Activation of mammalian phosphofructokinases by ribose 1,5-bisphosphate

Ribose 1,5-bisphosphate (Rib-1,5-P₂), a newly discovered activator of rat brain phosphofructokinase, forms rapidly during the initiation of glycolytic flux and disappears within 20 s (Ogushi, S., Lawson, J.W.R., Dobson, G.P., Veech, R.L., and Uyeda, K. (1990) J. Biol. Chem. 265, 10943-10949). Activation of various mammalian phosphofructokinases and plant pyrophosphate-dependent phosphofructokinases by Rib-1,5-P₂ was investigated. The order of decreasing potency for activation of rabbit muscle phosphofructokinase was: fructose (Fru) 2,6-P₂, Rib-1,5-P₂, Fru-1,6-P₂, Glc-1,6-P₂, phosphoribosylpyrophosphate, ribulose-1,5-P₂, sedoheptulose-1,7-P₂, and myoinositol 1,4-P₂. The K_0.5 values for activation by Rib-1,5-P₂ of rat brain, rat liver, and rabbit muscle phosphofructokinases and potato and mung bean pyrophosphate-dependent phosphofructokinases were 64 nM, 230 nM, 82 nM, 710 nM, and 80 μM, respectively. The corresponding K_0.5 values for Fru-2,6-P₂ were 9, 8.6, 10, 7, and 65 nM, respectively. Rib-1,5-P₂ was a competitive inhibitor of Fru-2,6-P₂, binding to the muscle enzyme with K(i) of 26 μM. Citrate increased the K_0.5 for Rib-1,5-P₂ without affecting the maximum activation, and AMP lowered the K_0.5 for Rib-1,5-P₂ without affecting the maximum activation. These effects of citrate and AMP were similar to those observed with Fru-2,6-P₂ and different from those with Fru-1,6-P₂. Rib-1,5-P₂ is the second most potent activator of phosphofructokinase thus far discovered. The Rib-1,5-P₂-activated conformation of the enzyme seems to be similar to that induced by Fru-2,6-P₂, but different from that induced by Fru-1,6-P₂.