9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
413
2 moles of ATP are used for the formation of HDP (Reactions 1 and 3)
while 4 moles of ADP are converted to ATP (Reactions 7 and 10), resulting in a net gain of 2 moles of ATP. For the glycolysis of glycogen
only 1 mole of ATP is required with a net gain of 3 moles of ATP.
Thus, energy-rich phosphate bonds are generated during glycolysis.
These energy relationships and balances have been discussed by Krebs
and Kornberg (i). The energy yield from glycolysis is low when compared with that produced by the oxidation of glucose. Glycolysis results
not only in a net gain of ATP, but also in a corresponding loss of inorganic phosphate. The effects of these changes on the regulation and
on the rate of glycolysis in various tissues and cells will be considered
below (Sections II, B and II, C).
In the glycolysis of glycogen to lactic acid every reaction is reversible
with the exception of the conversion of fructose-6-phosphate to fructose1,6-diphosphate (Reaction 3). The occurrence in certain tissues and
cells of a specific fructose-l,6-diphosphatase (2-4), catalyzing the dephosphorylation of fructose-l,6-diphosphate to fructose-6-phosphate and
inorganic phosphate, affords an opportunity for the synthesis of glycogen
from lactate, or from other intermediates of glycolysis, through a reversal of the reactions of the Embden-Meyerhof scheme. Another irreversible reaction of glycolysis is the phosphorylation of glucose, catalyzed by hexokinase. However, glucose can be formed from glucose-6phosphate, produced either from glycogen or by a reversal of the
glycolytic reactions, through the action of a specific glucose-6-phosphatase (5, 6).
A variety of factors may affect glycolysis of a particular tissue or cell.
If any one of them should become limiting, this would determine the
rate at which glucose is converted to lactic acid. Some of these factors
will be discussed below.
II. Regulatory Factors
A. GLUCOSE TRANSPORT INTO THE CELL
An important source of intracellular glucose is provided by the
transport of this hexose. The passage of glucose into the cell from its
surrounding medium proceeds at a faster rate than can be accounted
for by simple diffusion; thus, a mechanism for the active transport of
glucose must exist. Although the process is rapid, the intracellular concentration in many tissues and cells is extremely low. In these cells the
capacity of the glycolytic enzymes to utilize glucose is higher than the
rate at which this hexose is entering the cell. This phenomenon is probably the basis for the observation that the rate of glycolysis in cell-free
413
2 moles of ATP are used for the formation of HDP (Reactions 1 and 3)
while 4 moles of ADP are converted to ATP (Reactions 7 and 10), resulting in a net gain of 2 moles of ATP. For the glycolysis of glycogen
only 1 mole of ATP is required with a net gain of 3 moles of ATP.
Thus, energy-rich phosphate bonds are generated during glycolysis.
These energy relationships and balances have been discussed by Krebs
and Kornberg (i). The energy yield from glycolysis is low when compared with that produced by the oxidation of glucose. Glycolysis results
not only in a net gain of ATP, but also in a corresponding loss of inorganic phosphate. The effects of these changes on the regulation and
on the rate of glycolysis in various tissues and cells will be considered
below (Sections II, B and II, C).
In the glycolysis of glycogen to lactic acid every reaction is reversible
with the exception of the conversion of fructose-6-phosphate to fructose1,6-diphosphate (Reaction 3). The occurrence in certain tissues and
cells of a specific fructose-l,6-diphosphatase (2-4), catalyzing the dephosphorylation of fructose-l,6-diphosphate to fructose-6-phosphate and
inorganic phosphate, affords an opportunity for the synthesis of glycogen
from lactate, or from other intermediates of glycolysis, through a reversal of the reactions of the Embden-Meyerhof scheme. Another irreversible reaction of glycolysis is the phosphorylation of glucose, catalyzed by hexokinase. However, glucose can be formed from glucose-6phosphate, produced either from glycogen or by a reversal of the
glycolytic reactions, through the action of a specific glucose-6-phosphatase (5, 6).
A variety of factors may affect glycolysis of a particular tissue or cell.
If any one of them should become limiting, this would determine the
rate at which glucose is converted to lactic acid. Some of these factors
will be discussed below.
II. Regulatory Factors
A. GLUCOSE TRANSPORT INTO THE CELL
An important source of intracellular glucose is provided by the
transport of this hexose. The passage of glucose into the cell from its
surrounding medium proceeds at a faster rate than can be accounted
for by simple diffusion; thus, a mechanism for the active transport of
glucose must exist. Although the process is rapid, the intracellular concentration in many tissues and cells is extremely low. In these cells the
capacity of the glycolytic enzymes to utilize glucose is higher than the
rate at which this hexose is entering the cell. This phenomenon is probably the basis for the observation that the rate of glycolysis in cell-free