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CASE FILES: BIOCHEMISTRY
Figure 26-1. Schematic diagram showing the events that lead to the export of
glucose from the liver cell during times of low blood glucose.
anticipate subsequent periods of decreased glucose availability (e.g., overnight
fast). During the latter period of time, glycogen will be mobilized as a readily
available source of glucose. In the case of muscle, glycogen is used selfishly,
as an energy source by the myocyte only. In contrast, liver glycogen will be
mobilized to help maintain blood glucose levels. The biochemical pathways of
glycogenesis and glycogenolysis are illustrated in Figure 26-2.
As glycogenesis is the reciprocal pathway to glycogenolysis, gluconeogen-
esis is the reciprocal pathway to glycolysis. Glycolysis, the “lysis” of glucose
to two pyruvate molecules, is a ubiquitous metabolic pathway, whereas gluco-
neogenesis occurs in only a select number of tissues, including the liver. During
periods of increased glucose availability, flux through the glycolytic pathway
increases, thereby utilizing this readily available fuel source. In contrast, during
periods of decreased glucose availability, rates of gluconeogenesis increase, in
an attempt to maintain blood glucose levels. The sources of carbon for gluco-
neogenesis depend on the given metabolic situation (e.g., exercise, starvation,
diabetes mellitus), and include glycerol, amino acids, and lactate. Although the
majority of fatty acid-derived carbon cannot be used for the net synthesis of
glucose, fatty acid metabolism plays a central role in gluconeogenesis.
Gluconeogenesis is an energetically demanding process, which is driven by the
P-oxidation of fatty acids. When the rate of acetyl-CoA generation through
fatty acid P-oxidation exceeds the rate of acetyl-CoA oxidation via the Krebs
cycle and oxidative phosphorylation (ox phos), acetyl-CoA accumulates. This
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