An introduction to the glycolysis
Glycolysis is the lysis of glucose molecules. It is a pathway that cleaves one molecule of glucose into two molecules of pyruvate. Glycolysis occurs in the cytosol and involves a set of ten reactions. There are 10 different enzymes loosely associated with each other. These enzymes catalyze each reaction; one enzyme per reaction.
Glycolysis or anaerobic respiration doesn’t use molecular oxygen. It is the initial step of the catabolism of glucose and other hexoses. This pathway channels different types of hexoses through the various steps and finally converts them to an end product pyruvate. Glycolysis occurs in the cytosol and has two phases; preparatory phase and recovery phase.
The preparatory phase includes five reactions from step 1 to step 5. In this phase, the conversion of one glucose molecule into two molecules of glyceraldehyde-3-phosphate consumes two molecules of ATP. However, the recovery phase produces four molecules of ATP with the release of one molecule of NADH.
The recovery phase includes other reactions from step 5 to step 10. In this phase, two molecules of glyceraldehydes 3-phosphate are converted into two molecules of pyruvate. Each of the steps is described below.
Different reactions of the glycolysis
Step 1: This is the first reaction of glycolysis. In this step, hexokinase catalyzes the phosphorylation of glucose to form glucose 6-phosphate. It is an irreversible step that consumes one molecule of ATP. The enzyme is Mg++ ion-dependent. This is the committed step of the first control point of regulation.
Step 2: The second reaction is reversible one in which phosphoglucose isomerase converts the glucose 6-phosphate into fructose 6-phosphate.
Step 3: This is the third step in which phosphofructokinase phosphorylates the fructose 6-phosphate into fructose-1, 6-bisphosphate. It consumes one molecule of ATP and is another control point of regulation. The enzyme is Mg++ ion-dependent.
Step 4: In this step, aldolase cleaves the fructose 1, 6-bisphosphate into two molecules of glyceraldehydes 3-phosphate and dihydroxyacetone phosphate.
Step 5: This is an interconversion reaction in which triosephosphate isomerase catalyzes the reversible interconversion of dihydroxyacetone phosphate and glyceraldehydes 3-phosphate.
Step 6: Glyceraldehydes 3-phosphate dehydrogenase catalyzes the conversion of glyceraldehydes 3-phosphate into 1,3-bisphosphoglycerate. The given enzyme is an NAD+ dependent enzyme that produces one molecule of NADH along with the formation of each molecule of the 1, 3-bisphosphoglycerate.
This reaction is sensitive to arsenic poisoning. Since arsenate is an analog of inorganic phosphate that reacts with the glyceraldehyde 3-phosphate to form 1-arseno-3-phosphoglycerate. Hydrolysis of this compound produces 3-phosphoglycerate. Thus, arsenate causes a bypass of the formation of 1, 3-bisphosphoglycerate and thus there is no ATP formed.
Step 7: Phosphoglycerate kinase catalyzes the conversion of 1, 3-bisphosphoglycerate into 3-phosphoglycerate. This step is substrate-level phosphorylation because substrate-level phosphorylation of ADP produces a molecule of ATP. This is the Mg++ dependent enzyme.
Step 8: After then, Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate into the 2-phosphoglycerate.
Step 9: In this step, Enolase catalyzes the conversion of 2-phosphoglycerate into phosphoenolpyruvate. This is a dehydration reaction.
Step 10: Pyruvate kinase catalyzes the last reaction of glycolysis. It converts phosphoenolpyruvate into pyruvate. In this step, the substrate-level phosphorylation of ADP forms a molecule of ATP. This step is substrate-level phosphorylation as well as the third control point of regulation.