Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
of ATP and high level of AMP and calcium ions. Thus, an acceleration of glycolysis
takes place on increased energy demand of cells and as an effect of nervous stimuli.
In the next step of glycolysis, F-1,6-BP is catalytically cleaved by aldolase into
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Up to this point, glycolysis
does not produce energy; on the contrary, it consumes two ATP molecules.
The energy is liberated in subsequent steps. Glyceraldehyde-3-phosphate directly
enters further stages of glycolysis, and isomeric dihydroxyacetone phosphate can
reversibly transform into glyceraldehyde-3-phosphate. In vivo, one F-1,6-BP molecule
produces two molecules of glyceraldehyde-3-phosphate.
The redox reaction of glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate
produces acetylophosphate, the mixed anhydride of phosphoric and acetic acids. The
anhydride has a high potential of phosphate group transfer. The subsequent transfer
of the phosphate group from acetylophosphate (1,3-bisphosphoglycerate) to ADP
produces 3-phosphoglycerate and ATP.
In the last phase of glycolysis, 3-phosphoglycerate is intramolecularly rearranged
into pyruvate and the second ATP molecule. ATP allosterically inhibits glycolysis
at low energy demand of cells.
Glucokinase, which is located in liver, phosphorylates excessive glucose.
Glucokinase produces G-6-P for synthesis of glycogen. Because the K m of glucokinase
is high, the production of glucose on its low level in blood is put into
priority. The sequence of reactions leading from G-6-P to pyruvate is generally
common for all organisms and types of cells but the fate of produced pyruvate is
different.
In animal tissues, pyruvate is transformed into lactate under anaerobic conditions.
This reduction with NADH from the oxidation of glyceraldehyde-3-phosphate
is catalyzed by lactate dehydrogenase. Recovery of NAD + in this process provides
continuous glycolysis. A portion of energy liberated on glycolysis under anaerobic
conditions, although not high, is responsible for a postmortem increase in temperature
of muscles.
Under aerobic conditions, pyruvate undergoes irreversible oxidative decarboxylation
to acetyl-CoA:
Pyruvate + NAD + + CoA → acetyl-CoA + CO 2 + NADH (16.7)
Postmortem glycogen degradation in muscles proceeds at a high rate until the
resistance and capacitance of cell and plasma membranes (sarcolemma, sarcoplasmic
reticulum, mitochondria, and lysosomes) are eliminated. 4 When pH from resting
antemortem muscles decreases from approximately 7.4 to below 6.5, there is a free
diffusion of ions through formerly semipermeable plasma membranes. In this state,
the muscle loses its ability to contract, and there is a free diffusion of ions; this
results in the equalization of pH throughout the tissue. From this point onward,
glycolysis continues at a reduced rate until either glycogen deposits are completely
depleted or the pH is low enough to completely inhibit the glycolytic enzymes. This
occurs at a pH slightly below 5.4. Even though at this level of pH ample glycogen
might still be present, glycogen breakdown ceases.