Pyrophosphate activation in hypoxanthine--guanine phosphoribosyltransferase with transition state analogue

Isotope-edited difference Raman and FTIR studies complemented by ab initio calculations have been applied to the transition state analogue complex of HGPRT.ImmHP.MgPP(i) to determine the ionic states of the 5'-phosphate moiety of ImmHP and of PP(i). These measurements characterize electrostatic interactions within the enzyme active site as deduced from frequency shifts of the phosphate groups. The bound 5'-phosphate moiety of ImmHP is dianionic, and this phosphate group exists in two different conformations within the protein complex. In one conformation, a hydrogen bond between the 5'-phosphate of ImmHP and the OH group of Tyr104 in the catalytic loop appears to be stronger. With the stronger H-bond, the OH of Tyr104 approaches one of the P..O bonds from the bridging oxygen side to cause distortion of the PO(3) moiety, as indicated by a lowered symmetric P..O stretch frequency. The asymmetric stretch frequencies are similar in both phosphate conformations. Bound PP(i) in this complex is fully ionized to P(2)O(7)(4-). Bond frequency changes for bound PP(i) indicate coordination to Mg(2+) ions but show no indication of significant P..O bond polarization. Extrapolation of these results to reaction coordinate motion for HGPRT suggests that bond formation between C1' of the nucleotide ribose and the oxygen of PP(i) is accomplished by migration of the ribocation toward immobilized pyrophosphate.