Carbohydrate catabolism provides ATP as well as precursors for a variety of biosynthetic processess. It is crucial to a cell to maintain a sufficient concentration of ATP, at a nearly constant level, regardless of which fuel is used to produce ATP and regardless of the rate at which ATP is consumed. An organism that undergoes a change in circumstances, such as increased muscular activity, decreased availability of oxygen, or decreased dietary intake of carbohydrate, must alter its catabolic patterns to change the flow of carbohydrate fuel, whether from stored reserves or from extracellular sources, through glycolysis. These changes in catabolic patterns are accomplished by the regulation of key enzymes in the catabolic pathways. In glycolysis in muscle and liver tissue, four enzymes play a regulatory role: glycogen phosphorylase, hexokinase, phosphofructokinase-l, and pyruvate kinase. Our discussion of the regulation of glycolysis necessarily involves some details of the reciprocally regulated process of glucose synthesis (gluconeogenesis), which is more fully discussed in
Before describing the regulation of glucose catabolism, we will consider some general principles that apply to the regulation of all biochemical pathways.
Regulatory Enzymes Act as Metabolic Valves
Although not at equilibrium with their surroundings, adult organisms generally exist in a steady state. A constant influx of fuel and nutrients and a constant release of energy and waste products allow the organism to maintain a constant composition. When the steady state is disturbed by some change in external circumstances or fuel supply, the temporarily altered fluxes through individual metabolic pathways trigger regulatory mechanisms intrinsic to each pathway. The net ef fect of all of these adjustments is to return the organism to the steady state-to achieve homeostasis. Because of the central role of ATP in cellular activities, evolution has produced catabolic enzymes with regulatory properties that ensure a high steady-state concentration of ATP, "high" in this context meaning high relative to the breakdown products ADP and AMP.The flux through a biochemical pathway depends on the activities of the enzymes that catalyze each reaction. For some of the enzymes in a pathway such as glycolysis, the reaction is essentially at equilibrium within the cell; the activity of such an enzyme is sufficiently high that the substrate is converted to product as fast as the substrate is supplied. The flux through this step is essentially substrate-limiteddetermined by the instantaneous concentration of the substrate.
Other cellular reactions are far from equilibrium. In the glycolytic pathway, the equilibrium constant (K'eq) for the reaction catalyzed by phosphofructokinase-1 is about 250, but the mass action ratio [fructose-1,6-bisphosphate][ADP]/[fructose-6-phosphate][ATP] in a typical cell in the steady state is about 0.04. (The intracellular concentrations of some glycolytic enzymes and reactants
Regulation of Glucose Metabolism Is Different in Muscle and Liver
The regulatory enzymes that control the rate of breakdown of carbohydrates via glycolysis illustrate these general principles of metabolic regulation. Glucose catabolism is doubtless regulated in all organisms, but the regulatory mechanisms have been studied especially well in vertebrate muscle and liver.
In muscle the end served by glycolysis is ATP production, and the rate of glycolysis increases as muscle contracts more vigorously or more frequently, demanding more ATP. The liver has a different role in whole-body metabolism, and glucose metabolism in the liver is correspondingly different. The liver serves to keep a constant level of glucose in the blood, producing and exporting glucose when the tissues demand it, and importing and storing glucose when it is provided in excess in the diet.
Glycogen Phosphorylase of Muscle Is Regulated Allosterically and Hormonally
In skeletal muscle cells (myocytes), the mobilization of stored glycogen to provide fuel for glycolysis is brought about by glycogen phosphorylase, which degrades glycogen to glucose-1-phosphate (Fig. 14-11). The case of glycogen phosphorylase is an especially instructive example of enzyme regulation. It was the first enzyme shown to be allosterically regulated and the first shown to be controlled by reversible phosphorylation. It is also one of only a few allosteric enzymes for which the detailed three-dimensional structures of the active and inactive forms are known from x-ray crystallographic studies.
In skeletal muscle, glycogen phosphorylase occurs in two forms: a catalytically active form, phosphorylase a, and a usually inactive form, phosphorylase b (Fig. 14-17); the latter predominates in resting muscle. The rate of glycogen breakdown in muscle depends in part on the ratio of phosphorylase a (active) to phosphorylase b (less active), which is adjusted by the action of hormones such as epinephrine. Phosphorylase a consists of two identical subunits (Mr 94,500), in each of which the Ser residue at position 14 is phosphorylated. Phosphorylase b is structurally identical except that the Ser14 residues are not phosphorylated. Phosphorylase a is converted into the less active phosphorylase b by dephosphorylation, catalyzed by phosphorylase a phosphatase (Fig. 14-17). Phosphorylase b is converted back into phosphorylase a by the enzyme phosphorylase b kinase, which catalyzes phosphate transfer from ATP to Ser14.
Hormones ultimately regulate the interconversion of phosphorylase
a and b by regulating the activities of phosphorylase a phosphatase and
phosphorylase b kinase. Epinephrine is released into the blood
by the adrenal gland when an animal is suddenly confronted by a situation that
requires vigorous muscular activity. Epinephrine is a signal to skeletal muscle
to turn on the processes that lead to production of ATP, which will be needed
for muscle contraction. Glycogen phosphorylase is activated to provide
glucose-1-phosphate to be fed into the glycolytic pathway. By the cascade of
events shown in Figure 14-18, the binding of epinephrine to its specific
receptor in the plasma membrane of a muscle cell activates phosphorylase b
kinase and inactivates phosphorylase a phosphatase, tipping the balance toward
formation of the active (a) form of glycogen phosphorylase. The cascade of
activations allows one molecule of hormone to cause activation of many molecules
of target enzyme (glycogen phosphorylase).
When the emergency is over, release of epinephrine ceases, the
phosphorylase b kinase reverts to its original, lower activity, and the ratio of
phosphorylase a to phosphorylase b returns to that in resting muscle.
Superimposed on the hormonal control is faster, allosteric
regulation of glycogen phosphorylase b by ATP and AMP. Phosphorylase b, the
relatively inactive form, is activated by its allosteric effector AMP (Fig.
14-17), which increases in concentration in muscle during the ATP breakdown
accompanying contraction. The stimulation of phosphorylase b by AMP can be
prevented by high concentrations of ATP, which blocks the AMP binding site. The
activity of phosphorylase b thus reflects the ratio of AMP to ATP. Phosphorylase
a, which is not stimulated by AMP, is sometimes referred to as the
AMP-independent form, and phosphorylase b as the AMP-dependent form.
In resting muscle nearly all the phosphorylase is in the b form,
which is inactive because ATP is present at a much higher concentration than
AMP. Vigorous muscular activity increases the AMP : ATP ratio, very rapidly
activating (in milliseconds) phosphorylase b by allosteric means. On a longer
time scale (seconds to minutes) hormonetriggered phosphorylation of
phosphorylase b converts it into phosphorylase a, the activity of which is
independent of the AMP : ATP ratio.
There is yet a third type of control on glycogen phosphorylase in
skeletal muscle. Calcium, the intracellular signal for muscle contraction, is
also an allosteric activator of phosphorylase b kinase. When a transient rise in
intracellular Ca2+ triggers muscle contraction, it
also accelerates conversion of phosphorylase b to the more active phosphorylase
