Glycolosis is the sequence of reactions which converts a glucose molecule into two pyruvate molecules with the production of NADH and ATP. Specific enzymes control each of the different reactions. There is a net gain of 2 ATP at the end of glycolosis. All the reactions of glycolosis take place entirely in the cytosol due to the abundance of free-floating ingredients such as ADP, NAD+, and inorganic phosphates. Glycolosis itself does not require oxygen and can proceed aerobically or anaerobically. It is divided into two phases : Phase I and Phase II.
The different enzymes involved in glycolysis act as kinases, mutases, and dehydrogenases, cleaving enzymes, isomerases or enolases. They act in concert to split or rearrange the intermediates, to add on phosphate groups, and to move those phosphate groups onto ADP to make ATP. Several of the reactions involve the phosphorylation of intermediates, which is important not only for the production of ATP from ADP, but also as a useful handle on the substrate for enzyme binding, to trap intermediates within the cell, and to drive pathways in one direction by making phosphorylation and dephosphorylation reactions irreversible. The different enzymes have been split into two groups, those in phase I and those in phase II, simply for convenience.
Enzymes in Phase I
Hexokinase, Phosphoglucose isomerase, Phosphofructokinase, Fructose-bisphosphate aldolase, Triosephosphate isomerase. (5 enzymes)
Enzymes in Phase II
The second phase of glycolysis involves the extraction of energy in the form of 4 ATP per molecule of glucose, a net gain of 2 ATP molecules. The product of glycolysis, pyruvate, can then be further broken either aerobically ( to carbon dioxide and water through the TCA cycle) or anaerobically (to lactate or alcohol
Glyceraldehyde 3-phosphate dehydrogenase, Phosphoglycerate kinase,
Phosphoglycerate mutase, Enolase, Pyruvate kinase. (5 enzymes)
So a minimum of 10 enzymes are required for glycolysis to take place.
The detailed mechanism is as follows.
Catalyses: a-D-Glucose + ATP à Glucose-6-phosphate (G6P) + ADP
The first step in glycolysis is a priming reaction, where a phosphate group is added to glucose using ATP. This reaction is important for its ability to trap glucose within the cell. Whereas glucose can easily traverse the plasma membrane, the negatively charged phosphate group prevents G6P from crossing, so cells can stock up on glucose while levels are high. However, the hexokinase reaction is highly regulated, with G6P providing a feedback inhibition of the enzyme, thereby preventing excessive stockpiling until glycolysis depletes G6P levels.
In mammals, there are four isozymes of hexokinase: types I, II, III and IV (glucokinase). These isozymes differ in their catalysis, localisation and regulation, thereby contributing to the different patterns of glucose metabolism in different tissues. Type I, II and III hexokinases can phosphorylate a variety of hexose sugars, including glucose, fructose and mannose, and as such are involved in a number of metabolic pathways. It is thought that type I hexokinase may have a catabolic function, producing G6P for energy production in glycolysis, whereas types II and III may have an anabolic function, providing G6P for glycogen or lipid synthesis. Type I hexokinase binds to the mitochondrial membrane, thereby enabling the coordination of the rate of glycolysis with that of the TCA cycle. Type IV hexokinase (glucokinase) is a liver/pancreatic b-cell enzyme that is specific for a-D-glucose, and whose level is controlled by insulin, not G6P. Due to the lack of inhibition by G6P, during times of high blood glucose levels the liver can stockpile G6P, converting it to glycogen ...