Species Differences in Amino Acid Metabolism

Metabolic pathways for most amino acids are generally similar between microorganisms and animals, but important differences do occur. For example, N-acetylglutamate is an intermediate of and an allosteric activator for arginine synthesis in microorganisms and animal cells, respectively.[2,5] Second, deiminase plays a significant role in microbial arginine degradation to form citrulline and ammonia; animal cells, however, lack this pathway. Third, the conversion of proline into pyrroline-5-carbox-ylate is catalyzed by NAD(P)+-dependent proline dehy-drogenase in microorganisms, but by oxygen-dependent proline oxidase in animal cells. Regarding differences among animals, most mammals (except for cats and ferrets) can convert glutamine, glutamate, and proline into citrulline in enterocytes, whereas birds do not. Similarly, ammonia detoxification pathways differ remarkably between ureotelic and uricotelic organisms.

Table 1 Reactions initiating amino

acid catabolism in animals

Reactions

Examples

Transamination

Leucine + a Ketoglutarate^ a Ketoisocaproate +Glutamate

(1)

Deamidation

Glutamine + H2O ^Glutamate + NH+

(2)

Oxidative deamination

Glutamate + NAD+ ^ a Ketoglutarate+NH3+NADH + H+

(3)

Decarboxylation

Ornithine !Putrescine + CO2

(4)

Hydroxylation

Arginine + O2+ NADPH + H+!NO + Citrulline+NADP+

(5)

Reduction

Lysine + a Ketoglutarate + NADPH + H+!Saccharopine+NADP+

(6)

Dehydrogenation

Threonine+NAD+!2 Amino 3 ketobutyrate+NADH + H+

(7)

Hydrolysis

Arginine + H2O ^Ornithine + Urea

(8)

Dioxygenation

Cysteine + O2!Cysteinesulfinate

(9)

One carbon unit transfer

Glycine+N5 N10 methylene THF ^ Serine + THF

(10)

Condensation

Methionine+Mg ATP !S Adenosylmethionine+Mg PPi + Pi

(11)

Oxidation

Proline+1/2O2!Pyrroline 5 carboxylate + H2O

(12)

Enzymes that catalyze the indicated reactions are: 1) BCAA transaminase; 2) glutaminase; 3) glutamate dehydrogenase; 4) ornithine decarboxylase; 5) NO synthase; 6) lysine:a ketoglutarate reductase; 7) threonine dehydrogenase; 8) arginase; 9) cysteine dioxygenase; 10) hydroxymethyltransferase; 11) S adenosylmethionine synthase; and 12) proline oxidase. THF, tetrahydrofolate. Tetrahydrobiopterin is required for hydroxylation of arginine, phenylalanine, tyrosine, and tryptophan.

Enzymes that catalyze the indicated reactions are: 1) BCAA transaminase; 2) glutaminase; 3) glutamate dehydrogenase; 4) ornithine decarboxylase; 5) NO synthase; 6) lysine:a ketoglutarate reductase; 7) threonine dehydrogenase; 8) arginase; 9) cysteine dioxygenase; 10) hydroxymethyltransferase; 11) S adenosylmethionine synthase; and 12) proline oxidase. THF, tetrahydrofolate. Tetrahydrobiopterin is required for hydroxylation of arginine, phenylalanine, tyrosine, and tryptophan.

Table 2 Important nitrogenous products of amino acid metabolism in animals

Precursors Products Functions

Precursors Products Functions

Table 2 Important nitrogenous products of amino acid metabolism in animals

Arginine

NO

Vasodilator; neurotransmitter, signaling molecule; angiogenesis; cell

metabolism; apoptosis (programmed cell death); immune response

Agmatine

Signaling molecule; inhibitor of NO synthase and ornithine

decarboxylase; brain and renal function

Cysteine

Taurine

Antioxidant; muscle contraction; bile acid conjugates; retinal function

Glutamate

g Aminobutyrate

Neurotransmitter; inhibitor of glutamatergic, serotonin, and NEPN activities

Glutamine

Glu and Asp

Neurotransmitters; fuels for enterocytes; components of the malate shuttle

Glucosamine

Glycoprotein and ganglioside formation; inhibitor of NO synthesis

Ammonia

Renal regulation of acid base balance; synthesis of carbamoylphosphate,

glutamate and glutamine

Glycine

Serine

One carbon unit metabolism; ceramide and phosphatidylserine formation

Heme

Hemoproteins (e.g., hemoglobin, myoglobin, catalase, cytochrome C)

Histidine

Histamine

Allergic reaction; vasodilator; gastric acid and central

acetylcholine secretion

Methionine

Homocysteine

Oxidant; inhibitor of NO synthesis; risk factor for cardiovascular disease

Betaine

Methylation of homocysteine to methionine; one carbon unit metabolism

Choline

Synthesis of betaine, acetylcholine (neurotransmitter and vasodilator)

and phosphatidylcholine

Cysteine

An important sulfur containing amino acid; formation of disulfide bonds

Phenylalanine

Tyrosine

A versatile aromatic amino acid containing a hydroxyl group

Serine

Glycine

Antioxidant; bile acid conjugates; neurotransmitter; immunomodulator

Tryptophan

Serotonin

Neurotransmitter; smooth muscle contraction; hemostasis

N acetylserotonin

Inhibitor of sepiapterin reductase and thus tetrahydrobiopterin synthesis

Melatonin

Circadian and circannual rhythms; free radical scavenger; antioxidant

Tyrosine

Dopamine

Neurotransmitter; apoptosis; lymphatic constriction

EPN and NEPN

Neurotransmitters; smooth muscle contraction; cAMP production;

glycogen and energy metabolism

Melanin

Dark color pigment; free radical scavenger; chelator of metals

T3 and T4

Gene expression; tissue differentiation and development; cell metabolism

Arg and Met

Polyamines

Gene expression; DNA and protein synthesis; ion channel function;

apoptosis; signal transduction; antioxidants; cell function, proliferation,

and differentiation

Gln and Asp

Nucleic acids

Gene expression; cell cycle and function; protein and uric acid synthesis

Gln and Trp

NAD(P)

Coenzymes for oxidoreductases; substrate of poly(ADP ribose) polymerase

Arg, Pro or Gln

Ornithine

Glutamate, glutamine, and polyamine synthesis; mitochondrial integrity

Arg, Met, Gly

Creatine

Energy metabolism in muscle and nerve; antioxidant; antiviral; antitumor

Cys, Glu, and Gly

Glutathione

Free radical scavenger; antioxidant; formation of leukotrienes,

mercapturate, glutathionylspermidine, glutathione NO adduct and

glutathionylproteins; signal transduction; gene expression; apoptosis;

spermatogenesis; sperm maturation; cellular redox state

Gln, Glu, and Pro

Citrulline

Free radical scavenger; arginine synthesis

Lys, Met, and Ser

Carnitine

Transport of long chain fatty acids into mitochondria; storage of energy

as acetylcarnitine

EPN, epinephrine; NEPN, norepinephrine; T3, triiodothyronine; T4, thyroxine.

EPN, epinephrine; NEPN, norepinephrine; T3, triiodothyronine; T4, thyroxine.

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