336
CASE FILES: BIOCHEMISTRY
[36.3]
During starvation muscle activity decreases, and muscle protein is bro-
ken down to provide a carbon source for the liver production of glucose
via gluconeogenesis. Which of the following amino acids remains in
the muscle cell to provide a source of energy for the muscle?
A. Alanine
B. Aspartate
C. Leucine
D. Glutamate
E. Threonine
A nsw ers
[36.1]
C. In starvation a major metabolic adjustment is that the brain acti-
vates the ketone body metabolic pathway and uses ketone bodies for
energy, thus sparing somewhat the breakdown of body proteins to
generate amino acid carbon skeletons for gluconeogenesis in the
liver. In starvation liver derives most of its energy from ß-oxidation
of fatty acids. Muscle proteins are broken down to generate carbon
skeletons during starvation. Triglyceride stores in adipose tissue are
being used to provide fatty acids for ß-oxidation in the liver. Red
blood cells are not able to use ketone bodies because they have no
mitochondria.
[36.2]
B. The synthesis of ketone bodies begins with the combination of two
molecules of acetyl-CoA to generate one molecule of free CoA and a
molecule of acetoacetyl-CoA, which combines with another mole-
cule of acetyl-CoA to yield another free CoA molecule and ß-
hydroxy-ß-methylglutaryl-CoA (HMG-CoA). HMG-CoA undergoes
hydrolysis to produce one molecule acetyl-CoA and one molecule of
acetoacetate, which can be reduced to ß-hydroxybutyrate. The reac-
tion of succinyl-CoA and acetoacetate is a reaction in the pathway of
ketone body utilization but not in the pathway of ketone body
formation.
[36.3]
C. Leucine but none of the other amino acids listed is a branched-
chain amino acid. The muscle has a very active branched-chain
amino acid metabolic pathway and uses that pathway to provide
energy for its own use. The products of leucine metabolism are
acetyl-CoA and acetoacetate, which are used in the tricarboxylic acid
cycle. Acetoacetate is activated by succinyl-CoA and cleaved to two
molecules of acetyl-CoA in the ß-ketothiolase reaction. The other
branched-chain amino acids, valine, and isoleucine, yield succinyl-
CoA and acetyl-CoA as products of their catabolism.
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