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 P-O 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 P-O 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 kcat in the phosphoglucomutase reaction. By contrast, the stretching frequency of the V-O 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 V-O 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 VO3(2-) 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Deng
H
H
Department of Physics, City College of the City University of New York, New York 10031.
Ray
W J
WJ
Burgner
J W
JW
Callender
R
R
D000818:Animals; D001665:Binding Sites; D002413:Cations, Divalent / Q000737:chemistry; D005958:Glucosephosphates / Q000737:chemistry / Q000378:metabolism; D066298:In Vitro Techniques; D009132:Muscles / Q000201:enzymology; D010710:Phosphates / Q000378:metabolism; D010733:Phosphoglucomutase / Q000737:chemistry*; D011817:Rabbits; D013059:Spectrum Analysis, Raman; D014638:Vanadates / Q000737:chemistry
D002413:Cations, Divalent; D005958:Glucosephosphates; D010710:Phosphates; D014638:Vanadates; D010733:Phosphoglucomutase