bio.lesson document 20

8/6/2019 bio.lesson document 20

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BIO 202 Biochemistry II

bySeyhun YURDUGL

Lecture 9

Amino Acid Metabolism I:Amino Acid Biosynthesis


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C

ontent Outline Introduction

Types of aminoacids in brief Important intermediary compounds(like S-adenosylmethionine)

Examples from different aminoacids.


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Introduction

All tissues have some capability:

for synthesis of the non-essential aminoacids,

amino acid remodeling,

and conversion of non-amino acid carbon

skeletons: into amino acids and other derivatives that

contain nitrogen.


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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 iseliminated:

via transaminations, deamination,

and urea formation;


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Introduction

the carbon skeletons are generallyconserved as: carbohydrate,

via gluconeogenesis,

or as fatty acid via fatty acid synthesispathways.


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Another type of classification of

amino acids

In this respect amino acids fall into threecategories:

glucogenic,

ketogenic,

or glucogenic and ketogenic.


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Glucogenic amino acids give rise to a net production of pyruvate;

or TCA cycle intermediates,

such as -ketoglutarate or oxaloacetate;

all of which are precursors to glucose viagluconeogenesis.


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Glucogenic amino acids All amino acids;

except lysine and leucine:

at least partly glucogenic.


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K

etogenic amino acids Lysine and leucine:

are the only amino acids;

that are solely ketogenic,

giving rise only to acetyl-CoA oracetoacetylCoA,

neither of which can bring about net glucoseproduction.


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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 acidprecursors; and are thus characterized as being glucogenic and

ketogenic.


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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 CO2 andH2O.


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Nonessential Essential

Alanine Arginine*

Asparagine Histidine

Aspartate Isoleucine

Cysteine Leucine

Glutamate Lysine

Glutamine Methionine*

Glycine Phenylalanine*

Proline Threonine

Serine Tryptophan

Tyrosine Valine

Essential vs. Nonessential Amino Acids


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Essential group The amino acids:

arginine,

methionine,

and phenylalanine:

considered essential for reasons not directlyrelated to lack of synthesis.


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Essential group Arginine:

synthesized by mammalian cells;

but at a rate that is insufficient to meet thegrowth:

needs of the body;

and the majority that is synthesized:

cleaved to form urea.


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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.


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Glutamate and Aspartate

Biosynthesis

Glutamate and aspartate:

synthesized from their widely distributed -keto acid precursors;

by simple one-step transaminationreactions.


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Biosynthesis

The former:

catalyzed by glutamate dehydrogenase;

and the latter:

by aspartate aminotransferase, AST.


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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


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Alanine and theGlucose-

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.


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Alanine and theGlucose-Alanine

Cycle

Alanine:

transferred to the circulation by manytissues,

but mainly by muscle,

in which alanine:

formed from pyruvate at a rate proportionalto intracellular pyruvate levels.


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Cycle

Liver accumulates plasma alanine,

reverses the transamination that occurs inmuscle,

and proportionately increases ureaproduction.


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Pyruvate and theGlucose-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.


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Cycle

The key feature of the cycle is thatmolecule, alanine,

peripheral tissue exports pyruvate andammonia (which are potentially rate-limiting for metabolism) to the liver,

where the carbon skeleton: recycled and most nitrogen eliminated.


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Cycle

There are 2 main pathways to production ofmuscle alanine:

directly from protein degradation,

and via the transamination of pyruvate byalanine transaminase, ALT (also referred to

as serum glutamate-pyruvate transaminase,SGPT).


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glutamate + pyruvate -KG +alanine

Alanine and theGlucose-Alanine Cycle


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Cys

teine

Biosy

nthes

is

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).


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S-AdoMet


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S-adenosylmethionine SAM serves as a precursor for numerous

methyl transfer reactions (e.g. the

conversion of norepinephrine toepinephrine,

The result of methyl transfer:

the conversion of SAM to S-adenosylhomocysteine.


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S-adenosylhomocysteine:

then cleaved by adenosylhomocysteinase:

to yield homocysteine and adenosine.


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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.


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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 oftransmethylase activity).


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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 tomethionine and not AMP.


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C

ysteine synthesis Homocysteine:

condenses with serine to produce

cystathionine, which is subsequently cleaved by

cystathionase;

to produce cysteine and -ketobutyrate. The sum of the latter two reactions:

known as trans-sulfuration.


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C

ysteine synthesis Cysteine is used for protein synthesis and

other body needs,

while the -ketobutyrate:

decarboxylated and converted to propionyl-CoA.


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C

ysteine 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 cystineby a non-enzymatic reduction reaction.


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Utilization of methionine in thesynthesis of cysteine


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C

ysteine 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.


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C

ysteine synthesis Cystathionase is under negative allosteric

control by cysteine,

as well, cysteine inhibits the expression ofthe cystathionine synthase gene.


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Tyrosine Biosynthesis

Tyrosine is produced in cells by:

hydroxylating the essential amino acidphenylalanine.

This relationship is much like that betweencysteine and methionine.


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T

yro

sin

eBio

synth

esi

s Half of the phenylalanine required:

goes into the production of tyrosine;

if the diet is rich in tyrosine itself,

the requirements for phenylalanine arereduced by about 50%.


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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).


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Ornithine and Proline

Biosynthesis

Glutamate:

the precursor of both proline and ornithine,

with glutamate semialdehyde being abranch point intermediate,

leading to one or the other of these 2products.


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Biosynthesis

While ornithine is not one of the 20 aminoacids used in protein synthesis,

it plays a significant role, as the acceptor ofcarbamoyl phosphate in the urea cycle.


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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.


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Biosynthesis

The fate of glutamate semialdehyde:

depends on prevailing cellular conditions.

Ornithine production:

occurs from the semialdehyde;

via a simple glutamate-dependenttransamination, producing ornithine.


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Biosynthesis When arginine concentrations become

elevated,

the ornithine contributed from the ureacycle;

plus that from glutamate semialdehyde:

inhibit the aminotransferase reaction, withaccumulation of the semialdehyde as aresult.


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Biosynthesis

The semialdehyde cyclizes spontaneously toD1-pyrroline-5-carboxylate;

which is then reduced to proline;

by an NADPH-dependent reductase.


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Serine Biosynthesis

Aminotransferase activity with glutamate;

as a donor produces 3-phosphoserine,

which is converted to serine byphosphoserine phosphatase


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Glycine Biosynthesis

The main pathway to glycine:

a 1-step reaction catalyzed by serinehydroxymethyltransferase.

This reaction involves the transfer of thehydroxymethyl group;

from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF.


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Glycine produced from serine;

or from the diet:

can also be oxidized by glycine cleavagecomplex, GCC,

to yield a second equivalent ofN5,N10-

methylene-tetrahydrofolate; as well as ammonia and CO2.


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Glycine:

involved in many anabolic reactions;

other than protein synthesis; including the synthesis of purine

nucleotides,

heme, glutathione, creatine and serine.


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Aspartate/Asparagine and

Glutamate/Glutamine Biosynthesis

Glutamate:

synthesized by the reductive amination of -ketoglutarate;

catalyzed by glutamate dehydrogenase;

it is thus a nitrogen-fixing reaction.


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Glutamate/Glutamine Biosynthesis In addition, glutamate arises by

aminotransferase reactions,

with the amino nitrogen being donated by anumber of different amino acids.

Thus, glutamate:

a general collector of amino nitrogen.


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Glutamate/Glutamine Biosynthesis Aspartate:

formed in a transamination reaction;

catalyzed by aspartate transaminase, AST.

This reaction uses the aspartate;

-keto acid analog, oxaloacetate,

and glutamate as the amino donor.


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Glutamate/Glutamine Biosynthesis Aspartate can also be formed by:

deamination of asparagine;

catalyzed by asparaginase.


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Glutamate/Glutamine Biosynthesis Asparagine synthetase;

and glutamine synthetase,

catalyze the production of asparagine and;

glutamine from their respective -aminoacids.


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Glutamate/Glutamine Biosynthesis Glutamine is produced from glutamate;

by the direct incorporation of ammonia;

and this can be considered another nitrogenfixing reaction.

Asparagine, however:

formed by an amidotransferase reaction


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Glutamate/Glutamine Biosynthesis Aminotransferase reactions:

are readily reversible.

The direction of any individualtransamination;

depends principally on the concentration

ratio of reactants and products.


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Glutamate/Glutamine Biosynthesis By contrast, transamidation reactions,

which are dependent on ATP,

are considered irreversible.


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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 bedegraded to aspartate.


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LITERATURE CITED

Devlin,T.M. Textbook of Biochemistry withClinical Correlations,Fifth Edition,Wiley-LissPublications,New York, USA, 2002.

Lehninger, A. Principles of Biochemistry, Secondedition, Worth Publishers Co., New York, USA,1993.

Matthews, C.K. and van Holde, K.E.,Biochemistry, Second edition, Benjamin /Cummings Publishing Company Inc., SanFrancisco, 1996.

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