|
|
|
|
|
|||||
|
Biochemistry Page |
|
|||||||
|
|
Introduction All tissues have some capability for synthesis of the non-essential
amino acids, amino acid remodeling, and conversion of non-amino
acid carbon skeletons into amino acids and other derivatives that contain
nitrogen. However, the liver is the major site of nitrogen
metabolism in the body. In times of dietary surplus, the
potentially toxic nitrogen of amino acids is eliminated via transaminations,
deamination, and urea formation; the carbon skeletons are generally conserved
as carbohydrate, via gluconeogenesis,
or as fatty acid via fatty acid
synthesis pathways. In this respect amino acids fall into three
categories: glucogenic, ketogenic, or glucogenic and
ketogenic. Glucogenic amino acids are those that give rise to a net
production of pyruvate or TCA cycle
intermediates, such as a-ketoglutarate or oxaloacetate, all of which are
precursors to glucose via gluconeogenesis. All amino acids except lysine and
leucine are at least partly glucogenic. Lysine and leucine are the only amino
acids that are solely ketogenic, giving rise only to acetylCoA or
acetoacetylCoA, neither of which can bring about net glucose production. A small group of amino acids comprised of isoleucine,
phenylalanine, threonine, tryptophan, and tyrosine give rise to both glucose
and fatty acid precursors and are thus characterized as being glucogenic and
ketogenic. Finally, it should be recognized that amino acids have a third
possible fate. During times of starvation the reduced carbon skeleton is used
for energy production, with the result that it is oxidized to CO2
and H2O. Amino Acid
Biosynthesis Glutamate and
Aspartate Glutamate and aspartate are synthesized from their widely
distributed a-keto acid precursors by simple 1-step transamination reactions.
The former catalyzed by glutamate dehydrogenase and the latter
by aspartate aminotransferase, AST.
Aspartate is also derived from asparagine through the action of asparaginase.
The importance of glutamate as a common intracellular amino donor for
transamination reactions and of aspartate as a precursor of ornithine for the
urea cycle
is described in the Nitrogen
Metabolism page. Alanine and the
Glucose-Alanine Cycle Aside from its role in protein synthesis, alanine is second only
to glutamine in prominence as a circulating amino acid. In this capacity 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. The pyruvate is either oxidized or converted to glucose via gluconeogenesis.
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. The key feature of the cycle is that in 1 molecule,
alanine, peripheral tissue exports pyruvate and ammonia (which are
potentially rate-limiting for metabolism) to the liver, where the carbon
skeleton is recycled and most nitrogen eliminated. There are 2 main pathways to production of muscle alanine:
directly from protein degradation, and via the transamination of pyruvate by glutamate-pyruvate
aminotransferase (also called alanine transaminase, ALT).
glutamate + pyruvate <-------> a-KG
+ alanine Cysteine
Biosynthesis 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).
SAM serves as a precurosor for numerous methyl transfer
reactions (e.g. the conversion of norepinephrine to epinenephrine, see Specialized
Products of Amino Acids). The result of methyl transfer is the
conversion of SAM to S-adenosylhomocysteine. S-adenosylhomocysteine is then
cleaved by adenosylhomocyteinase to yield homocysteine and
adenosine. Homocysteine can be converted back to methionine by methionine
synthase, a reaction that occurs under methionine-sparing conditions
and requires N5-methyl-tetrahydrofolate as methyl donor.
This reaction was discussed in the context of vitamin B12-requiring
enzymes in the Vitamins
page. Transmethylation reactions employing SAM are extremely
important, but in this case the role of S-adenosylmethionine in
transmethylation is secondary to the production of homocysteine (essentially
a by-product of transmethylase activity). 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 and not AMP. In cysteine synthesis, homocysteine condenses with serine to
produce cystathionine, which is subsequently cleaved by cystathionase
to produce cysteine and a-ketobutyrate. The sum of the latter two reactions
is known as trans-sulfuration. Cysteine is used for protein synthesis and other body needs,
while the a-ketobutyrate is decarboxylated and converted to propionyl-CoA.
While cysteine readily oxidizes in air to form the disulfide cystine, cells
contain little if any free cystine because the ubiquitous reducing agent, glutathione
effectively reverses the formation of cystine by a non-enzymatic reduction
reaction. The 2 key enzymes of this pathway, cystathionine synthase
and cystathionase (cystathionine lyase), both use pyridoxal
phosphate as a cofactor, and both are under regulatory control. Cystathionase
is under negative allosteric control by cysteine, as well, cysteine inhibits
the expression of the cystathionine synthase gene. Genetic defects are known for both the synthase and the lyase.
Missing or impaired cystathionine synthase leads to homocystinuria
and is often associated with mental retardation, although the complete
syndrome is multifaceted and many individuals with this disease are mentally
normal. Some instances of genetic homocystinuria respond favorably to
pyridoxine therapy, suggesting that in these cases the defect in cystathionine
synthase is a decreased affinity for the cofactor. Missing or
impaired cystathionase leads to excretion of cystathionine in
the urine but does not have any other untoward effects. Rare cases are known
in which cystathionase is defective and operates at a low
level. This genetic disease leads to methioninuria with no other
consequences. 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 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%. Phenylalanine hydroxylase is a
mixed-function 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. Missing or
deficient phenylalanine hydroxylase leads to the genetic
disease known as phenlyketonuria
(PKU), which if untreated leads to severe mental retardation. 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 a-ketoglutarate. This absence of a-ketoglutarate 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, phenylpyruvic acid, is reduced to phenylacetate
and phenyllactate, and all 3 compounds appear in the urine. The presence of
phenylacetate in the urine imparts a "mousy" odor. If the problem
is diagnosed early, the addition of tyrosine and restriction of phenylalanine
from the diet can minimize the extent of mental retardation. In other pathways,
tetrahydrobiopterin is a cofactor. The effects of missing or defective dihydropteridine
reductase cause even more severe neurological difficulties than those
usually associated with PKU caused by deficient hydroxylase activity. Ornithine and
Proline Biosynthesis Glutamate is the
precursor of both proline and ornithine, with glutamate semialdehyde being a
branch point intermediate leading to one or the other of these 2 products.
While ornithine is not one of the 20 amino acids used in protein synthesis,
it plays a significant role as the acceptor of carbamoyl phosphate in the urea cycle.
Ornithine serves an additional important role as the precursor for the
synthesis of the polyamines.
The production of ornithine from glutamate is important when dietary
arginine, the other principal source of ornithine, is limited. The fate of
glutamate semialdehyde depends on prevailing cellular conditions. Ornithine
production occurs from the semialdehyde via a simple glutamate-dependent
transamination, producing ornithine. When arginine concentrations become
elevated, the ornithine contributed from the urea cycle
plus that from glutamate semialdehyde inhibit the aminotransferase reaction,
with accumulation of the semialdehyde as a result. The semialdehyde cyclizes
spontaneously to D1pyrroline-5-carboxylate which is then reduced
to proline by an NADPH-dependent reductase. Serine Biosynthesis The main pathway to
serine starts with the glycolytic intermediate 3-phosphoglycerate. 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. Glycine
Biosynthesis The main pathway to
glycine is a 1-step reaction catalyzed by serine
hydroxymethyltransferase. This reaction involves the transfer of the
hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF),
producing glycine and N5,N10-methylene-THF.
Glycine produced from serine or from the diet can also be oxidized by glycine
cleavage complex, GCC, to yield a second equivalent of N5,N10-methylene-tetrahydrofolate
as well as ammonia and CO2. Glycine is involved
in many anabolic reactions other than protein synthesis including the
synthesis of purine
nucleotides, heme,
glutathione,
creatine and serine. Aspartate/Asparagine
and Glutamate/Glutamine Biosynthesis Glutamate is
synthesized by the reductive amination of a-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. Aspartate is formed
in a transamintion reaction catalyzed by aspartate transaminase,
AST. This reaction uses the aspartate a-keto acid analog, oxaloacetate,
and glutamate as the amino donor. Aspartate can also be formed by deamination
of asparagine catalyzed by asparaginase. Asparagine
synthetase and glutamine synthetase,
catalyze the production of asparagine and glutamine from their respective a-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. By contrast, transamidation reactions, which are dependent on
ATP, are considered irreversible. As a consequence, the degradation of
asparagine and glutamine take place by a hydrolytic pathway rather than by a
reversal of the pathway by which they were formed. As indicated above,
asparagine can be degraded to aspartate. 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 NH3+,
with the NH3+ 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 a-ketoglutarate, making glutamine a glucogenic
amino acid. 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 a-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 as described
above 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 a-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,
was described above. Serine can be catabolized back to the glycolytic
intermediate, 3-phosphoglycerate, by a pathway that is essentially a reversal
of serine biosynthesis. However, the enzymes are different. Serine can also
be converted to pyruvate through a deamination reaction catalyzed by serine/threonine
dehydratase. Threonine
Catabolism There are at least
3 pathways for threonine catabolism. One involves a pathway initiated by threonine
dehydrogenase yielding a-amino-b-ketobutyrate. The a-amino-b-ketobutyrate
is either converted to acetyl-CoA and glycine or spontaneously degrades to
aminoacetone which is converted to pyruvate. The second pathway involves serine/threonine
dehydratase yielding a-ketobutyrate which is further catabolized to
propionyl-CoA and finally the TCA cycle
intermediate, succinyl-CoA. The third pathway utilizes threonine
aldolase. The products of this reaction are both ketogenic
(acetyl-CoA) and glucogenic (pyruvate). 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 or to pyruvate by serine/threonine
dehydratase. Nevertheless, the main glycine catabolic pathway leads
to the production of CO2, ammonia, and one equivalent of N5,N10-methyleneTHF
by the mitochondrial glycine cleavage complex. 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 more important catabolic pathway is via a cytochrome-P450-coupled enzyme, 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 to b-sulfinylpyruvate which is in undergoes desulfuration yielding bisulfite, (HSO3-) and the glucogenic product, pyruvate. The enzyme sulfite oxidase uses O2 and H2O to convert HSO3- to sulfate, (SO4-) and H2O2. The resultant sulfate is used as a precursor for the formation of 3'-phosp |
|||||||
