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Amino acid Catabolism
Amino Acids Metabolism
Introduction
• amino acids are not stored by the body, that is, no protein exists whose
sole function is to maintain a supply of amino acids for future use.
Therefore, amino acids must be obtained from the diet, synthesized de
novo, or produced from normal protein degradation. Any amino acids in
excess of the biosynthetic needs of the cell are rapidly degraded.
•
The first phase of catabolism involves the removal of the α-amino groups
(usually by transamination and subsequent oxidative deamination), forming
ammonia and the corresponding α-keto acid—the “carbon skeletons” of
amino acids.
• A portion of the free ammonia is excreted in the urine, but most is used in
the synthesis of urea, which is quantitatively the most important route for
disposing of nitrogen from the body.
• In the second phase of amino acid catabolism, the carbon skeletons of the
α-ketoacids are converted to common intermediates of energy producing,
metabolic pathways. These compounds can be metabolized to CO2 and
water, glucose, fatty acids, or ketone bodies by the central pathways of
metabolism
Key concepts of amino acid metabolism
Amino acid Catabolism
The amino acids recovered from protein turnover, or obtained
from the diet or de novo synthesis, are used to :
• Support ongoing protein synthesis in cells.
• they are also used as metabolic precursors for numerous
biomolecules, including heme groups (hemoglobin and
cytochromes), nucleotide bases (purines and pyrimidines)
and a variety of signaling molecules (neurotransmitters,
hormones, nitric oxide).
• Can be used as energy source.
• Glucose, fatty acids and ketone bodies can be formed from
amino acids.
• The liver is the major site of protein degradation in mammals.
Amino acid Catabolism
• There are three major steps in catabolism of AAs.
1. Removal of amino group; deamination by:
I. Transamination : Transfer of amino gp to a-ketoglutarate
yielding glutamate
II. Oxidative deamination: removal of amino gp from
glutamate to release ammonia
III. Other deamination processes.
2. Urea Cycle: Conversion of NH3 to urea for excretion
3. Metabolic break down of carbon skeleton to generate
common intermediates that can be catabolized to CO2
or used in anabolic pathways to be stored as glucose or
fat.
Transamination
Introduction
• The presence of the α-amino group keeps
amino acids safely locked away from oxidative
breakdown.
• Removing the α-amino group is essential step
in the catabolism of all amino acids.
• Once removed, this nitrogen can be
incorporated into other compounds or
excreted, with the carbon skeletons being
metabolized.
Transamination
• The first step in the catabolism of most L-amino acids, once they
have reached the liver, is removal of the -amino groups, promoted
by enzymes called aminotransferases or transaminases.
• In these transamination reactions, the α -amino group is
transferred to the -carbon atom of α-ketoglutarate, leaving behind
the corresponding α-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.
• The glutamate then functions as an amino group donor for
biosynthetic pathways of nonessential amino acids. or for excretion
pathways that lead to the elimination of nitrogenous waste
products.
• All amino acids, with the exception of lysine and threonine,
participate in transamination [These two amino acids lose their αamino groups by deamination].
Serine and Threonine
The β–hydroxy amino acids,
serine and threonine, can
be directly deaminated
(a) Overview of catabolism of amino
groups (shaded) in vertebrate liver.
(b) Excretory forms of nitrogen.
Substrate specificity of
aminotransferases
• Cells contain different types of aminotransferases.
• These enzymes are found in the cytosol and mitochondria of cells
throughout the body—especially those of the liver, kidney,
intestine, and muscle.
• Each aminotransferase is specific for one or, at most, a few amino
group donors.
• Aminotransferases are named after the specific amino group donor,
because the acceptor of the amino group is almost always αketoglutarate. (e.g alanine aminotransferase, aspartate
aminotransferase).
• The reactions catalyzed by aminotransferases are freely reversible .
• The two most important aminotransferase reactions are catalyzed
by alanine aminotransferase (ALT) and aspartate aminotransferase
AST
Transamination
• Typically, α-ketoglutarate accepts amino
groups.
• L-Glutamine acts as a temporary
storage of nitrogen.
• L-Glutamine can donate the amino
group when needed for amino acid
biosynthesis.
• All aminotransferases rely on the
pyridoxal phosphate cofactor.
Aminotransferases
– Alanine aminotransferase (ALT): Formerly called glutamate-pyruvate
transaminase, ALT is present in many tissues. The enzyme catalyzes
the transfer of the amino group of alanine to α-ketoglutarate, resulting
in the formation of pyruvate and glutamate. The reaction is readily
reversible. However, during amino acid catabolism, this enzyme (like
most aminotransferases) functions in the direction of glutamate
synthesis. Thus, glutamate, in effect, acts as a “collector” of nitrogen
from alanine.
– Aspartate aminotransferase (AST): AST formerly called glutamateoxaloacetate transaminase, AST is an exception to the rule that
aminotransferases funnel amino groups to form glutamate. During
amino acid catabolism, AST transfers amino groups from glutamate to
oxaloacetate, forming aspartate, which is used as a source of nitrogen
in the urea cycle.The AST reaction is also reversible.
COO−
COO−
COO−
CH2
COO−
CH2
CH2
CH2
CH2
CH2
HC
NH3+
COO−
+
C
O
COO−
C
O
+
COO−
HC
NH3+
COO−
aspartate α-ketoglutarate oxaloacetate glutamate
Aminotransferase (Transaminase)
Example of a Transaminase reaction:
Aspartate donates its amino group, becoming the
α-keto acid oxaloacetate.
α-Ketoglutarate accepts the amino group,
becoming the amino acid glutamate.
CH3
HC
COO−
COO−
CH2
CH2
CH2
NH3+
COO−
alanine
+
C
CH3
O
COO−
α-ketoglutarate
C
CH2
O
COO−
pyruvate
+
HC
NH3+
COO−
glutamate
Aminotransferase (Transaminase)
In another example, alanine becomes pyruvate as
the amino group is transferred to α-ketoglutarate.
Mechanism of action of
aminotransferases
• 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 which is covalently linked to the ε-amino group
of a specific lysine residue at the active site of the enzyme through an
aldimine (Schiff base)
• Pyridoxal phosphate functions as an intermediate carrier of amino groups
at the active site of aminotransferases.
• Aminotransferases act by transferring the amino group of an amino acid to
the pyridoxal part of the coenzyme to generate pyridoxamine phosphate.
The pyridoxamine form of the coenzyme then reacts with an α-keto acid
to form an amino acid, at the same time regenerating the original
aldehyde form of the coenzyme.
• 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 keto acid.
Ping-Pong reaction of
aminotransferases
• Aminotransferases are classic examples of enzymes catalyzing
bimolecular Ping-Pong reactions in which the first substrate reacts
and the product must leave the active site before the second
substrate can bind.
• Thus the incoming amino acid binds to the active site, donates its
amino group to pyridoxal phosphate, and departs in the form of an
-ketoacid.
• The incoming -keto acid then binds, accepts the amino group from
pyridoxamine phosphate, and departs in the form of an amino acid.
• Measurement of the alanine aminotransferase and aspartate
aminotransferase levels in blood serum is important in some
medical diagnoses.
Structure of Pyridoxal Phosphate and
Pyridoxamine Phosphate
• Intermediate, enzymebound carrier of
amino groups
• Aldehyde form can
react reversibly with
amino groups
• Aminated form can
react reversibly with
carbonyl groups
Pyridoxal Phosphate is
Covalently Linked to the
Enzyme at Rest
• The linkage is made via an
nucleophilic attack of the amino
group an active-site lysine side
chain
• After dehydration, a Schiff base
linkage is formed
• The covalent complex is called
internal aldimine because the
Schiff base connects PLP to the
enzyme
Mechanism of Transamination
Reaction: role of PLP
R
H
C
COO
Enz
−
(C H 2 ) 4
NH2
A m in o a c id
N+
O−
−O
P
O
HC
H2
C
H
O
−
O
+
N
H
CH3
E n z y m e (L y s )-P L P S c h iff b a s e
1- In the resting state, the aldehyde group of
pyridoxal phosphate is in a Schiff base linkage to
the ε-amino group of an enzyme lysine side-chain.
Enz−Lys−NH2
R
H
C
COO−
N+
2- The α-amino
group of a substrate
amino acid displaces
the enzyme lysine,
to form a Schiff base
linkage to PLP.
O−
−O
P
O
HC
H2
C
H
O−
O
+
N
H
CH3
Amino acid-PLP Shiff base (aldimine)
3- The active site lysine extracts H+, promoting
tautomerization, followed by reprotonation &hydrolysis.
O
Enz−Lys−NH2
4- an amino
acid leaves as
an α-keto
acid.
O−
−O
P
O
NH2
CH2
H2
C
R
C COO−
α-keto acid
OH
O
+
N
CH3
H
Pyridoxamine phosphate (PMP)
The amino group remains on what is now pyridoxamine
phosphate (PMP).
5- A different α-keto acid reacts with PMP and the
process reverses, to complete the reaction.
Mechanism of Transamination reaction: role of
PLP
•Reactions begin with formation of a new Schiffbase (aldimine) between
the -amino group of the amino acid and PLP, which substitutes for the
enzyme-PLP linkage.
• A quinonoid intermediate is involved in the reaction.
• a second -keto acid replaces the one that is released, and this is converted
to an amino acid in a reversal of the reaction steps (right to left).
Biochemistry in Medicine
Assays for Tissue Damage
• Analyses of certain enzyme activities in blood serum give valuable
diagnostic information for a number of disease conditions.
• Alanine aminotransferase (ALT; also called glutamate-pyruvate
transaminase, GPT) and aspartate aminotransferase (AST; also
called glutamateoxaloacetate transaminase, GOT) are important in
the diagnosis of heart and liver damage caused by heart
attack, drug toxicity, or infection.
• After a heart attack,a variety of enzymes, including these
aminotransferases,leak from the injured heart cells into the
bloodstream.
• Measurements of the blood serum concentrations of the two
aminotransferases and of another enzyme, creatine kinase, Provide
information about the severity of the damage.
Biochemistry in Medicine
• The SGOT and SGPT tests are also important in
occupational medicine, to determine whether people
exposed to carbon tetrachloride, chloroform, or other
industrial solvents have suffered liver damage.
• Liver degeneration caused by these solvents is
accompanied by leakage of various enzymes from
injured hepatocytes into the blood.
• Aminotransferases are most useful in the monitoring
of people exposed to these chemicals,because these
enzyme activities are high in liver and can be detected
in very small amounts.