Amino Acid Metabolism
All tissues in the body have limited capability for synthesis of:
•Non-essential or dispensable amino acids
•Amino acid remodeling
•Conversion of non-amino acid carbon skeletons into amino acids
•Conversion of non-animo acid carbon skeletons into derivatives that contain nitrogen
Amino Acid Metabolism
The liver is the major site of nitrogen metabolism
•In times of dietary surplus, the potentially toxic nitrogen of amino acids is
eliminated via transaminations, oxidative deamination, and urea formation.
Carbon skeletons are generally conserved:
•carbohydrate, via gluconeogenesis
•fatty acid via fatty acid synthesis
•During times of starvation the carbon skeletons of amino acids are
reduced and used for energy production (oxidized to CO2 and H2O).
Amino Acid Metabolism
Amino acids fall into three categories:
1. Glucogenic: which give rise to a “net” production of pyruvate or TCA cycle
intermediates, such as α-ketoglutarate or oxaloacetate (precursors to glucose via
gluconeogenesis)
All amino acids except Lysine and Leucine are at least partly glucogenic.
2. Ketogenic: which give rise to acetylCoA or acetoacetylCoA
Lysine and Leucine are solely ketogenic
3. Glucogenic and Ketogenic: which give rise to both glucose and fatty acid
precursors
Isoleucine, Phenylalanine, Threonine, Tryptophan, and Tyrosine
Non-essential Amino Acid Biosynthesis
Glutamate
Glutamate is synthesized from the widely distributed α-keto acid precursor by a simple 1step transamination reaction catalyzed by glutamate dehydrogenase
Non-essential Amino Acid Biosynthesis
Aspartate
Aspartate is synthesized by a simple 1-step transamination reaction catalyzed by
aspartate aminotransferase, AST (formerly referred to as serum glutamate-oxalate
transaminase, SGOT).
Non-essential Amino Acid Biosynthesis
Aspartate
Aspartate can also be derived from asparagine through the action of asparaginase.
Aspartate is also a precursor of ornithine (urea cycle).
Non-essential Amino Acid Biosynthesis
Alanine
There are 2 main pathways for production of alanine: directly from protein degradation or
transamination of pyruvate by alanine transaminase, ALT (also referred to as serum
glutamate-pyruvate transaminase, SGPT).
Non-essential Amino Acid Biosynthesis
Alanine
Alanine is second only to glutamine in circulating amino acid abundance.
Alanine is transferred to circulation mainly by muscle
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
Pyruvate is either oxidized or converted
to glucose
When alanine transfer from muscle to liver is coupled with glucose transport from liver
back to muscle, the process is known as the glucose-alanine cycle.
This serves to essentially transfer nitrogen from peripheral tissue to the liver.
Muscle gets rid of nitrogen and replenishes its energy supply
Non-essential Amino Acid Biosynthesis
Cysteine
The sulfur for cysteine synthesis comes from the essential amino acid methionine. A
condensation of ATP and methionine catalyzed by methionine adenosyltransferase
yields S-adenosylmethionine (SAM or AdoMet).
Non-essential Amino Acid Biosynthesis
Cysteine
SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the conversion of
norepinephrine to epinenephrine).
Methyl Transfer
The result of methyl transfer is the conversion of SAM to S-adenosylhomocysteine which is then
cleaved by adenosylhomocyteinase to yield homocysteine and adenosine.
Trans-sulfuration
Homocysteine condenses with serine to produce cystathionine, which is cleaved by
cystathionase to produce cysteine and α-ketobutyrate.
Cysteine is used for protein synthesis
α-ketobutyrate is first converted to propionyl-CoA then to succinyl-CoA.
Cysteine readily oxidizes in air to form the disulfide cystine but, cells contain little or no free
cystine
Non-essential Amino Acid Biosynthesis
Tyrosine
Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine. Half
of the phenylalanine required goes into the production of tyrosine; if the diet is rich in
tyrosine itself, the requirements for phenylalanine are reduced by about 50%.
PKU in Humans
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 phenylketonuria or PKU. Patients suffering from PKU have plasma phenylalanine levels
>1000μM, whereas the non-PKU humans exhibit levels of plasma phenylalanine <1000μM.
Untreated PKU leads to severe mental retardation which is caused by the accumulation of
phenylalanine and depletes neural tissue of α-ketoglutarate. This absence of α-ketoglutarate in
the brain shuts down the TCA cycle.
Non-essential Amino Acid Biosynthesis
Proline
Glutamate is the precursor of both proline and ornithine.
Glutamate semialdehyde is the branch point intermediate leading to one of these 2 products.
The production of ornithine from glutamate is important when dietary arginine (the other
principal source of ornithine) is limited.
Non-essential Amino Acid Biosynthesis
Serine
The main pathway to de novo biosynthesis of serine starts with the glycolytic intermediate 3phosphoglycerate. An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto
acid, 3-phosphopyruvate, suitable for subsequent transamination. Aminotransferase activity
with glutamate as a donor produces 3-phosphoserine, which is converted to serine by
phosphoserine phosphatase.
Non-essential Amino Acid Biosynthesis
Glycine
The main pathway to glycine is a 1-step reversible reaction catalyzed by serine
hydroxymethyltransferase (SHMT). This reaction involves the transfer of the hydroxymethyl
group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10methylene-THF.
There are mitochondrial and cytosolic versions of serine hydroxymethyltransferase.
The cytosolic enzyme is SHMT1
The mitochondrial enzyme is SHMT2.
Glycine is involved in many anabolic reactions other than protein synthesis including the
synthesis of purine nucleotides, heme, glutathione, creatine and serine.
Glycine produced from serine or from the diet can also be oxidized by glycine decarboxylase
(also referred to as the glycine cleavage complex, GCC) to yield a second equivalent of
N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Non-essential Amino Acid Biosynthesis
Aspartate/Asparagine and Glutamate/Glutamine Biosynthesis
Glutamate is synthesized by the reductive amination of α-ketoglutarate catalyzed by glutamate
dehydrogenase; it is thus a nitrogen-fixing 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.
Non-essential Amino Acid 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 fixing reaction.
Asparagine, however, is formed by an amidotransferase reaction.
Aminotransferase reactions are readily reversible. The direction of any individual
transamination depends principally on the concentration ratio of reactants and products.
Transamidation reactions, which are dependent on ATP, are considered irreversible.
Thus, degradation of asparagine and glutamine take place by a hydrolytic pathway rather than
by a reversal of the pathway by which they were formed.
Amino Acid Catabolism
Glutamine/Glutamate and Asparagine/Aspartate Catabolism
Glutaminase is an important kidney tubule enzyme involved in converting glutamine (from liver
and from other tissue) to glutamate and NH4+, with the NH4+ being excreted in the urine.
Glutaminase activity is present in many other tissues as well, although its activity is not nearly as
prominent as in the kidney. The glutamate produced from glutamine is converted to αketoglutarate, making glutamine a glucogenic amino acid.
Glutamine/Glutamate and Asparagine/Aspartate Catabolism
Asparaginase is also widely distributed within the body, where it converts asparagine into
ammonia and aspartate. Aspartate transaminates to oxaloacetate, which follows the
gluconeogenic pathway to glucose.
Glutamate and aspartate are important in collecting and eliminating amino nitrogen via
glutamine synthetase and the urea cycle, respectively. The catabolic path of the carbon
skeletons involves simple 1-step aminotransferase reactions that directly produce net quantities
of a TCA cycle intermediate.
The glutamate dehydrogenase reaction operating in the direction of α-ketoglutarate production
provides a second avenue leading from glutamate to gluconeogenesis.
Alanine Catabolism
Alanine is also important in intertissue nitrogen transport as part of the glucose-alanine
cycle.
Alanine's catabolic pathway involves a simple aminotransferase reaction that directly
produces pyruvate. Generally pyruvate produced by this pathway will result in the
formation of oxaloacetate, although when the energy charge of a cell is low the
pyruvate will be oxidized to CO2 and H2O via the PDH complex and the TCA cycle.
This makes alanine a glucogenic amino acid.
Arginine, Ornithine and Proline Catabolism
The catabolism of arginine begins within the context of the urea cycle. It is hydrolyzed to
urea and ornithine by arginase.
Ornithine, in excess of urea cycle needs, is transaminated to form glutamate
semialdehyde.
Glutamate semialdehyde can serve as the precursor for proline biosynthesis or it can be
converted to glutamate.
Proline catabolism is a reversal of its synthesis process.
The glutamate semialdehyde generated from ornithine and proline catabolism is oxidized
to glutamate by an ATP-independent glutamate semialdehyde dehydrogenase.
The glutamate can then be converted to α-ketoglutarate in a transamination reaction.
Thus, arginine, ornithine and proline, are glucogenic.
Serine Catabolism
The conversion of serine to glycine and then glycine oxidation to CO2 and NH3, with
the production of two equivalents of N5,N10-methyleneTHF, (glycine biosynthesis).
Serine can be catabolized back to the glycolytic intermediate, 3-phosphoglycerate, by
a pathway that is essentially a reversal of serine biosynthesis.
It has been demonstrated in rodents and dogs that serine can be converted to
pyruvate through a deamination reaction catalyzed by serine/threonine dehydratase.
Glycine Catabolism
Glycine is classified as a glucogenic amino acid, since it can be converted to serine by
serine hydroxymethyltransferase and serine can be converted back to the glycolytic
intermediate, 3-phosphoglycerate .
Serine can also be converted to pyruvate by serine/threonine dehydratase. This does not
occur in humans.
The main glycine catabolic pathway leads to the production of CO2, ammonia, and one
equivalent of N5,N10-methyleneTHF by the mitochondrial glycine decarboxylase, also
called the glycine cleavage complex, GCC.
Threonine Catabolism
The principal threonine catabololizing pathway in humans involves glycine-independent
serine/threonine dehydratase yielding α-ketobutyrate which is further catabolized to propionylCoA and finally to succinyl-CoA.
Serine/threonine dehydratase is expressed at high levels only in the liver. It appears that in
newborn infants catabolism of threonine occurs exclusively via the action of the
serine/threonine dehydratase. Therefore, it is presumed that this is the predominant threonine
catabolizing pathway in humans.
The second pathway of threonine catabolism utilizes serine hydroxymethyltransferase. The
products of this reaction are acetyl-CoA and glycine. The glycine can be converted to serine via
the same enzyme and the serine is then catabolized yielding pyruvate and NH4+. Thus, via this
catabolic pathway threonine yields ketogenic and glucogenic byproducts.
In humans it appears that threonine aldolase is actually encoded by a non-functional
pseudogene, whereas in other vertebrates the threonine aldolase gene encodes a functional
threonine catabolizing enzyme.
An additional pathway occurs in the mitochondria and is initiated by threonine dehydrogenase
eventually yielding to acetyl-CoA and glycine. The threonine dehydrogenase gene in humans
appears to be non-functional due to the incorporation of three inactivating mutations.
Cysteine Catabolism
There are several pathways for cysteine catabolism. The simplest, but least important
pathway is catalyzed by a liver desulfurase and produces hydrogen sulfide, (H2S) and
pyruvate. The major catabolic pathway in animals is via cysteine dioxygenase that oxidizes
the cysteine sulfhydryl to sulfinate, producing the intermediate cysteinesulfinate.
Cysteinesulfinate can serve as a biosynthetic intermediate undergoing decarboxylation
and oxidation to produce taurine. Catabolism of cysteinesulfinate proceeds through
transamination and eventualy leads to pyruvate.
Other than protein, the most important product of cysteine metabolism is the
bile salt precursor taurine, which is used to form the bile acid conjugates
taurocholate and taurochenodeoxycholate.
Methionine Catabolism
Methionine is:
Incorporated into polypeptide chains
Used in the production of α-ketobutyrate
Used in the production of cysteine via SAM
The transulfuration reactions that produce cysteine from homocysteine and serine also
produce α-ketobutyrate, the latter being converted first to propionyl-CoA and then via a
3-step process to succinyl-CoA.
Valine, Leucine and Isoleucine Catabolism
This group of essential amino acids are identified as the branched-chain amino acids, BCAAs.
Because this arrangement of carbon atoms cannot be made by humans, these amino acids are
an essential element in the diet.
The catabolism of all three compounds initiates in muscle and yields NADH and FADH2 which
can be utilized for ATP generation.
The catabolism of all three of these amino acids uses the same enzymes in the first two steps.
The first step in each case is a transamination using a single BCAA aminotransferase, with αketoglutarate as amine acceptor. As a result, three different α-keto acids are produced and are
oxidized using a common branched-chain α-keto acid dehydrogenase (BCKD), yielding the three
different CoA derivatives.
Valine breakdown produces propionylCoA, the glucogenic precursor of succinyl-CoA.
Isoleucine catabolism terminates with production of acetylCoA and propionylCoA; thus
isoleucine is both glucogenic and ketogenic.
Leucine gives rise to acetylCoA and acetoacetylCoA, and is thus classified as strictly ketogenic.
Phenylalanine and Tyrosine Catabolism
Phenylalanine has only two fates:
incorporation into polypeptide chains
production of tyrosine
Phenylalanine catabolism always follows the pathway of tyrosine catabolism.
The main pathway for tyrosine degradation involves conversion to fumarate and
acetoacetate, allowing phenylalanine and tyrosine to be classified as both glucogenic and
ketogenic.
Tyrosine is equally important for protein biosynthesis as well as an intermediate in the
biosynthesis of several physiologically important metabolites e.g. dopamine, norepinephrine
and epinephrine.
Lysine Catabolism
Lysine catabolism is unusual in the way that the ε-amino group is transferred to α-ketoglutarate
and into the general nitrogen pool. The reaction is a transamination in which the ε-amino group
is transferred to the α-keto carbon of α-ketoglutarate forming the metabolite, saccharopine.
Unlike the majority of transamination reactions, this one does not employ pyridoxal phosphate
as a cofactor. Because this transamination reaction is not reversible, lysine is an essential amino
acid. The ultimate end-product of lysine catabolism is acetoacetyl-CoA.
Lysine is also important as a precursor for the synthesis of carnitine, required for the transport
of fatty acids into the mitochondria for oxidation. Free lysine does not serve as the precursor for
this reaction, rather the modified lysine found in certain proteins. Some proteins modify lysine
to trimethyllysine using SAM as the methyl donor to transfer methyl groups to the ε-amino of
the lysine side chain. Hydrolysis of proteins containing trimethyllysine provides the substrate for
the subsequent conversion to carnitine.
Histidine Catabolism
Histidine catabolism begins with release of the α-amino group catalyzed by histidase,
introducing a double bond into the molecule. The end product of histidine catabolism is
glutamate, making histidine one of the glucogenic amino acids.
Another key feature of histidine catabolism is that it serves as a source of ring nitrogen to
combine with tetrahydrofolate (THF).
Tryptophan Catabolism
A number of important side reactions occur during the catabolism of tryptophan on the
pathway to acetoacetate.
Kynurenine is the first key branch point intermediate in the catabolic pathway leading to 3 fates:
Kynurenine can undergo deamination in a standard transamination reaction yielding kynurenic
acid.
High levels of kynurenic acid have been found in the urine of individuals suffering from
schizophrenia.
Aside from its role as an amino acid in protein biosynthesis, tryptophan also serves as a
precursor for the synthesis of serotonin and melatonin.
The Urea Cycle
Kidney glutaminase is responsible for converting excess
glutamine from the liver to urine ammonium. However, about
80% of the excreted nitrogen is in the form of urea which is also
largely made 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.
CPS-I is carbamoyl phosphate synthetase-I, OTC is ornithine transcarbamoylase.
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.
With long-term changes in the quantity of dietary protein, changes of 20-fold or
greater in the concentration of cycle enzymes are observed. 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.
Regulation of the Urea Cycle
Short-term regulation of the cycle occurs principally at CPS-I, which is
relatively inactive in the absence of its allosteric activator N-acetylglutamate.
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 synthetase.