INTRODUCTION
Glycolysis-Derived from the Greek stem glyk, "sweet," and the word lysis, "dissolution."
Glycolysis is the sequence of reactions that converts glucose into pyruvate with the concomitant production of a relatively small small amount of ATP. In aerobic organisms, glycolysis is the prelude to the citric acid cycle and the electron transport chain, which together harvest most of the energy contained in glucose. Under aerobic conditions, pyruvate enters mitochondria, where it is completely oxidized to carbon dioxide and water. Under anaerobic conditions, as in actively contracting muscle, pyruvate is converted into lactate. Under anaerobic conditions, yeast transforms pyruvate into ethanol. The formation of ethanol and lactate from glucose are examples of fermentations. (5)
The 10 reactions of glycolysis occur in the cytosol. In the first stage, glucose is converted into fructose 1,6-bisphosphate by phosphorylation, an isomerization, and a second phosphorylation reaction. Two molecules of ATP are consumed per molecule of glucose in these reactions, which are the prelude to the net synthesis of ATP. In the second stage, fructose 1,6-bisphosphate is cleaved by adolase into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, which are readily interconvertible. Glyceraldehyde 3-phosphate is then oxidized and phosphorylated to form 1,3-bisphosphoglycerate, an acyl phosphate with a high phosphoryl transfer potential. 3-phosphoglycerate is then formed as one ATP is generated. In the last stage of glycolysis, phosphoenolpyruvate, a second intermediate with a high phosphoryl transfer potential, is formed by a phosphoryl shift and a dehydration. Another ATP is generated as phosphoenolpyruvate is converted into pyruvate. There is a net gain of two molecules of ATP in the formation of two molecules of pyruvate from one molecule of glucose. (5)
Overall mechanism
Glucose + 2 ADP + 2 Pi + 2 NAD+ ---> 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O (3)
The glycolytic pathway has a dual role: it degrades glucose to generate ATP, and it provides building blocks for synthetic reactions, such as the formation of long-chain fatty acids. The rate of conversion of glucose into pyruvate is regulated to meet these two major cellular needs. In metabolic pathways, enzymes catalyzing essentially irreversible reactions are potential sites of control. In glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are virtually irreversible. Hence, these glycolytic enzymes would be expected to have regulatory as well as catalytic roles. (5)
The enzyme of interest at this web site is phosphofructokinase. Phosphofructokinase is the most important control element in the glycolytic pathway of mammals. The enzyme (a 340-Kd tetramer) is inhibited by high levels of ATP, which lower its affinity for fructose 6-phosphate. A high concentration of ATP converts the hyperbolic binding curve of fructose 6-phosphate into a sigmoidal one. This allosteric effect is elicited by the binding of ATP to a specific regulatory site that is distinct from the catalytic site. The inhibitory action of ATP is reversed by AMP, and so the activity of the enzyme increases when the ATP/AMP ratio is lowered. A second control feature comes into play when the pH drops. The inhibition of phosphofructokinase by H+ prevents excessive formation of lactate and a precipitous drop in blood pH. This condition described is know as acidosis. (5)
As stated earlier, glycolysis serves a second function of furnishing carbon skeletons for biosynthesis. Therefore, phosphofructokinase should also be regulated by a signal indicating whether building blocks are abundant or scarce. Indeed, phosphofructokinase is inhibited by citrate, an early intermediate in the citric acid cycle. Citrate inhibits phosphofructokinase by enhancing the inhibitory effect of ATP. A high level of citrate means that biosynthetic precursors are abundant and so additional glucose should not be degraded for this purpose. (5)