Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis: The Central Pathway of Glucose Catabolism
Phosphorylated intermediates are formed during glycolysis

When examining The sequence of glycolytic reactions, we must first emphasize the crucial fact that all nine intermediates on the pathway from glucose to Pyruvate are phosphorylated compounds (Fig. 15-2). Phosphate groups apparently serve three Functions.

1. At pH 7, phosphate groups are fully ionized, thereby imparting a net negative charge to each glycolytic intermediate. Because cellular membranes are generally impermeable to electrically charged molecules, glycolytic intermediates cannot leak out of The Cell. Glucose enters Cells, and lactate or pyruvate leave them, solely because the cell membranes contain specialized transport systems capable of translocating these specific molecules.

2. The second function of phosphate groups is self-evident: they are essential components in the enzymatic conservation of metabolic energy, as they are ultimately transferred to ADP to yield ATP.

3. Phosphate groups act as recognition tags; they are binding groups that enable the glycolytic intermediates to properly position themselves within the active sites of their respective Enzymes.

Nearly all glycolytic enzymes require Mg²⁺ ions for catalytic activity. It is well established that Mg²⁺ ions form complexes with the phosphate groups of glycolytic intermediates, as well as with those of ADP and ATP (Section 14.8). This strongly suggests that the substrate-binding sites of many glycolytic enzymes are specific not so much for the phosphorylated intermediates themselves, but rather for their complexes with Mg²⁺ ions.