CONVERSION OF AMINO

ACIDS TO SPECIALIZED

PRODUCTS

Raymond Oliver A. Cruz, MD

BIOMEDICAL IMPORTANCE

Many proteins contain amino acids that

have been posttranslationally modified for

a specific function

These include heme, certain hormones,

and neurotransmitters

The synthesis of purines and pyrimidines

also begins with amino acids which

contribute a portion of their backbone

Glycine Metabolites and pharmaceuticals excreted as

water-soluble glycine conjugates include

glycocholic acid (a primary bile acid) and hippuric

acid formed from the food additive benzoate

Glycine is incorporated into creatine

Biosynthesis and metabolism of creatine and creatinine.

Glycine

The entire glycine molecule becomes

atoms 4, 5, and 7 of purines

GLYCINE

The nitrogen and

a-carbon of glycine

are incorporated

into the pyrrole

rings and the

methylene bridge

carbons of heme

a-Alanine

Together with glycine, a-alanine

constitutes a major fraction of the free

amino acids in plasma.

b-Alanine

Present in coenzyme A

b-Alanine

Also present as b-alanyl dipeptides,

principally carnosine

Mammalian tissues form b-alanine from

cytosine, carnosine, and anserine

b-Alanyl Dipeptides

The b-alanyl

dipeptides carnosine

and anserine (N-

methylcarnosine)

activate myosin

ATPase, chelate

copper, and enhance

copper uptake

Serine

Participates in

the biosynthesis

of sphingosine, a

component of

sphingomyelin

Phosphorylated Serine,

Threonine, & Tyrosine

The phosphorylation

and

dephosphorylation of

seryl, threonyl, and

tyrosyl residues

regulate the activity of

certain enzymes of

lipid and carbohydrate

metabolism

Methionine

S-Adenosylmethionine is the principal

source of methyl groups in the body

It also contributes its carbon skeleton for

the biosynthesis of the 3-diaminopropane

portions of the polyamines spermine and

spermidine

Cysteine

L-Cysteine is a

precursor of the

thioethanolamine

portion of

coenzyme A and of

the taurine that

conjugates with

bile acids such as

taurocholic acid

Histidine

Decarboxylation of histidine to histamine is

catalyzed by a broad-specificity aromatic

L-amino acid decarboxylase that also

catalyzes the decarboxylation of dopa, 5-

hydroxytryptophan, phenylalanine,

tyrosine, and tryptophan

Histidine

Arginine

Arginine is the formamidine donor for

creatine synthesis and via ornithine to

putrescine, spermine, and spermidine

The 24-hour urinary excretion of creatinine

is proportionate to muscle mass and is a

useful indicator of kidney function

Glycine, arginine, and methionine participate in creatinine synthesis

Arginine is also the

precursor of the

intercellular signaling

molecule nitric oxide

(NO) that serves as a

neurotransmitter,

smooth muscle

relaxant, and

vasodilator

Polyamines

The polyamines

spermidine and

spermine function

in cell proliferation

and growth,

CATABOLISM OF POLYAMINES

Tryptophan

Following hydroxylation of tryptophan to 5-hydroxytryptophan by liver tyrosine hydroxylase, subsequent decarboxylation forms serotonin (5-hydroxytryptamine or 5-HT), a potent vasoconstrictor and stimulator of smooth muscle contraction

Kidney tissue, liver tissue, and fecal bacteria all convert tryptophan to tryptamine, then to indole 3-acetate.

Tyrosine

Neural cells convert tyrosine to epinephrine

and norepinephrine

Tyrosine is also a precursor of triiodothyronine

and thyroxine

g-Aminobutyrate

g-Aminobutyrate (GABA) functions in brain

tissue as an inhibitory neurotransmitter by

altering transmembrane potential

differences

It is formed by decarboxylation of L-

glutamate, a reaction catalyzed by L-

glutamate decarboxylase

PLP-pyridoxal phosphate

TAPOS NA PO!

GISING PA KAYO?