Structure and role for active site lid of lactate monooxygenase from Mycobacterium smegmatis

摘要

Lactate monooxygenase (LMO) catalyzes the FMN‐dependent “coupled” oxidation of lactate and O to acetate, carbon dioxide, and water, involving pyruvate and hydrogen peroxide as enzyme‐bound intermediates. Other α‐hydroxy acid oxidase family members follow an “uncoupled pathway,” wherein the α‐keto acid product quickly dissociates before the reduced flavin reacts with oxygen. Here, we report the structures of wild‐type LMO and a wild‐type‐like C203A variant at 2.1 Å and 1.7 Å resolution, respectively. The overall LMO fold and active site organization, including a bound sulfate mimicking substrate, resemble those of other α‐hydroxy acid oxidases. Based on structural similarity, LMO is similarly distant from lactate oxidase, glycolate oxidase, mandelate dehydrogenase, and flavocytochrome b and is the first representative enzyme of its type. Comparisons with other α‐hydroxy acid oxidases reveal that LMO has a longer and more compact folded active site loop (Loop 4), which is known in related flavoenzymes to undergo order/disorder transitions to allow substrate/product binding and release. We propose that LMO's Loop 4 has an enhanced stability that is responsible for the slow product release requisite for the coupled pathway. We also note electrostatic features of the LMO active site that promote substrate binding. Whereas the physiological role of LMO remains unknown, we document what can currently be assessed of LMO's distribution in nature, including its unexpected occurrence, presumably through horizontal gene transfer, in halophilic archaea and in a limited group of fungi of the genus .