Amino Acid Metabolism Overview

Study Notes

Glycine is a non-essential, glucogenic amino acid.
Glycine's fate includes the formation of purine bases (carbon atoms 4, 5, and nitrogen atom 7 are derived from glycine) and glutathione (a tripeptide of glutamate, cysteine, and glycine; acts as a hydrogen carrier).
Creatine (methyl-guanido-acetate) is used by muscles as creatine phosphate, a source of energy during exercise. It's synthesized from glycine, arginine, and methionine.
Glycine reacts with succinyl CoA to form heme during heme synthesis.
Collagens have a triple helical structure with glycine present in every third position (Gly-X-Y-Gly-X-Y-Gly-X-Y...).
Glycine conjugates with primary bile acids (e.g., cholic acid) to form glycocholic acid (bile salts).
Glycine conjugates with toxic benzoate to form the non-toxic hippuric acid.
Glycine acts as an inhibitory neurotransmitter in the spinal cord.
Glycine participates in the formation of glyoxylic acid.

Phenylalanine is an essential, mixed glucogenic/ketogenic amino acid.
It is involved in the structure of body proteins and the formation of tyrosine primarily in the liver.
The conversion of phenylalanine to tyrosine is an irreversible reaction catalyzed by phenylalanine hydroxylase.
Deficiency in any component of this pathway can lead to phenylketonuria.
Dihydrobiopterin reductase and tetrahydrobiopterin are enzymes involved in the process, along with oxygen.

Tyrosine is a non-essential amino acid derived from phenylalanine.
It is a mixed glucogenic/ketogenic amino acid.
Important derivatives of tyrosine include catecholamines (dopamine, noradrenaline, adrenaline) in the adrenal medulla and adrenal neurons.
It's also a precursor for thyroid hormones (T3 and T4) in the thyroid gland.
Tyrosine is involved in the synthesis of melanin pigments and the putrefactive substances phenol, cresol, and tyramine in the intestines.

Tryptophan is an essential, mixed glucogenic/ketogenic amino acid.
Catabolism of tryptophan produces alanine and acetoacetyl CoA.
It contributes to niacin (a B vitamin complex) synthesis.
Tryptophan is a precursor in the synthesis of serotonin (5-hydroxytryptamine), a neurotransmitter.
It is involved in melatonin synthesis, a hormone produced by the pineal gland.
Tryptophan is used to synthesize indol and skatol.

Arginine is a semi-essential, glucogenic amino acid.
Microorganisms can synthesize arginine from glutamate.
Arginine participates in the synthesis of α-ketoglutarate.
Arginine plays a role in creatine synthesis.
It's involved in the synthesis of streptomycin, a component of some antibiotics.
Arginine is used in the synthesis of urea and nitric acid as well as arginine phosphate in invertebrates.

Serine is a non-essential, glucogenic amino acid involved in the synthesis of glycine, cysteine, ethanolamine, and choline.
It's crucial for sphingosine formation (from serine and palmitoyl-CoA).
Serine is a precursor to the methyl groups of pyrimidine bases (thymine) and purine bases (C2 and C8).
It's also involved in the synthesis of phosphoproteins, such as casein.

Methionine is an essential, glucogenic amino acid.
Its function is as a methyl donor (S-adenosyl methionine, SAM).
SAM is important for the synthesis of creatine, adrenaline, noradrenaline, choline, and other compounds.

Glutamic acid is a non-essential, glucogenic amino acid.
It's synthesized from α-ketoglutarate.
It's involved in the synthesis of α-ketoglutarate, providing a glucose precursor.
Glutamic acid is involved in glutamine, proline, hydroxyproline, ornithine, glutathione, and GABA (γ-aminobutyric acid) synthesis.
GABA is an inhibitory neurotransmitter.

Aspartic acid is a non-essential, glucogenic amino acid.
It's synthesized from oxaloacetate via transamination.
Aspartic acid is essential for synthesizing purines (N1 group) and pyrimidines (N1, C4, C5, C6).
It is a precursor to urea synthesis.
It's involved in the synthesis of β-alanine and asparagine.