CHAPTER
15
Carbohydrate Metabolism II:
Gluconeogenesis, Glycogen
Synthesis and Breakdown, and
Alternative Pathways
In this chapter, carbohydrate metabolism is discussed
in terms of nondietary sources of glucose and nongly-
colytic pathways. Gluconeogenesis and glycogen syn-
thesis and breakdown make up the first category, while
the pentose phosphate pathway
(also called hexose
monophosphate shunt) and the glucuronic acid path-
way make up the second. Metabolic pathways of fruc-
tose, galactose, and some other sugars belong in both
categories.
15.1
Gluconeogenesis
Metabolic Role
Gluconeogenesis
(literally, “formation of new sugar”) is
the metabolic process by which glucose is formed from
noncarbohydrate sources, such as lactate, amino acids,
and glycerol. Gluconeogenesis provides glucose when di-
etary intake is insufficient to supply the requirements of
the brain and nervous system, erythrocytes, renal medulla,
testes, and embryonic tissues, all of which use glucose as a
major source of fuel. Gluconeogenesis has three additional
functions.
1.
Control o f acid-base balance.
Production of lactate
in excess of its clearance causes metabolic acidosis,
and resynthesis of glucose from lactate is a major
route of lactate disposal. Since glycolysis is almost
totally anaerobic in erythrocytes, renal medulla, and
some other tissues, even under normal conditions
lactate is continually released. Other tissues,
particularly muscle during vigorous exercise, can
produce large amounts of lactate, which must be
removed or lactic acidosis will result (Chapter 21).
The continuous conversion of lactate to glucose in the
liver and of glucose to lactate by anaerobic glycolysis,
particularly in muscle, forms a cyclical flow of carbon
called the
Cori cycle
(Chapter 22). Deamination of
amino acids prior to gluconeogenesis in the kidney
also provides a supply of NH
3
to neutralize acids
excreted in the urine (Chapter 39).
2.
Maintenance o f amino acid balance.
Metabolic
pathways for the degradation of most amino acids and
for the synthesis of nonessential amino acids involve
some steps of the gluconeogenic pathway. Imbalances
of most amino acids, whether due to diet or to an
altered metabolic state, are usually corrected in the
liver by degradation of the excess amino acids or by
synthesis of the deficient amino acids through
gluconeogenic intermediates.
3.
Provision o f biosynthetic precursors.
In the absence
of adequate dietary carbohydrate intake,
gluconeogenesis supplies precursors for the synthesis
of glycoproteins, glycolipids, and structural
carbohydrates.
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