Download Lehninger Principles of Biochemistry

David L. Nelson and Michael M. Cox
LEHNINGER
PRINCIPLES OF BIOCHEMISTRY
Sixth Edition
CHAPTER 18
Amino Acid Oxidation and the
Production of Urea
© 2013 W. H. Freeman and Company
Excretion
as
Urea
Conversion to:
pyruvate
acetyl-CoA
citric acid cycle intermediates
Digestion by
proteases
Digestion by
proteases
cytosol
mitochondria
NH4+
NH4+
Forms of excreted nitrogen
Digestion of dietary protein
Protein in stomach stimulates production of hormone gastrin.
Gastrin stimulates secretion of HCl and the protease pepsin.
Pepsin hydrolyzes proteins on amino side of Phe, Trp, Tyr.
In small intestine acidic contents stimulate secretion of
the hormone secretin.
Secretin stimulates bicarbonate secretion, bringing pH up to 7.
Amino acids stimulate release of hormone cholecystokinin.
Cholecystokinin stimulates secretion of the proteases:
trypsin - hydrolyzes proteins on carboxyl side of Lys, Arg
chymotrypsin - carboxyl side of Phe, Trp, Tyr
carboxypeptidase A – removes carboxy-terminal amino acids
aminopeptidase – removes amino terminal amino acids
cytosol
mitochondria
NH4+
NH4+
Aminotransferases
catalyze transfer of amino
groups
of amino acids to
a-ketoglutarate
Pyridoxal phosphate (PLP) functions as an amino group
carrier in aminotransferases
Glutamate is transported to the mitochondria where
it undergoes oxidative deamination to a-ketoglutarate
cytosol
mitochondria
NH4+
NH4+
Glucose-Alanine
Cycle
Nitrogen excretion
and
The Urea Cycle
The Urea Cycle
The citric acid cycle and the urea cycle are linked
The Cost of Urea Synthesis
2 ATPs are used in carbamoyl-phosphate synthesis.
1 ATP is used in arginosuccinate synthesis.
The overall equation for the urea cycle is:
2NH4+ + HCO3- + 3ATP + H2O  urea + 2ADP + 4Pi + AMP + 5H+
However, fumarate was produced which feeds into the
citric acid cycle and is converted to oxaloacetate,
producing a molecule of NADH (2.5 ATP)