Glycolysis Essay

The word glycolysis comes from the two Greek words glykys, meaning sweet, and lysis, meaning splitting [8]. In this process a molecule of glucose is degraded into two molecules of pyruvate. Glycolysis consists of ten enzyme catalysed reactions. Cells can only utilise phosphorylated glucose. The first reaction in glycolysis is therefore a phosphorylation of glucose to glucose 6-phosphate. In E. coli this is generally performed by the glucose specific phosphotransferase system (PTS) [9].

This system consists of several enzymes that use phosphoenolpyruvate (PEP) produced during glycolysis as the phosphate source. The products of the reaction are glucose 6-phosphate and pyruvate (PYR). The phosphorylated glucose is converted further to fructose 6-phosphate by phosphoglucose isomerase [8]. In the third reaction fructose 6-phosphate undergoes an additional phosphorylation to fructose 1,6-diphosphate by phosphofructokinase-1. A molecule of ATP acts as the phosphate source. The next step is the cleavage of the six carbon molecule to two three carbon molecules. It is from this reaction that the pathway obtained its name. The enzyme aldolase performs the splitting, which yield one molecule of glyceraldehyde 3-phosphate and one molecule of dihydroxyacetone phosphate. These two substances are in equilibrium. As glyceraldehyde 3-phosphate is utilised in glycolysis, dihydroxyacetone phosphate converts to glyceraldehyde 3-phosphate by triose phosphate isomerase. Glyceraldehyde 3-phosphate is oxidised with inorganic phosphate to form 1,3-biphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase. In this oxidation step one molecule of NADH is reduced to one molecule of NAD+. During the seventh reaction 1,3-biphos...

... middle of paper ...

...osphate acetyltransferase and acetate kinase are the two enzymes used in the second reaction pathway to produce acetate via acetylphosphate. From these reactions one molecule of ATP is gained. The third route for pyruvate degradation is directly to acetate by pyruvate oxidase. Phosphoenolpyruvate, which also is a product of glycolysis, can too enter the mixed acid fermentation. It can form pyruvate and a molecule of ATP or form oxaloacetate in the presence of carbon dioxide by phosphoenolpyruvate carboxylase. Oxaloacetate is then hydrogenated by malate dehydrogenase and NADH to malate and NAD+. The enzyme fumarase turns malate into fumarate and water. These last two enzymes are the same as in the citric acid cycle. Finally fumarate is converted to succinate by fumarate reductase. Figure 4 presents a summary of the reactions during mixed acid fermentation.