The mechanism of octahedral complex formation by labile metal ions

Biochemical aspects of molybdenum coordination chemistry

The Coordination and Bioinorganic Chemistry of Molybdenum

Electrochemical studies of oxo- and sulfido-bridged molybdenum(VI), -(V), and -(IV) diethyldithiocarbamate complexes in aprotic solvents

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain.

Molybdenum-containing enzymes

Using the temperature-jumps relaxation technique the kinetics of complex formation between molybdate and 8-hydroxyquinoline have been studied in the pH range 7.9–8.9. A reaction scheme for formation of the 1:1 complex is proposed which accounts for the observed pH dependence of the overall forward and backward rate constants. The specific rate constants evaluated are discussed in terms of the detailed reaction mechanism. Complex formation between molybdate and 8-hydroxyquinoline may be regarded as a model for the binding of substrate by the enzyme xanthine oxidase. This enzyme contains molybdenum which probably is involved in substrate binding. The experimental data are compatible with this hypothesis.

P. F. Knowles

Papers

Kinetics of complex formation of molybdate with 8-hydroxyquinoline. A model system for enzyme-substrate binding in xanthine oxidase