Evidence for two catalytic sites on 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase. Dynamics of substrate exchange and phosphoryl enzyme formation.

The bifunctional enzyme 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase appears to be the only enzyme catalyzing the formation and hydrolysis of Fru-2,6-P2. The enzyme as we isolate it, contains a trace of tightly bound Fru-6-P. In this condition, it exhibited an ATPase activity comparable to its kinase activity. Inorganic phosphate stimulated all of its activities, by increasing the affinity for all substrates and increasing the Vmax of ATP and Fru-2,6-P2 hydrolysis. The enzyme catalyzed ADP/ATP and Fru-6-P/Fru-2,6-P2 exchanges at rates comparable to net reaction rates. It was phosphorylated by both [gamma-32P]ATP and [2-32P] Fru-2,6-P2, and the label from either donor was chased by either unlabeled donor, showing that the bound phosphate is hydrolyzed if not transferred to an acceptor ligand. The rate of labeling of the enzyme by [2-32P]Fru-2,6-P2 was 2 orders of magnitude greater than the maximal velocity of the bisphosphatase and therefore sufficiently fast to be a step in the hydrolysis. Both inorganic phosphate and Fru-6-P increased the rate and steady state of enzyme phosphorylation by ATP. Fru-2,6-P2 inhibited the ATPase and kinase reactions and Fru-6-P inhibited the Fru-2,6 bisphosphatase reaction while ATP and ADP had no effect. Removal of the trace of Fru-6-P by Glu-6-P isomerase and Glu-6-P dehydrogenase reduced enzyme phosphorylation by ATP to very low levels, greatly inhibited the ATPase, and rendered it insensitive to Pi, but did not affect ADP/ATP exchange. (alpha + beta)Methylfructofuranoside-6-P did not increase the rate or steady state labeling by ATP. These results suggest that labeling of the enzyme by ATP involved the production of [2-32P]Fru-2,6-P2 from the trace Fru-6-P. The 6-phosphofructo-2-kinase, fructose 2,6-bisphosphatase, and ATP/ADP exchange were all inhibited by diethylpyrocarbonate, suggesting the involvement of histidine residues in all three reactions. These results can be most readily explained in terms of two catalytic sites, a kinase site whose phosphorylation by ATP is negligible (or whose E-P is labile) and a Fru-2,6 bisphosphatase site which is readily phosphorylated by Fru-2,6-P2.