease known as
phenlyketonuria (PKU).
• Patients suffering from PKU have plasma
phenylalanine levels >1000μM, whereas the nonPKU hyperphenylalaninemias exhibit levels
of
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Ornithine and Proline Biosynthesis
• Glutamate serves as the precursor for the synthesis
of both ornithine and proline which are derived
from the Δ1-pyrroline-5-carboxylate intermediate in
the pathway.
• Formation of Δ1-pyrroline-5-carboxylate occurs via
the action of the bi-functional enzyme, aldehyde
dehydrogenase
• The transamination of the tautomeric form of Δ1pyrroline-5-carboxylate (glutamate γ-semialdehyde)
results in the generation of ornithine.
• The reduction of Δ1-pyrroline-5-carboxylate to
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proline occurs via the action of pyrroline-5-
Ornithine and Proline Biosynthesis
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Serine biosynthesis
Serine can be derived from the glycolytic intermediate, 3phosphoglycerate, in a three-step reaciton pathway.
The first reaction is catalyzed by phosphoglycerate dehydrogenase
(PHGDH).
The second reaction is a simple transamination catalyzed by
phosphoserine aminotransferase 1 (PSAT1) which utilizes glutamate as
the amino donor and releases 2-oxoglutarate (α-ketoglutarate).
The last step in the reaction pathway is catalyzed by phosphoserine
phosphatase (PSPH).
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Glycine Biosynthesis
• The main pathway to glycine is a one-step reversible
reaction catalyzed by serine
hydroxymethyltransferase (SHMT).
• This enzyme is a member of the family of onecarbon transferases and is also known as glycine
hydroxymethyltransferase.
• This reaction involves the transfer of the
hydroxymethyl group from serine to the cofactor
tetrahydrofolate (THF), producing glycine and
N5,N10-methylene-THF
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Glycine Biosynthesis
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Glycine Biosynthesis
Glycine as a Neurotransmitter
• Glycine is involved in many anabolic reactions other
than protein synthesis including the synthesis of
purine nucleotides, heme, glutathione, creatine and
serine.
• Also, glycine functions in the central nervous system
as an inhibitory neurotransmitter where it
participates in regulating signals that process motor
and sensory information that permit movement,
vision and audition.
• Glycine is co-released with GABA which is the
primary inhibitory neurotransmitter.
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Aspartate/Asparagine and Glutamate/Glutamine
Biosynthesis
Glutamate is synthesized by the reductive amination of 2oxoglutarate (α-ketoglutarate) catalyzed by glutamate
dehydrogenase
it is thus a nitrogen-incorporating reaction.
In addition, glutamate arises by aminotransferase reactions,
with the amino nitrogen being donated by a number of
different amino acids.
Thus, glutamate is a general collector of amino nitrogen
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Aspartate/Asparagine and Glutamate/Glutamine
Biosynthesis
Aspartate is formed in a transamination reaction catalyzed by
aspartate transaminase, AST.
This reaction uses the aspartate α-keto acid analog,
oxaloacetate, and glutamate as the amino donor.
Aspartate can also be formed by deamination of asparagine
catalyzed by asparaginase
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Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
•Asparagine synthetase and glutamine synthetase, catalyze the
production of asparagine and glutamine from their respective α-amino
acids.
•Glutamine is produced from glutamate by the direct incorporation of
ammonia; and this can be considered another nitrogen incorporating
reaction.
• Asparagine, however, is formed by an amidotransferase reaction.
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Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
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Specialized products of amino acids
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Specialized products of amino acids
• Important products derived from amino acids
include heme, purines, pyrimidines, hormones,
neurotransmitters, and biologically active peptides.
• Small peptides or peptide-like molecules not
synthesized on ribosomes fulfill specific functions in
cells.
• Histamine plays a central role in many allergic
reactions.
• Neurotransmitters derived from amino acids
include γ-aminobutyrate, 5-hydroxytryptamine
(serotonin), dopamine, norepinephrine, and
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epinephrine.
Tyrosine-Derived Neurotransmitters /
Hormones
• Much of the tyrosine that does not get incorporated
into proteins is catabolized for energy production.
• Another significant fate of tyrosine is conversion to
the catecholamines.
• The catecholamines are dopamine, norepinephrine,
and epinephrine.
• All three catecholamines exert effects in numerous
locations in the body as either a neurotransmitter
or as a hormone.
• Within the brain the catecholamines exert their
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effects as neurotransmitters, in the periphery they
Tyrosine-Derived Neurotransmitters /
Hormones
• Tyrosine is transported into catecholaminesecreting neurons and adrenal medullary cells
where catecholamine synthesis takes place.
• The first step in the process requires tyrosine
hydroxylase which, like phenylalanine hydroxylase
(of tyrosine synthesis), requires tetrahydrobiopterin
(H4B, or written as BH4) as cofactor.
• The tyrosine hydroxylase reaction represents the
rate-limiting reaction of catecholamine biosynthesis.
• The dependence of tyrosine hydroxylase on H4B
necessitates the coupling to the action115of
Tyrosine-Derived Neurotransmitters /
Hormones
• The product of the tyrosine hydroxylase reaction is
3,4-dihydrophenylalanine (L-DOPA; more commonly
just DOPA).
• The enzyme DOPA decarboxylase then converts
DOPA to dopamine.
• The enzyme dopamine β-hydroxylase then converts
dopamine to norepinephrine.
• Dopamine β-hydroxylase is a major vitamin C and
copper (Cu2+)-dependent enzyme.
• The last step of catecholamine biosynthesis
is
the
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Tyrosine-Derived Neurotransmitters /
Hormones
• The enzyme phenylethanolamine Nmethyltransferase catalyzes this methylation
reaction utilizing SAM as a methyl donor.
• In addition to epinephrine synthesis, the last
reaction generates S-adenosylhomocysteine.
• Within the substantia nigra locus of the brain, and
some other regions of the brain, synthesis proceeds
only to dopamine.
• Within the locus coeruleus region of the brain the
end product of the pathway is norepinephrine.
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• Within adrenal medulla chromaffin cells,
tyrosine is
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Tyrosine-Derived Neurotransmitters /
Hormones
• The actions of norepinephrine and epinephrine are
exerted via receptor-mediated signal transduction
events.
• The receptors to which epinephrine and
norepinephrine bind are referred to as adrenergic
receptors.
• The adrenergic receptors are members of the Gprotein coupled receptor (GPCR) family.
• There are two distinct classes of adrenergic
receptor identified as the α (alpha) and β (beta)
receptors.
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Tyrosine-Derived Neurotransmitters /
Hormones
• The α1 class contains the α1A, α1B, and α1D receptors.
• The α1 receptor class are coupled to Gq-type Gproteins that activate PLCβ resulting in increases in
IP3 and DAG release from membrane PIP2.
• The α2 class contains the α2A, α2B, and α2C receptors.
• The α2 class of adrenergic receptors are coupled to
Gi-type G-proteins that inhibit the activation of
adenylate cyclase and therefore, receptor activation
results in reduced levels of cAMP and consequently
reduced levels of active PKA.
• The β class of receptors is composed of
three
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Tyrosine-Derived Neurotransmitters /
Hormones
• Dopamine binds to dopamineric receptors
identified as D-type receptors and there are five
subclasses identified as D1, D2, D3, D4, and D5.
• All five dopamine receptors belong the the Gprotein coupled receptor (GPCR) family.
• The D1 and D5 dopamine receptors are coupled to
the activation of Gs-type G-proteins and, therefore,
receptor activation results in activation of adenylate
cyclase.
• The D2, D3, and D4 dopamine receptors are coupled
to Gi-type G-proteins and, therefore, receptor
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•
Tryptophan-Derived
Neurotransmitters
Tryptophan serves as the precursor for the synthesis
of serotonin (5-hydroxytryptamine, 5-HT) and
melatonin (N-acetyl-5-methoxytryptamine)
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serotonin
• Serotonin is synthesized through a two-step process
involving a tetrahydrobiopterin-dependent
hydroxylation reaction (catalyzed by tryptophan-5monooxygenase, also called tryptophan hydroxylase)
and then a decarboxylation catalyzed by aromatic Lamino acid decarboxylase.
• Tryptophan hydroxylase represents the rate-limiting
step in serotonin and melatonin synthesis.
• READ ON THE FUNCTIONS OF SEROTONIN
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Melatonin
• Melatonin is derived from serotonin within the
pineal gland and the retina, where the necessary Nacetyltransferase enzyme is found.
• The pineal parenchymal cells secrete melatonin into
the blood and cerebrospinal fluid.
• Synthesis and secretion of melatonin increases
during the dark period of the day and is maintained
at a low level during daylight hours.
• This diurnal variation in melatonin synthesis is
brought about by norepinephrine secreted by the
postganglionic sympathetic nerves that innervate
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the pineal gland.
Melatonin
• The effects of norepinephrine are exerted through
interaction with β-adrenergic receptors.
• This leads to increased levels of cAMP, which in turn
activate the N-acetyltransferase required for
melatonin synthesis.
• Melatonin functions by inhibiting the synthesis and
secretion of other neurotransmitters such as
dopamine and GABA.
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Creatine Biosynthesis
• Creatine synthesis begins in the kidneys using the
amino acids arginine and glycine.
• The formation of guanidinoacetate from these two
amino acids is catalyzed by the enzyme glycine
amidinotransferase, also called L-arginine:glycine
amidinotransferase.
• Guanidinoacetate is transported to the blood and
picked up by heptocytes where it is methylated
forming creatine.
• The methyl donor for this reation is Sadenosylmethionine (SAM) and the reaction is
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catalyzed by the enzyme guanidinoacetate N-
Creatine Biosynthesis
• Creatine is released to the blood where is is picked
up by the brain and skeletal muscle cells through
the action of the transporter.
• Within these cells creatine is phosphorylated by
creatine kinases (CK; also called creatine
phosphokinase, CPK) that generate the high-energy
storage compound, creatine phosphate.
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Creatine Biosynthesis
• Creatine is used as a storage form of high energy phosphate.
• The phosphate of ATP is transferred to creatine, generating creatine
phosphate, through the action of creatine phosphokinase.
• The reaction is reversible such that when energy demand is high (e.g.
during muscle exertion) creatine phosphate donates its phosphate to
ADP to yield ATP.
• Both creatine and creatine phosphate are found in muscle, brain and
blood.
• Creatinine is formed in muscle from creatine phosphate by a
nonenzymatic dehydration and loss of phosphate.
• The amount of creatinine produced is related to muscle mass and
remains remarkably constant from day to day.
• Creatinine is excreted by the kidneys and the level of excretion
(creatinine clearance rate) is a measure of renal function.
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Glutathione
• Glutathione (GSH) is a tripeptide composed of glutamate,
cysteine and glycine.
• Glutathione serves as a potent reductant eliminating
hydroxy radicals, peroxynitrites, and hydroperoxides.
• It is conjugated to drugs to make them more water soluble.
• It is involved in amino acid transport across cell
membranes (the γ-glutamyl cycle)
• It is a substrate for the peptidoleukotrienes.
• it serves as a cofactor for some enzymatic reactions.
• It serves as an aid in the rearrangement of protein disulfide
bonds.
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Glutathione
• GSH is synthesized in the cytosol of all mammalian
cells via the two-step reaction.
• The rate of GSH synthesis is dependent upon the
availability of cysteine and the activity of the ratelimiting enzyme, γ-glutamylcysteine synthetase
(also called glutamate-cysteine ligase, GCL).
• The second reaction of GSH synthesis involves the
enzyme, glutathione synthetase, which condenses
γ-glutamylcysteine with glycine.
• Both reactions of GSH synthesis require ATP.
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