Download Nitrogen Balance

Nitrogen Balance
N balance = Nin - Nout
• In normal adults on adequate diet, nitrogen intake equal
nitrogen excreted,this is the nitrogen balance.
• Major dietary source of N is Protein (>95).
• AA are used for protein synthesis & N containing
compounds.
• AA in excess are degraded (used for energy)
N is disposed of in urea (80%), ammonia, uric acid or
creatinine in urine with small amounts in fecal matter
(undigested)
Positive Nitrogen Balance
Positive nitrogen balance,an excess of ingested over excreted nitrogen,
occurs during growth, pregnancy, lactation and recovery from diseases.
Negative Nitrogen Balance
• Negative nitrogen
balance, where output
exceeds intake, may occur
after surgery, in advanced
cancer, low protein diet,
malabsorption,starvation
and some hormonal
imbalance(eg.hyperthyrio
-dism).
Overall Nitrogen Metabolism
• Amino acid catabolism is part of the larger process of
the metabolism of nitrogen-containing molecules.
Nitrogen enters the body in a variety of compounds
present in food, the most important being amino acids
contained in dietary protein.
• Nitrogen leaves the body as urea, ammonia, and other
products derived from amino acid metabolism.
• The role of body proteins in these transformations
involves two important concepts: the amino acid pool
and protein turnover.
Amino acid pool
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Free amino acids are present throughout the body, for example, in cells, blood,
and the extracellular fluids. all these amino acids are belonged to a single entity,
called the amino acid pool.
This pool is supplied by three sources: 1) amino acids provided by the degradation
of body proteins, 2) amino acids derived from dietary protein, and 3) synthesis of
nonessential amino acids from simple intermediates of metabolism .
Conversely, the amino pool is depleted by three routes: 1) synthesis of body
protein, 2) amino acids consumed as precursors of essential nitrogen-containing
small molecules, and 3) conversion of amino acids to glucose, glycogen, fatty acids
or CO2.
Although the amino acid pool is small (comprised of about 90–100 g of amino
acids) in comparison with the amount of protein in the body (about 12 kg in a 70kg man), it is at the center of whole-body nitrogen metabolism.
In healthy, well fed individuals, the input to the amino acid pool is balanced by the
output, that is, the amount of amino acids contained in the pool is constant. The
amino acid pool is said to be in a steady state.
Protein turnover
• Most proteins in the body are constantly being synthesized and then
degraded, permitting the removal of abnormal or unneeded proteins.
• For many proteins, regulation of synthesis determines the concentration
of protein in the cell, with protein degradation assuming a minor role.
• For other proteins, the rate of synthesis is relatively constant, and cellular
levels of the protein are controlled by selective degradation.
• Rate of turnover: In healthy adults, the total amount of protein in the
body remains constant, because the rate of protein synthesis is just
sufficient to replace the protein that is degraded. This process, called
protein turnover, leads to the hydrolysis and resynthesis of 300–400 g of
body protein each day.
• The rate of protein turnover varies widely for individual proteins. Shortlived proteins (for example, many regulatory proteins and misfolded
proteins) are rapidly degraded, having half-lives measured in minutes or
hours. Long-lived proteins, with half-lives of days to weeks, constitute the
majority of proteins in the cell. Structural proteins, such as collagen, are
metabolically stable, and have half-lives measured in months or years.
Examples of Non-protein
Nitrogenous Compounds
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heme
pyrimidines
purines
choline (serine)
creatine
bile salts (glycine)
Melanin (tyrosine)
porphyrins
epinephrine (phenylalanine)
nicotinic acid (tryptophan)
Almost all nitrogen in human metabolism comes from
dietary amino acids.
Amino acid pool
Catabolism of the Carbon Skeletons of Amino
Acids
– The catabolism of the 20 amino acids found in proteins involves the removal
of α-amino groups, followed by the breakdown of the resulting carbon
skeletons.
– The breakdown of the resulting carbon skeletons funnel into 7 metabolic
intermediates.
– These intermediates directly enter the pathways of intermediary metabolism,
resulting either in the synthesis of glucose or lipid or in the production of
energy through their oxidation to CO2 and water by the citric acid cycle.
• Acetyl–CoA
Ketogenic
• Acetoacetyl–CoA
• Pyruvate
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α-Ketoglutarate
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Succinyl–CoA
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Fumarate
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Oxaoloacetate
Glucogenic
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Glucogenic and Ketogenic A A
• Amino acids can be classified as glucogenic, ketogenic, or both based on
which of the seven intermediates are produced during their catabolism
Glucogenic amino acids
• Amino acids whose catabolism yields pyruvate or one of the intermediates
of the citric acid cycle are termed glucogenic or glycogenic. These
intermediates are substrates for gluconeogenesis and, therefore, can give
rise to the net formation of glucose or glycogen in the liver and glycogen in
the muscle.
Ketogenic amino acids
• Amino acids whose catabolism yields either acetoacetate or one of its
precursors (acetyl CoA or acetoacetyl CoA) are termed ketogenic.
• Carbon skeletons of ketogenic amino acids can be catabolized for energy
in Krebs Cycle, or converted to ketone bodies or fatty acids. They cannot
be converted to glucose.
• Leucine and lysine are the only exclusively ketogenic amino acids found in
proteins.
Glucogenic and Ketogenic A A
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Amino acids are grouped
according to their major
degradative end product.
Some amino acids are listed
more than once because
different parts of their carbon
skeletons are degraded to
different end products.
14 amino acids are exclusively
glucogenic.
Four of the amino acids are
both glucogenic and ketogenic.
Only two amino acids, leucine
and lysine, are exclusively
ketogenic.
Oxaloacetate
↓
phosphoenolpyruvate
↓
glucose
Two Amino Acids are Converted to
Oxaloacetate
• Asparagine and aspartate are
degraded to Oxaloacetate.
• The carbon skeletons of
asparagine and aspartate
ultimately enter the citric acid
cycle as oxaloacetate.
• The enzyme asparaginase
catalyzes the hydrolysis of
asparagine to aspartate.
• Aspartate undergoes
transamination with
ketoglutarate to yield
glutamate and oxaloacetate.
Two Amino Acids Can form
Fumarate
• Phenylalanine and tyrosine: Hydroxylation of
phenylalanine leads to the formation of tyrosine .
• This reaction, catalyzed by phenylalanine
hydroxylase, is the first reaction in the catabolism
of phenylalanine.
• Thus, the metabolism of phenylalanine and
tyrosine merge, leading ultimately to the
formation of fumarate and acetoacetate.
Phenylalanine and tyrosine are, therefore, both
glucogenic and ketogenic.
Catabolism of Phenylalanine and tyrosine
Five Amino Acids Are Converted to
α-Ketoglutarate
• The carbon skeletons of five amino acids (proline, glutamate,
glutamine, arginine, and histidine) enter the citric acid cycle as α ketoglutarate.
• Glutamate is converted to α-ketoglutarate by transamination, or
through oxidative deamination by glutamate dehydrogenase.
• Proline,glutamine, arginine, and histidine are first converted to
glutamate which is oxidatively deaminated to α –ketoglutarate by
glutamate dehydrogenase.
• Glutamine is converted to glutamate and ammonia by the enzyme
glutaminase . Glutamate is converted to α-ketoglutarate by
transamination, or through oxidative deamination by glutamate
dehydrogenase
• Proline,glutamate, and glutamine have five-carbon skeletons while
arginine and histidine have 6 carbon skeletons.
Proline
• The cyclic structure of proline is opened by
oxidation of the carbon most distant from the
carboxyl group to create a Schiff base, then
hydrolysis of the Schiff base to a linear
semialdehyde, glutamate -semialdehyde.
• This intermediate is further oxidized at the
same carbon to produce glutamate.
• Transamination or deamination of glutamate
produces -ketoglutarate.
Arginine and Histidine
• Arginine and histidine contain five adjacent carbons and a sixth carbon
attached through a nitrogen atom.
• The catabolic conversion of these amino acids to glutamate is therefore
slightly more complex than proline or glutamine.
• Arginine is converted to the five-carbon skeleton of ornithine by arginase
in the urea cycle, and the ornithine is transaminated to glutamate ᵞsemialdehyde which is converted to glutamate.
• glutamate ᵞ-semialdehyde is a common intermediate in proline and
arginine metabolism.
• Conversion of histidine to the five-carbon glutamate occurs in a multistep
pathway; the extra carbon is removed in a step that uses tetrahydrofolate
as cofactor.
• Histidine is oxidatively deaminated by histidase to urocanic acid, which
subsequently forms N-formiminoglutamate (FIGlu). FIGlu donates its
formimino group to tetrahydrofolate, leaving glutamate, which is
degraded as described above.
Six Amino Acids Are Degraded to
Pyruvate
• The carbon skeletons of six amino acids are converted in
whole or in part to pyruvate.
• These are alanine, tryptophan, cysteine, serine, glycine, and
threonine.
• All carbon atoms of gly, ala, cys, and ser are converted to
pyruvate.
• Only two carbon atoms of thr and three of trp form
pyruvate.
• The pyruvate can then be converted to either acetyl-CoA (a
ketone body precursor) or oxaloacetate (a precursor for
gluconeogenesis) or oxidized to CO2 and water.
• Thus amino acids catabolized to pyruvate are both
ketogenic and glucogenic.
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Catabolic pathway of Glycine,
serine, threonine,
cysteine,alanine and
tryptophan
Alanine
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Alanine yields pyruvate directly on transamination withketoglutarate.
Tryptophan
• Tryptophan the side chain of tryptophan is
cleaved to yield alanine and aromatic part.
• Alanine is transaminated to pyruvate.
• The aromatic part can be converted to
acetoacetyl COA.
Serine
• Serine can be converted to Gly. and also Pyruvate.
• This amino acid can be converted to glycine and N5,N10methylenetetrahydrofolate by serine hydroxymethyl transferase .
• Serine can also be converted
to pyruvate by serine
dehydratase
• Both the
β-hydroxyl and the
α-amino groups of serine
are removed in this single
pyridoxal phosphate–dependent reaction. In this reaction the loss
of water is followed by hydrolytic loss of ammonia.
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Cysteine :Two different pathways then convert cysteine to
pyruvate.
1st step transamination.
2nd step removal of sulfer atom.
Threonine
• There are two main pathways
for threonine degradation.
• One pathway leads to
pyruvate via glycine.
• The conversion to glycine
occurs in two steps.
• This is a relatively minor
pathway in humans,
accounting for 10% to 30% of
threonine catabolism, but is
more important in some other
mammals.
• The major pathway in humans
leads to succinyl-CoA.
Therionine to succinyl-CoA
Glycine
• Glycine is degraded via three pathways, only one leads to pyruvate.
• 1- Glycine can be converted to serine by addition of a methylene group
from N5,N10-methylenetetrahydrofolic acid. This reaction, catalyzed by
serine hydroxymethyl transferase, requires the coenzymes
tetrahydrofolate and pyridoxal phosphate.
Glycine cont…
• 2- Glycine undergoes oxidative cleavage to CO2, NH4 ,
and a methylene group (-CH2-).
• This reversible reaction, catalyzed by glycine cleavage
enzyme, and requires tetrahydrofolate, which accepts
the methylene group.
• In this oxidative cleavage pathway the two carbon atoms
of glycine do not enter the citric acid cycle.
• One carbon is lost as CO2 and the other becomes the
methylene group of N5,N10-methylenetetrahydrofolate,
(one carbon group donor in biosynthetic pathways).
• This pathway predominates in animals.
Glycine cont..
• 3- Glycine is a substrate for the enzyme D-amino acid
oxidase. The glycine is converted to glyoxylate, an
alternative substrate for hepatic lactate
dehydrogenase.
• Glyoxylate is oxidized in an NAD-dependent reaction
to oxalate.