Amino acids in biology University, Exams of Biochemistry

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Amino acids degradation and synthesis

Shyamal D. Desai Ph.D.Department of Biochemistry & Molecular BiologyMEB. Room # 7107Phone- 504-568-4388sdesai@lsuhsc.edu

Nitrogen metabolism

N

Atmospheric nitrogen N2 is most abundant but is tooinert for use in most biochemical processes.

Dietary proteins

Atmospheric nitrogen is acted upon by bacteria (nitrogenfixation) and plants to nitrogen containing compounds. Weassimilate these compounds as proteins (amino acids) inour diets.

Amino acids

Body proteins

Other nitrogencontaining compounds

Carbon skeletons

NH4+

Urea

excreted

Lecture I

Lecture III

α Lecture II

-amino groups

Conversion of nitrogeninto specialized products

Enters various metabolic pathways

Disposal of Nitrogen

Amino acids synthesis& degradation

EssentialEssential

versusversus

Nonessential Amino AcidsNonessential Amino Acids

Cannot be synthesizedde novo,

hence, must

be

supplied in the diet.

Synthesized by body

Glucogenic and Ketogenic Amino acids

Amino acids are classified as glucogenic, ketogenic, or both based on which of theseven intermediates are produced during their catabolism.

Glucogenic

Ketogenic

Amino acids that can beconverted into glucosethrough gluconeogenesis

Amino acids that can beconverted into ketone bodiesthrough ketogenesis

Amino acids whose catabolism yields pyruvate or one of the intermediates of the citric acid cycle are termed glucogenic or glycogenic Amino acids whose catabolism yields either acetoacetate or one of its precursor, (acetyl CoA or acetoacetyl CoA) are termed ketogenic.

Some amino acids are both glucogenic or ketogenic

Glucogenic and Ketogenic Amino acids

or glycogenic

Catabolism of the carbon skeletons of amino acids Amino acids that enter metabolism as oxaloacetate

(Aspargine and Aspartate)

Asparagine is hydrolyzed by Asparaginase, liberating ammonia and AspartateAspartate loses its amino group by transamination to form oxaloacetate

condenses with acetyl CoA to form citratein the first reaction of the Krebs cycle.

Glucogenic

Amino acids that enter metabolism as pyruvate

Glucogenic

1. Alanine

Alanine, Serine, Glycine, Cystine Threonine Alanine loses its amino group by transamination to form

pyruvate

2. Serine and 3) Glycine

Serine can be converted to glycine and N5, N10-methylenetetrahydorfolate or topyruvate by serine dehydratase. Inter conversion of serine and glycine

4. Cystine

Cysteine

reduced by NADH + H+

Cystine

desulfuration

pyruvate

5. Threonine

Threonine

pyruvate α

-ketobutyrate

Succinyl CoA

Amino Acids that enter metabolism as fumarate

Phenylalanine and Tyrosine

1. Phenylalanine and 2) Tyrosine

Fumarate Acetoacetate

hydroxylated

Phenylalanine

Tyrosine

Hence these two aa are both glucogenic and ketogenic

Amino acids that form succinyl CoA Valine, Isoleucine and Threonine

1. Valine and Isoleucine

Valine and Isoleucine

Propionyl CoA

Metabolism of IsoleucineAlso give Acetyl CoA and henceIs both glucogenic and ketogeic

Requires vitamin B12 and Biotin

Succinyl CoA

TCA cycle

2. Threonine

Threonine

dehydrated

Propionyl CoASuccinyl CoA

TCA cycle

Amino acids that form acetyl CoA or acetoacetyl CoA

2. Isoleucine

3. Lysine

4. Tryptophan

5. Leucine

Glucogenic and ketogenic

Ketogenic and glucogenic

Exclusively Ketogenic

Since its metabolism yields both alanine and Acetoacetyl CoA

Acetyl CoA Exclusively Ketogenic

Lysine is unusual in that neither of its amino groupsundergoes transamination as the first step of in catabolism

Catabolism of the branched chain amino acids

Branched chain AA are: Isoleucine, Leucine, Valine* Essential AA•Metabolized primarily by the peripheral tissues (muscles) and notIn the liver like other amino acids.*All three have similar route of catabolism

Transamination Oxidative decarboxylationDehydrogenase

Catalyzed by a single Vitamin B6-requiring enzyme,Branched-chain

α

-amino acid aminotransferase.

The removal of carboxyl group of the

α

-keto acids from these three AAs

is catalyzed by the same branched-chain

α

-keto acid dehydrogenase

complex.This enzyme uses thiamine pyrophosphate, lipoic acid, FAD, NAD+, andCoA as cooenzymes).

Oxidation of the products formed in the decarboxylation reactionyields

α

• β

-unsaturated acyl CoA derivatives.

Role of Folic aid in Amino acid metabolism

Tetrahydrofolic acid, an active form of Folic acid that carries single carbon unit. This carbon unit is transferred^ to specific structures that are being synthesized or modified. One

• carbon

metabolism comprises a network of integrated biochemical pathways that donate, and regenerate,

the

one

• carbon

moieties needed for physiologic processes.

Synthesis by amidation

Glutamine:

Glutamine:•contains an amide linkage with ammonia at the

γ

-carboxyl

•Is formed from glutamate•Reaction is driven by glutamine synthetase•Requires ATP•Reaction serves as a major step for detoxification of ammonia inaddition to the synthesis of Glutamine for protein synthesis.

Biosynthesis of nonessential amino acids

Aspargine:

Aspargine: •contains an amide linkage with ammonia at the

β

-carboxyl

•Is formed from Aspratate •Reaction is driven by asparagine synthatase using glutamine as a amide donor. •Requires ATP

Proline: Glutamate is converted to proline by cyclization and reduction reactions.

Biosynthesis of nonessential amino acids

Serine: Synthesized from glycolysis intermediate 3-phosphogylcerate.

Or

Removal ofmethyl groupfrom serine

Glycine: Cysteine:

Is synthesized by two consecutive reactions1) Homocysteine + serine

Cystathionine

hydrolysis

α

-ketobutyrate + cysteine