Nitrogen Cycle

Summary of Protein and Amino Acid Metabolism

Transamination

Common acceptors: -ketoglutarate (glutamate) pyruvate (alanine); oxaloacetate (aspartate)

Important transaminases: Aspartate transaminase (AST) (also called glutamate-oxaloacetate transaminase, GOT) High activity in heart; tool for diagnosis of heart damage

(transaminase)

Alanine transaminase (ALT) (also called glutamate-pyruvate transaminase, GPT) High activity in liver; tool for diagnosis of liver damage

Oxidative Deamination

Alanine –Glucose Cycle

Urea Cycle

The Urea Cycle

glutamine(from extrahepatictissues)

ornithinecarbamoylphosphate

H2N C OPO32–

O

alanine(from muscle)

-ketoglutarate

pyruvate

glutamate

glutamine

glutamate

NH4+

oxaloacetate

aspartate

-ketoglutarate

aspartate

2 ATP

2 ADP+ Pi

HCO3–

citrulline

Pi

citrulline

arginosuccinate

2Pi

ATP

AMP

fumarate

arginine

ureaH2N C NH2

O

ornithine

H2O

malate

NADH

NAD+

-ketoglutarate

mitochondrialmatrix

cytosol

cytosol

malate

H2O

Fate of Carbon Atoms from Amino Acids

Tryptophan

glucogenic amino acids ketogenic amino acids

Biosynthesis of Nonessential Amino Acids

Methionine(essential amino acid)

Amino Acids as Precursors of Other Nitrogeneous Compounds

Amino Acid Product(s)Glycine Heme, Purines

Tyrosine Thyroid HormonesEpinephrineDopamineMelanin

Histidine Histamine

SeratoninTryptophan

Aspartate Purines, Pyrimidines

GABA (gamma-aminobutyrate)Glutamate