BIO 202 Biochemistry II

by
Seyhun YURDUGÜL
Lecture 9
Amino Acid Metabolism I:
Amino Acid Biosynthesis
 Content Outline
• Introduction
• Types of aminoacids in brief
• Important intermediary compounds(like S-
adenosylmethionine)
• Examples from different aminoacids.
 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.
 Introduction

• However, the liver:

• 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;
 Introduction
• the carbon skeletons are generally
conserved as: carbohydrate,
• via gluconeogenesis,
• or as fatty acid via fatty acid synthesis
pathways.
 Another type of classification of
amino acids

• In this respect amino acids fall into three

categories:
• glucogenic,
• ketogenic,
• or glucogenic and ketogenic.
 Glucogenic amino acids
• give rise to a net production of pyruvate;
• or TCA cycle intermediates,
• such as α-ketoglutarate or oxaloacetate;
• all of which are precursors to glucose via
gluconeogenesis.
 Glucogenic amino acids
• All amino acids;
• except lysine and leucine:
• at least partly glucogenic.
 Ketogenic amino acids
• Lysine and leucine:
• are the only amino acids;
• that are solely ketogenic,
• giving rise only to acetyl-CoA or
acetoacetylCoA,
• neither of which can bring about net
glucose production.
 Glucogenic and ketogenic amino
acids
• 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.
 Glucogenic and ketogenic amino
acids
• Finally, it should be recognized that amino
acids have a third possible fate.
• During times of starvation;
• the reduced carbon skeleton:
• used for energy production,
• with the result that it is oxidized to CO 2 and
H2O.
Essential vs. Nonessential Amino Acids

Nonessential Essential
Alanine Arginine*

Asparagine Histidine

Aspartate Isoleucine

Cysteine Leucine

Glutamate Lysine

Glutamine Methionine*

Glycine Phenylalanine*

Proline Threonine

Serine Tryptophan

Tyrosine Valine
 Essential group
• The amino acids:
• arginine,
• methionine,
• and phenylalanine:
• considered essential for reasons not directly
related to lack of synthesis.
 Essential group
• Arginine:
• 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:
• cleaved to form urea.
 Essential group
• Methionine is required in large amounts to
produce cysteine;
• if the latter amino acid:
• not adequately supplied in the diet.
Similarly, phenylalanine:
• needed in large amounts to form tyrosine;
• if the latter is not adequately supplied in the
diet.
 Glutamate and Aspartate
Biosynthesis

• Glutamate and aspartate:

• synthesized from their widely distributed α-
keto acid precursors;
• by simple one-step transamination
reactions.
 Glutamate and Aspartate
Biosynthesis
• The former:
• catalyzed by glutamate dehydrogenase;
• and the latter:
• by aspartate aminotransferase, AST.
 Glutamate and Aspartate
Biosynthesis
• Aspartate:
• 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
 Alanine and the Glucose-
Alanine Cycle
• Aside from its role in protein synthesis,
• Alanine:
• 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 and the Glucose-Alanine
Cycle
• Alanine:
• transferred to the circulation by many
tissues,
• but mainly by muscle,
• in which alanine:
• formed from pyruvate at a rate proportional
to intracellular pyruvate levels.
 Alanine and the Glucose-Alanine
Cycle
• Liver accumulates plasma alanine,
• reverses the transamination that occurs in
muscle,
• and proportionately increases urea
production.
Pyruvate and the Glucose-Alanine
Cycle
• The pyruvate:
• either oxidized or converted to glucose via
gluconeogenesis.
• When alanine transfer from muscle to liver:
• coupled with glucose transport from liver
back to muscle,
• the process is known as:
• the glucose-alanine cycle.
 Alanine and the Glucose-Alanine
Cycle
• The key feature of the cycle is that
molecule, alanine,
• peripheral tissue exports pyruvate and
ammonia (which are potentially rate-
limiting for metabolism) to the liver,
• where the carbon skeleton:
• recycled and most nitrogen eliminated.
 Alanine and the Glucose-Alanine
Cycle
• There are 2 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).
Alanine and the Glucose-Alanine Cycle

• glutamate + pyruvate <-------> α-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).
S-AdoMet
 S-adenosylmethionine
• SAM serves as a precursor for numerous
methyl transfer reactions (e.g. the
conversion of norepinephrine to
epinephrine,
• The result of methyl transfer:
• the conversion of SAM to S-
adenosylhomocysteine.
 S-adenosylhomocysteine:

• then cleaved by adenosylhomocysteinase:

• 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.
 Transmethylation
• Transmethylation reactions employing
SAM are extremely important,
• but in this case the role of S-
adenosylmethionine in transmethylation:
• secondary to the production of
homocysteine (essentially a by-product of
transmethylase activity).
 Transmethylation
• 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.
 Cysteine synthesis
• Homocysteine:
• condenses with serine to produce
cystathionine,
• which is subsequently cleaved by
cystathionase;
• to produce cysteine and α-ketobutyrate.
• The sum of the latter two reactions:
• known as trans-sulfuration.
 Cysteine synthesis
• Cysteine is used for protein synthesis and
other body needs,
• while the α-ketobutyrate:
• decarboxylated and converted to propionyl-
CoA.
 Cysteine synthesis
• 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.
Utilization of methionine in the synthesis of cysteine
 Cysteine synthesis
• 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.
 Cysteine synthesis
• Cystathionase is under negative allosteric
control by cysteine,
• as well, cysteine inhibits the expression of
the cystathionine synthase gene.
 Tyrosine Biosynthesis

• Tyrosine is produced in cells by:

• hydroxylating the essential amino acid
phenylalanine.
• This relationship is much like that between
cysteine and methionine.
 Tyrosine Biosynthesis
• 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:
• the tetrahydrofolate-related cofactor
tetrahydrobiopterin,
• which is maintained in the reduced state;
• by the NADH-dependent enzyme dihydropteridine
reductase (DHPR).
 Ornithine and Proline
Biosynthesis

• Glutamate:
• 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.
 Ornithine and Proline
Biosynthesis
• 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 and Proline
Biosynthesis
• 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.
 Ornithine and Proline
Biosynthesis
• The fate of glutamate semialdehyde:
• depends on prevailing cellular conditions.
• Ornithine production:
• occurs from the semialdehyde;
• via a simple glutamate-dependent
transamination, producing ornithine.
 Ornithine and Proline
Biosynthesis
• 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.
 Ornithine and Proline
Biosynthesis

• The semialdehyde cyclizes spontaneously

to D1-pyrroline-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.
 Serine Biosynthesis
• 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:

• 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 Biosynthesis
• 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 Biosynthesis
• Glycine:
• 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:
• synthesized by the reductive amination of
α-ketoglutarate;
• catalyzed by glutamate dehydrogenase;
• it is thus a nitrogen-fixing reaction.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• In addition, glutamate arises by
aminotransferase reactions,
• with the amino nitrogen being donated by a
number of different amino acids.
• Thus, glutamate:
• a general collector of amino nitrogen.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• Aspartate:
• 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/Asparagine and
Glutamate/Glutamine Biosynthesis
• Aspartate can also be formed by:
• deamination of asparagine;
• catalyzed by asparaginase.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• Asparagine synthetase;
• and glutamine synthetase,
• catalyze the production of asparagine and;
• glutamine from their respective α-amino
acids.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• Glutamine is produced from glutamate;
• by the direct incorporation of ammonia;
• and this can be considered another nitrogen
fixing reaction.
• Asparagine, however:
• formed by an amidotransferase reaction
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• Aminotransferase reactions:
• are readily reversible.
• The direction of any individual
transamination;
• depends principally on the concentration
ratio of reactants and products.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• By contrast, transamidation reactions,
• which are dependent on ATP,
• are considered irreversible.
 Aspartate/Asparagine and
Glutamate/Glutamine Biosynthesis
• 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.
 LITERATURE CITED
• Devlin,T.M. Textbook of Biochemistry with
Clinical Correlations,Fifth Edition,Wiley-Liss
Publications,New York, USA, 2002.
• Lehninger, A. Principles of Biochemistry, Second
edition, Worth Publishers Co., New York, USA,
1993.
• Matthews, C.K. and van Holde, K.E.,
Biochemistry, Second edition, Benjamin /
Cummings Publishing Company Inc., San
Francisco, 1996.