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Nitrogen Cycle
Summary of Protein and
Amino Acid Metabolism
Transamination
(transaminase)
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
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
-ketoglutarate
-ketoglutarate
glutamate
malate
NAD+
alanine
(from muscle)pyruvate
NADH
oxaloacetate
-ketoglutarate
aspartate
glutamate
NH4+
glutamine
(from extrahepatic
tissues)
malate
aspartate
citrulline
HCO3–
ATP
2 ATP
glutamine
citrulline
cytosol
2 ADP
+ Pi
2Pi
carbamoyl
phosphate
mitochondrial
matrix
AMP
Pi
arginosuccinate
O
H2N C OPO32–
H2O
ornithine
fumarate
H2O
arginine
ornithine
cytosol
O
H2N C NH2
urea
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
Glycine
Aspartate
Tyrosine
Histidine
Tryptophan
Glutamate
Product(s)
Heme, Purines
Purines, Pyrimidines
Thyroid Hormones
Epinephrine
Dopamine
Melanin
Histamine
Seratonin
GABA (gamma-aminobutyrate)