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Volume 272, Number 43, Issue of October 24, 1997 pp. 27065-27076
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

The Role of Glycine 99 in L-Lactate Monooxygenase from Mycobacterium smegmatis

(Received for publication, May 13, 1997, and in revised form, July 28, 1997)

Weimei Sun , Charles H. Williams Jr.^§ and Vincent Massey

From the  Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, Michigan 48109-0606 and the ^§ Department of Veterans Affairs Medical Center, Ann Arbor, Michigan 48105

Glycine 99 in L-lactate monooxygenase (LMO) from Mycobacterium smegmatis was mutated to serine and threonine, and the resultant mutants were studied extensively to explore the role of this residue in maintaining monooxygenase activity and in controlling the reactivity with molecular oxygen. Both mutants were observed to lose monooxygenase activity completely and generate H₂O₂ and pyruvate as reaction products. However, the mutants have much lower activities than a true L-lactate oxidase. The oxygen reactivities of the reduced and semiquinone forms of the mutant enzymes were significantly different from those of wild type enzyme. These results confirm our previous suggestion that the electronic interactions in the active site are a crucial factor that governs the oxygen reactivity of the enzyme (Sun, W., Williams, C. H., Jr., and Massey, V. (1996) J. Biol. Chem. 271, 17226-17233). In addition, the mutants cause a dramatic decrease of the rate of flavin reduction by L-lactate compared with the wild type enzyme, mainly due to the much lower stabilization of the transition state.

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