David L. Nelson and Michael M. Cox
Sixth Edition
CHAPTER 18
© 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
NH
4
+
NH
4
+

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
NH
4
+
NH
4
+

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
NH
4
+
NH
4
+

Glucose-Alanine
Cycle

The Urea Cycle

The citric acid cycle and the urea cycle are linked

2 ATPs are used in carbamoyl-phosphate synthesis.
1 ATP is used in arginosuccinate synthesis.
The overall equation for the urea cycle is:
2NH
4
+ + HCO 3+ 3ATP + H
2
O  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)