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 circu lar dichroic spectra in the pH range 6 to 8, These shifts are quite di stinct from those induced by the activators AMP and fructose 2,6-bisph osphate, Concomitant with these pH-induced spectral shifts, the enzyme also displays changes in its allosteric behavior, Inorganic ions such as K+, NH4+, SO42-, and PO43- also cause CD-spectral shifts similar t o 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 t he enzyme tetramer rather than due to a change in its aggretation stat e, Further, since the pK value of 6.4 obtained from pH dependence of i ncrease 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 (19 87) J, Biol, Chem, 262, 14068-14073), it is concluded that a single hi stidine residue in the ATP-inhibitory site acts as a trigger for the s tructural 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 i nvolves conformational transitions that are quite distinct from those induced by AMP and fructose 2,6-bisphosphate, A scheme is presented th at incorporates all of the different states of the enzyme dependent up on effecters and PH. (C) 1995 Academic Press, Inc.