Regulation of rat 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: Role of the NH₂-terminal region

The role of the NH₂-terminal region of the liver and skeletal muscle 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatases was investigated, as well that of a mutant of the liver isoform lacking the first 22 amino acids, by the overexpression of these enzymes in Escherichia coli and the comparison of their kinetic properties. The muscle isoform and the deletion mutant had K_m values for fructose 6-phosphate which were 50-and 20-fold higher, respectively, than that of the liver isoform, and the bisphosphatase maximal velocity of the liver deletion mutant was 4-fold higher than that of the native liver isoform. Phosphorylation of the liver isoform increased bisphosphatase activity by 2-3-fold and the K_m for fructose 6-phosphate of the 6-phosphofructo-2-kinase by 10-15-fold, but these kinetic effects were greatly diminished for the deletion mutant despite equivalent phosphorylation by cAMP-dependent protein kinase. Arg-173 of the skeletal muscle isoform was found to be functionally equivalent to the residue corresponding to the essential fructose 6- phosphate binding residue of the liver kinase domain, Arg-195. The results suggest that 1) the NH₂-terminal regions of the liver and skeletal muscle isoforms are important determinants of fructose 6-phosphate affinity, and 2) the initial 22 amino acids of the liver isoform exert an inhibitory influence on the bisphosphatase and mediate, at least in part, the response of both activities of the enzyme to cAMP-dependent phosphorylation.