Question

Your book has a wonderful term for the coordinated control of glycolysis and gluconeogenesis: reciprocal.

One is stimulated when the other is suppressed. But not only physiological conditions have such effect -- so do individual compounds found in cells.

A key control point in the pathways are the steps catalyzed by phosphofructokinase (in glycolysis) and fructose-1,6-bisphosphatase (in gluconeogenesis). Let's zero in on this spot.

A. Identify 3 compounds that act reciprocally on these two enzymes (inhibiting one and activating the other).

B. Now, explain why the buildup of each of these, in turn, makes sense physiologically. Why, for instance, does it make sense that AMP activates phosphofructokinase and but inhibits fructose-1,6-bisphosphatase? Answer that and then answer a similar question for the other two effector molecules that you identified in part A.

Answer 1

1) AMP stimulates phosphofructokinase, whereas ATP and citrate inhibit it. Fructose 1,6-bisphosphatase, on the other hand, is inhibited by AMP and activated by citrate.

2) A high level of AMP indicates that the energy charge is low and signals the need for ATP generation. Conversely, high levels of ATP and citrate indicate that the energy charge is high and that biosynthetic intermediates are abundant. Under these conditions, glycolysis is nearly switched off and gluconeogenesis is promoted.

PFK is inhibited by abundant cellular concentrations of citrate, another marker of a high energy state of a cell. When citrate levels are high, the cell can obtain more than enough energy from the citric acid cycle and does not need glycolysis to shovel more carbons into the citric acid cycle.

these enzymes have specific substrate binding sites for allosteric regulation of specific enzymes depending upon the molecule concentrztion in the cell.