Comparison of Vibrational Frequencies of Critical Bonds in Ground-State Complexes and in a Vanadate-Based Transition-State Analog Complex of Muscle Phosphoglucomutase. Mechanistic Implications

The symmetric stretching frequency of the [formula omited] bonds of the enzymic phosphate group in muscle phosphoglucomutase was measured via ¹⁶O/¹⁸O Raman difference spectroscopy. This frequency, and its shift on isotopic substitution, is characteristic of a dianionic phosphate ester. The [formula omited] stretching frequency is not detectably altered by the binding of the metal ion activators Mg²⁺, Zn²⁺, or Cd²⁺ nor by the subsequent binding of glucose phosphate. Hence, a binding-induced distortion/polarization of the enzymic phosphate group in the ground state, or enzyme-substrate complex, cannot serve as a rationale for the large value of k_cat in the phosphoglucomutase reaction. By contrast, the stretching frequency of the [formula omited] bonds within a vanadate group bound at the same site in the transition-state analog complex involving glucose 1-phosphate 6-vanadate is much lower than for a normal dianionic vanadate. This low [formula omited] stretching frequency is best rationalized in terms of the extensive polarization of all three nonbridging oxygens of the vanadate ester dianion plus the formation of a weak, fifth bond to the vanadium atom. This distortion/polarization of the VO₃²⁻ group depends on the metal ion activator, since it is largely abolished, and the involvement of the fifth ligand eliminated, by substitution of Li⁺ for Mg²⁺ at the metal activation site. To the extent that the vanadate-inhibitor complex mimics the transition state for the normal phosphoglucomutase reaction, as has been suggested [Ray, W. J., Jr., & Puvathingal, J. M. (1990) Biochemistry 29, 2790], the normal PO₃⁻ transfer is best described as a process with S_N2-like or associative character and thus is quite different from the process by which model phosphate ester dianions normally react in aqueous solution.