A pH-dependent allosteric transition in Ascaris suum phosphofructokinase distinct from that observed with fructose 2,6-bisphosphate.

Ascaris suum phosphofructokinase exhibits dramatic shifts in its circular dichroic spectra in the pH range 6 to 8. These shifts are quite distinct from those induced by the activators AMP and fructose 2,6-bisphosphate. Concomitant with these pH-induced spectral shifts, the enzyme also displays changes in its allosteric behavior. Inorganic ions such as K+, NH+4, SO4(2-), and PO4(3-) also cause CD-spectral shifts similar to those produced by a change in pH. Based on the evidence derived from gel filtration and sedimentation equilibrium studies, the observed CD-spectral shifts are interpreted as due to conformational changes in the enzyme tetramer rather than due to a change in its aggregation state. Further, since the pK value of 6.4 obtained from pH dependence of increase in ellipticity at 210 nm agrees very well with the pK value of 6.8 for the loss of ATP inhibition due to modification of a histidine residue (G. S. J. Rao, B. A. Wariso, P. F. Cook, and B. G. Harris (1987) J. Biol. Chem. 262, 14068-14073), it is concluded that a single histidine residue in the ATP-inhibitory site acts as a trigger for the structural changes accompanying ATP inhibition of the enzyme. This view is strongly supported by the observation that the enzyme desensitized to ATP inhibition by chemical modification of a histidine residue in the ATP-inhibitory site shows absolutely no change in its CD spectrum in the pH range 6 to 8. This study demonstrates that the mechanism of activation of phosphofructokinase at higher pH and by inorganic ions involves conformational transitions that are quite distinct from those induced by AMP and fructose 2,6-bisphosphate. A scheme is presented that incorporates all of the different states of the enzyme dependent upon effectors and pH.