Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
and glycogen, or to the heart, where it is metabolized to CO 2 and H 2O. The citric
acid cycle is active under aerobic conditions only. When one molecule of glucose
is split off glycogen and transformed through reactions under aerobic conditions,
the net energy yield is 30 molecules of ATP, whereas under anaerobic conditions
only 2 molecules of ATP are produced.
In the living muscle, ATP is the ultimate source of energy for the contractile
process, the pumping of Ca 2+ back into the sarcoplasmic reticulum during relaxation,
and the maintenance of Na + /K + gradients across the sarcolemma. Postmortem, as
the oxygen supply is depleted, aerobic metabolism through citric acid cycle begins
to fail. Less energy in the form of ATP is produced through anaerobic pathways.
ATP is required for muscle to be maintained in the relaxed state. When ATP becomes
limited, the muscle enters the onset phase of rigor mortis, loses its ability to relax,
and transforms to meat.
16.2 GLYCOGEN
Molecular weights of glycogen (see Chapter 1) isolated from various tissues range
from 103 to 10003 kDa. In the cytoplasm, glycogen forms 10- to 40-nm granules
containing proteins and enzymes catalyzing synthesis and degradation of glycogen.
In the period of animal growth and development, the number and the length of
branches of glycogen are almost constant.
Glycogen isolated from various animal species, and even various breeds within
a given species, shows significant differences. The structure of glycogen is a reflection
of the steric specificity of the branching enzyme and may be affected by the
physiological state of the animal. 2 For instance, a long-term tissue anoxia or stress
influences the length of branches (usually at every tenth D-glucose unit) and transglycosidation
in glycogen.
Peripheral chains of glycogen are longer than the chains in the interior of a
particle. 3 Within the central part of the molecule, branching can occur at every fourth
to fifth glucose unit, so the density of the interior of the molecule might be higher.
The terminal glucose units are the nonreducing ends, which are abstracted in the
process of phosphorylase-catalyzed phosphorylytic splitting of glycogen.
Even though glycogen is present in most body cells, it is most abundant in
mammalian liver and comprises 2 to 8% of its wet weight. The normal glycogen
content of mammalian muscle ranges from 0.5 to 1.0%, but because of the large
mass of muscle, most of the glycogen is present in this tissue. Diet has a marked
affect on glycogen stored in the liver, the muscle, and other tissues. Although the
glycogen content of mammalian liver and muscle is relatively stable in the normal
animal with free access to food, it is in constant flux, being degraded and synthesized
continuously. Liver glycogen stores are much more sensitive to inanition than those
of the muscle. Introducing fodder rich in glycogenic substance results in rapid return
of tissue glycogen to normal levels. The presence of glycogen in all tissue is lowered
by conditions, which increase energy demands such as exercise and exposure to
stress. The extent of glycogen depletion is correlated to the degree and duration of
such energy demands, and repletion is dependent on the length of recovery period.
A long-term deficiency in glycogen caused by exercise, stress, metabolic acidosis,