bio.lesson document 21

8/6/2019 bio.lesson document 21

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BIO 202 Biochemistry II

bySeyhun YURDUGL

Lecture 10

Amino Acid Metabolism II:Amino Acid Degradation


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C

ontent Outline

Catabolism of specific amino acids Urea Cycle


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Glutamine/Glutamate and

Asparagine/Aspartate Catabolism

Glutaminase:

an important kidney tubule enzyme;

involved in converting glutamine (fromliver and from other tissue);

to glutamate and NH3+, with the NH3

+beingexcreted in the urine.


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Glutaminase activity:

present in many other tissues as well,although its activity:

is not nearly as prominent as in the kidney.

The glutamate produced from glutamine isconverted to -ketoglutarate,

making glutamine a glucogenic amino acid.


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Asparaginase:

also widely distributed within the body,

where it converts asparagine into ammoniaand aspartate.

Aspartate transaminates to oxaloacetate,

which follows the gluconeogenic pathwayto glucose.


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Glutamate and aspartate:

important in collecting and eliminatingamino nitrogen;

via glutamine synthetase and the urea cycle,respectively.


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The catabolic path of the carbon skeletonsinvolves:

simple 1-step aminotransferase reactions;

that directly produce net quantities of aTCA cycle intermediate.


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The glutamate dehydrogenase reaction;

operating in the direction of -ketoglutarateproduction:

provides a second avenue leading fromglutamate to gluconeogenesis.


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Alanine Catabolism

Alanine:

also important in intertissue nitrogentransport;

as part of the glucose-alanine cycle.

Alanine's catabolic pathway: involves a simple aminotransferase reaction

that directly produces pyruvate.


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Alanine Catabolism 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 pyruvate dehydrogenase complex;

and the TCA cycle. This makes alanine: a glucogenic amino acid.


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Arginine, Ornithine and Proline

Catabolism

The catabolism of arginine:

begins within the context of the urea cycle:

hydrolyzed to urea;

and ornithine by arginase.


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Catabolism

Ornithine, in excess of urea cycle needs:

transaminated to form glutamatesemialdehyde.

Glutamate semialdehyde:

can serve as the precursor for prolinebiosynthesis as described above;

or it can be converted to glutamate.


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Catabolism

Proline catabolism: a reversal of its synthesisprocess.

The glutamate semialdehyde; generated from ornithine and proline catabolism:

oxidized to glutamate by an ATP-independent

glutamate semialdehyde dehydrogenase.


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Catabolism

The glutamate can then be converted to:

-ketoglutarate in a transamination reaction.

Thus arginine, ornithine and proline, areglucogenic.


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Serine Catabolism

The conversion of serine to glycine;

and then glycine oxidation to CO2

and NH3,

with the production of two equivalents ofN5,N10-methylene tetrahydrofolate(THF).


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Serine Catabolism

Serine can be catabolized back to the glycolyticintermediate,

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


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Threonine Catabolism

There are at least 3 pathways for threoninecatabolism.

One involves a pathway initiated bythreonine dehydrogenase;

yielding -amino--ketobutyrate.


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The -amino-ketobutyrate is eitherconverted to acetyl-CoA;

and glycine;

or spontaneously degrades to aminoacetonewhich is converted to pyruvate.


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The second pathway:

involves serine/threonine dehydratase;

yielding -ketobutyrate which is furthercatabolized to propionyl-CoA;

and finally the TCA cycle intermediate,succinyl-CoA.


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The third pathway utilizes threoninealdolase.

The products of this reaction are bothketogenic (acetyl-CoA);

and glucogenic (pyruvate).


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Glycine Catabolism

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.


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Glycine Catabolism

Nevertheless, the main glycine catabolicpathway:

leads to the production ofCO2, ammonia, and one equivalent ofN5,N10-

methylene THF:

by the mitochondrial glycine cleavagecomplex.


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Cysteine Catabolism

There are several pathways for cysteinecatabolism.

The simplest, but least important pathway:

catalyzed by a liver desulfurase andproduces hydrogen sulfide, (H2S) and

pyruvate.


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Cysteine Catabolism

The major catabolic pathway in animals:

via cysteine dioxygenase that oxidizes thecysteine sulfhydryl to sulfinate,

producing the intermediate cysteinesulfinate.


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Cysteine Catabolism Cysteine sulfinate: can serve as a biosynthetic intermediate

undergoing decarboxylation; and oxidation to produce taurine. Catabolism of cysteine sulfinate proceeds through

transamination to -sulfinylpyruvate;

which then undergoes desulfuration yieldingbisulfite, (HSO3

-);

and the glucogenic product, pyruvate.


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Cysteine Catabolism The enzyme sulfite oxidase uses O2 and

H2O to convert HSO3- to sulfate, (SO4

-) and

H2O2. The resultant sulfate:

used as a precursor;

for the formation of 3'-phosphoadenosine-5'-phosphosulfate, PAPS


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Cysteine Catabolism PAPS: used for the transfer of sulfate:

to biological molecules; such as the sugarsof the glycosphingolipids.


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Cysteine Catabolism Other than protein,

the most important product of cysteinemetabolism:

the bile salt precursor taurine,

which is used to form the bile acidconjugates taurocholate andtaurochenodeoxycholate.


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Cysteine Catabolism The enzyme cystathionase:

can also transfer the sulfur;

from one cysteine to another generatingthiocysteine and pyruvate.

Transamination of cysteine yields -

mercaptopyruvate which then reacts with sulfite,(SO32-),

to produce thiosulfate, (S2O32-) and pyruvate.


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Cysteine Catabolism Both thiocysteine and thiosulfate:

can be used by the enzyme rhodanese;

to incorporate sulfur into cyanide, (CN-),

thereby detoxifying the cyanide tothiocyanate.


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Methionine Catabolism

The principal fates of the essential aminoacid methionine:

incorporation into polypeptide chains,

and use in the production of -ketobutyrate;

and cysteine via SAM as described above.


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The transulfuration reactions that producecysteine from homocysteine and serine:

also produce -ketobutyrate,

the latter being converted to succinyl-CoA.


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Regulation of the methionine metabolic pathway: based on the availability of methionine and

cysteine. If both amino acids are present in adequate

quantities, SAM accumulates; and is a positive effector on cystathionine

synthase, encouraging the production of cysteine and -

ketobutyrate (both of which are glucogenic).


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However, if methionine is scarce,

SAM will form only in small quantities,

thus limiting cystathionine synthase activity. Under these conditions:

accumulated homocysteine is remethylated to

methionine, using N5-methyl THF and other compounds as

methyl donors


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Valine, Leucine and Isoleucine

Catabolism

This group of essential amino acids:

identified as the branched-chain aminoacids, BCAAs.

Because this arrangement of carbon atomscannot be made by humans,

these amino acids are an essential elementin the diet.


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Catabolism

The catabolism of all three compounds

initiates in muscle; and yields NADH and FADH2 which can be

utilized for ATP generation.


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Catabolism The catabolism of all three of these amino

acids:

uses the same enzymes in the first twosteps.

The first step in each case:

a transamination using a single BCAAaminotransferase,

with -ketoglutarate as amine acceptor.


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Catabolism

As a result,

three different -keto acids:

produced and are oxidized using a commonbranched-chain -keto acid dehydrogenase,

yielding the three different CoA derivatives.

Subsequently the metabolic pathwaysdiverge, producing many intermediates.


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Catabolism The principal product from valine is

propionylCoA,

the glucogenic precursor of succinyl-CoA. Isoleucine catabolism terminates with

production of acetylCoA andpropionylCoA;

thus isoleucine: both glucogenic andketogenic.


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Catabolism

Leucine gives rise to acetyl CoA;

and acetoacetyl CoA,

and classified as strictly ketogenic.


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Phenylalanine Catabolism

Phenylalanine normally has only two fates:

incorporation into polypeptide chains, and production of tyrosine;

via the tetrahydrobiopterin-requiring

phenylalanine hydroxylase. Thus, phenylalanine catabolism always

follows the pathway of tyrosine catabolism.


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Phenylalanine Catabolism The main pathway for tyrosine degradation:

involves conversion to fumarate andacetoacetate,

allowing phenylalanine and tyrosine to beclassified as:

both glucogenic and ketogenic.


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T

yrosine Catabolism Tyrosine is equally important for protein

biosynthesis;

as well as an intermediate in thebiosynthesis of several physiologicallyimportant metabolites

e.g. dopamine, norepinephrine andepinephrine


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Lysine Catabolism

Lysine catabolism:

unusual in the way that the Epsilon-aminogroup:

transferred to -ketoglutarate and into the

general nitrogen pool.


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Lysine Catabolism The reaction:

a transamination in which the epsilon-aminogroup is transferred to the -keto carbon;

of -ketoglutarate forming the metabolite,

saccharopine.


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Lysine Catabolism Unlike the majority of transamination

reactions,

this one does not employ pyridoxalphosphate as a cofactor.


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Lysine Catabolism

Saccharopine:

immediately hydrolyzed by the enzyme -

aminoadipic semialdehyde synthase; in such a way that the amino nitrogen

remains with the -carbon of -ketoglutarate,

producing glutamate and -aminoadipicsemialdehyde.


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Lysine Catabolism

Because this transamination reaction is notreversible,

Lysine: an essential amino acid.

The ultimate end-product of lysinecatabolism: acetoacetyl-CoA


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The Urea Cycle

Earlier it was noted that:

kidney glutaminase was responsible for convertingexcess glutamine;

from the liver to urine ammonium.


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The Urea Cycle

However, about 80% of the excreted nitrogen:

in the form of urea which is also largely made in theliver,

in a series of reactions;

that are distributed:

between the mitochondrial matrix;

and the cytosol.


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The Urea Cycle

The series of reactions that form urea:

known as the Urea Cycle;

or the Krebs-Henseleit Cycle.


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The Urea Cycle

The essential features of the urea cyclereactions;

and their metabolic regulation are asfollows:

Arginine from the diet or from proteinbreakdown:

cleaved by the cytosolic enzyme arginase,generating urea and ornithine.


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The Urea Cycle

In subsequent reactions of the urea cycle;

a new urea residue is built on the ornithine,

regenerating arginine;

and perpetuating the cycle.


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The Urea Cycle

Ornithine arising in the cytosol:

transported to the mitochondrial matrix,

where ornithine transcarbamoylase:

catalyzes the condensation of ornithine withcarbamoyl phosphate,

producing citrulline.


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The Urea Cycle

The energy for the reaction:

provided by the high-energy anhydride of

carbamoyl phosphate. The product, citrulline:

then transported to the cytosol, where the

remaining reactions of the cycle take place.


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The Urea Cycle

The synthesis of citrulline:

requires a prior activation of carbon and

nitrogen as carbamoyl phosphate (CP). The activation step:

requires 2 equivalents of ATP;

and the mitochondrial matrix enzymecarbamoyl phosphate synthetase-I (CPS-I).


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The Urea Cycle

There are two CP synthetases:

a mitochondrial enzyme, CPS-I,

which forms CP destined for inclusion inthe urea cycle,

and a cytosolic CP synthatase (CPS-II),

which is involved in pyrimidine nucleotidebiosynthesis.


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The Urea Cycle

CPS-I:

positively regulated by the allosteric

effector N-acetyl-glutamate, while the cytosolic enzyme:

acetylglutamate independent


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The Urea Cycle

In a two-step reaction,

catalyzed by cytosolic argininosuccinate

synthetase, citrulline and aspartate are condensed to

form argininosuccinate.


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The Urea Cycle

The reaction involves the addition of AMP(from ATP)

to the amido carbonyl of citrulline, forming an activated intermediate on the

enzyme surface (AMP-citrulline),

and the subsequent addition of aspartate toform argininosuccinate.


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The Urea Cycle

Arginine and fumarate:

produced from argininosuccinate;

by the cytosolic enzyme argininosuccinatelyase (also called argininosuccinase).


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The Urea Cycle

In the final step of the cycle;

arginase cleaves urea from aspartate,

regenerating cytosolic ornithine,

which can be transported to themitochondrial matrix

for another round of urea synthesis.


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The Urea Cycle

The fumarate,

generated via the action of

argininosuccinate lyase, reconverted to aspartate for use in the

argininosuccinate synthetase reaction.


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The Urea Cycle

This occurs through the actions of cytosolicversions of the TCA cycle enzymes,

fumarase (which yields malate) and malatedehydrogenase (which yields oxaloacetate).

The oxaloacetate:

then transaminated to aspartate by AST


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The Urea Cycle

Beginning and ending with ornithine,

the reactions of the cycle consumes 3

equivalents of ATP; and a total of 4 high-energy nucleotide

phosphates.


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The Urea Cycle

Urea:

the only new compound generated by the cycle;

all other intermediates and reactants are recycled. The energy consumed in the production of urea:

more than recovered;

by the release of energy formed; during the synthesis of the urea cycle

intermediates.


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The Urea Cycle

Ammonia released during the glutamatedehydrogenase reaction:

coupled to the formation of NADH. In addition, when fumarate is converted

back to aspartate,

the malate dehydrogenase reaction used toconvert malate;

to oxaloacetate generates a mole of NADH.


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The Urea Cycle

These two moles of NADH:

oxidized in the mitochondria yielding 6

moles of ATP.


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Histidine Catabolism

Histidine catabolism begins with release ofthe -amino group catalyzed by histidase,

introducing a double bond into themolecule. As a result, the deaminated product,

urocanate, is not the usual -keto acid associated with

loss of -amino nitrogens.


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The end product of histidine catabolism:

glutamate, making histidine one of the

glucogenic amino acids.


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Another key feature of histidine catabolism:

it serves as a source of ring nitrogen to

combine with tetrahydrofolate (THF), producing the 1-carbon THF intermediate

known as N5-formiminoTHF.

The latter reaction: one of two routes to N5-formiminoTHF.


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Tryptophan Catabolism

A number of important side reactions occur during the catabolism of tryptophan on the

pathway to acetoacetate. The first enzyme of the catabolic pathway: an iron porphyrin oxygenase that opens the indole

ring.

The latter enzyme: highly inducible, its concentration rising almost

10-fold on a diet high in tryptophan.


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Kynurenine: the first key branch point intermediate in the

pathway. undergoes deamination in a standardtransamination reaction;

yielding kynurenic acid.

Kynurenic acid and metabolites: have been shown to act as antiexcitotoxics and

anticonvulsives.


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A second side branch reaction:

produces anthranilic acid plus alanine.

Another equivalent of alanine: produced further along the main catabolic

pathway,

and it is the production of these alanine:

residues that allows tryptophan to be classifiedamong the glucogenic and ketogenic amino acids.


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The second important branch point convertskynurenine:

into 2-amino-3-carboxymuconicsemialdehyde, which has two fates.

The main flow of carbon elements from thisintermediate:

to glutarate.


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An important side reaction in liver:

a transamination;

and several rearrangements to producelimited amounts of nicotinic acid,

which leads to production of a small amount

of NAD+ and NADP+


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Aside from its role as an amino acid inprotein biosynthesis,

tryptophan also serves as a precursor: for the synthesis of serotonin;

and melatonin.


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bio.lesson document 21