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Metabolic
pathways producing ATP. ATP from glucose
In
skeletal muscle most of the ATP is produced in metabolic
pathways involving reactions of the sugar glucose or some
other carbohydrate derived from glucose. During contraction,
for example, glucose is made available for these reactions
by the breakdown of glycogen, the storage form of carbohydrate
in animal cells. The concentration of Ca2+ is transiently
increasedon activation of muscle. The ions are also activators
of the process of glycogen breakdown. During the recovery
period the glycogen supply is replenished by synthesizing
glycogen from glucose supplied to the muscle tissue by the
blood.
Striated
muscle. Metabolic pathways producing ATP
The glycolytic pathway
In all but the most brief, intense contractions—during which
phosphagen is utilized to form ATP—additional ATP is supplied
by a set of chemical reactions called the glycolytic pathway,
or the Embden–Meyerhof–Parnas pathway. These reactions are
especially important when the muscle is doing work at a
high rate. The enzymes that catalyze these reactions are
located in the sarcoplasm, as is glucose, the major starting
material. The glycolytic pathway can be summarized as follows:
reactions of glucose to lactate are coupled to formation
of ATP from ADP and phosphate.
C6H12O6+2ADP+2Pi=2C3H6O3+2ATP+2H2O
As already indicated, ATP is broken down in the contractile
reaction to form ADP and Pi , which can then be recycled
into the pathway. The lactate, a waste product, diffuses
out of the muscle and is transported by the blood to the
liver. The removal of lactate from the muscle is slower
than its production; if the muscle contracts for an extended
period, the accumulation of lactatein the muscle causes
fatigue and ultimately muscle cramps.
Most
of the lactate taken to the liver is converted to glucose,
from which glycogen is formed in aseries of reactions that
require ATP to provide more ATP. About one-sixth of the
lactate is oxidized to carbon dioxide by reactions that
require oxygen (see below) after the exercise is done; a
so-called oxygen debt accumulates during brief intense exercise.
Animals breathe hard after intense exercise to supply the
oxygen needed for the reconversion of lactate.
The
glycolytic pathway can operate up to 1,000 times faster
in a contracting muscle than it does in a resting muscle.
Two regulatory mechanisms play roles in the increase in
the rates of these reactions. First, reacting compounds
(reactants) accumulate; for example, more glucose is available
because glycogen breakdown increases during contraction,
and ADP and Pi accumulate. The second type of regulation
depends on the activity of the enzyme phosphofructokinase,
which catalyzes one of the early steps in glycolysis. This
enzyme can bind either ATP or adenosine monophosphate (AMP).
When ATP is bound, the activity of the enzyme is low; that
is, the reaction it catalyzes proceeds slowly. When AMP
is bound, the enzyme has a high activity. During contraction
the ATP level drops and that of ADP increases; ADP, in turn,
produces AMP as well as ATP, in a reaction catalyzed by
the enzyme adenylate kinase, or myokinase. The AMP then
can bind with the phosphofructokinase, thereby accelerating
glycolysis and the production of ATP. The formation of ATP
in the myokinase reaction enables the muscle to use the
energy in the ADP formed during contraction.
