Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis: The Central Pathway of Glucose Catabolism
Glycolysis consists of two stages

Before examining the individual enzymatic steps of Glycolysis, let us characterize the process in a more general way. The breakdown of a six-carbon glucose molecule into two three-carbon Pyruvate molecules involves ten Enzymes. All of them have been isolated in pure form from various organisms and thoroughly studied. The first five steps constitute the preparatory phase of glycolysis (Fig. 15-2). In these enzymatic reactions, glucose is phosphorylated at the expense of ATP, first at carbon 6 and then at carbon 1, yielding fructose-1,6-diphosphate, which is subsequently cleaved into two three-carbon molecules of glyceraldehyde-3-phosphate. Thus, the product of The First stage of glycolysis is glyceraldehyde-3-phosphate. Note that two ATP molecules must be invested to activate the glucose molecule and prepare it for Cleavage into two three-carbon fragments; this initial “investment” will later yield a very substantial return. Furthermore, as we will show, other hexoses—specifically D-fructose, D-galactose, and D-mannose—can also enter the preparatory phase of glycolysis after undergoing phosphorylation. The preparatory stage of glycolysis thus serves to convert the carbon skeletons of all metabolized hexoses into a single common product, glyceraldehyde-3-phosphate.

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Fig. 15-2. The two stages of anaerobic glycolysis. The numbers in parentheses indicate the number of molecules entering a given reaction.

The Second Stage of glycolysis, which also consists of five enzymatic reactions, can be metaphorically described as the collection of dividends: at this stage, The energy released during The conversion of two molecules of glyceraldehyde-3-phosphate into two molecules of pyruvate is captured (via the coupled phosphorylation of four ADP molecules) in the form of four ATP molecules (Fig. 15-2). The net yield of ATP in glycolysis, however, is not four but only two ATP molecules per cleaved glucose molecule, because two ATP molecules were already consumed in the first stage of glycolysis.

Glycolysis involves three distinct types of chemical transformations: (1) the degradation of the carbon Skeleton of glucose to form pyruvate (The pathway of carbon atoms); (2) the phosphorylation of ADP by high-energy phosphorylated compounds to yield ATP (the pathway of phosphoryl groups); and (3) The transfer of hydrogen atoms or electrons (the electron-transfer pathway). We will trace all these pathways as we proceed to discuss the individual reactions of glycolysis.

One final point is worth noting. In the vast majority of Cells, the enzymes catalyzing glycolytic reactions are present in soluble form within the Cytosol, i.e., the homogeneous aqueous phase of the Cytoplasm (Sec. 2.18). In contrast, the enzymes catalyzing the stages of carbohydrate oxidation that require oxygen are membrane-bound: in Eukaryotic cells, they are localized in the mitochondrial membranes, whereas in Prokaryotic Cells, they reside in Cell/30.html">The Plasma Membrane.