Fluorinated and deoxygenated substrates as probes of transition state structure in glycogen phosphorylase

Abstract

A series of deoxyfluoro- and deoxy-alpha-D-glucopyranosyl phosphates have been tested as substrates of rabbit muscle glycogen phosphorylase b. All are found to be utilized by the enzyme, but at substantially reduced rates. Values of Vm/Km for these analogues range from 10(2) to 10(5) times lower than that for the parent substrate. The large rate reductions are suggested to arise from a combination of intrinsic electronic effects and poorer binding of these substrates at the transition state. The data provide substantial evidence for an oxocarbonium-ion-like transition state. They also provide estimates of the strengths of hydrogen bonds to individual sugar hydroxyls at the transition state of the reaction. Further, comparison of such data with those obtained for glucose analogues binding as inhibitors to T-state phosphorylase suggests that these two glucose subsites are essentially identical; thus, the glucose pocket remains intact during the conformational transition associated with activation of the enzyme.