The enigmatic reaction of flavins with oxygen

Abstract

The reaction of flavoenzymes with oxygen remains a fascinating area of research because of its relevance for reactive oxygen species (ROS) generation. Several exciting recent studies provide consistent mechanistic clues about the specific functional and structural properties of the oxidase and monooxygenase flavoenzymatic systems. Specifically, the spatial arrangement of the reacting oxygen that is in direct contact with the flavin group is emerging as a crucial factor that differentiates between oxidase and monooxygenase enzymes. A challenge for the future will be to use these emerging concepts to rationally engineer flavoenzymes, paving the way to new research avenues with far-reaching implications for oxidative biocatalysis and metabolic engineering.

Role of Ser-257 in the Sliding Mechanism of NADP(H) in the Reaction Catalyzed by the Aspergillus fumigatus Flavin-dependent Ornithine N5-Monooxygenase SidA

“…This caged radical pair rapidly recombines to form a covalent flavin oxygen adduct at C4a of the flavin. Depending on the specific chemical outcome, a C4a-peroxyflavin or C4a-hydroperoxyflavin is formed (2,9,28). Formation of these intermediates does not ensure monooxygenation, as stabilization is required for optimal catalysis.…”

The Ala95‐to‐Gly substitution in Aerococcus viridansl‐lactate oxidase revisited – structural consequences at the catalytic site and effect on reactivity with O2 and other electron acceptors

“…The water molecule accomodated in the A95G variant reduces the hydrophobicity around the putative O 2 reacting site compared to the wild-type lactate oxidase that has the methyl group of Ala95 in an approximately analogous position. For different flavoenzymes, an optimum combination of hydrophobicity and steric constraint creates a physically confined microenvironment that can strongly facilitate the ability of O 2 to rapidly react with the reduced flavin [17,18]. A lowering of polarity of the flavin environment in (S)-mandelate dehydrogenase upon substitution of Gly81 by Ala was suggested to account for the slight increase in O 2 reactivity in the mutant compared to wild-type enzyme [44].…”

“…Small differences in microenvironment of the reactive C4a-N5 locus of FMN might be responsible [17,18], in part at least, for these starkly varied O 2 reactivities. Enzyme structures [3][4][5][6][7] reveal that strand b1 of the b barrel provides close contact contact with the re-face, the side of the flavin ring opposite to where the substrate binds (Fig.…”

Engineering of pyranose 2-oxidase for modified oxygen reactivity

“…Channels leading from the surface to the active site may affect oxidase reactivity [37,38]. Altering the oxygen reactivity is of considerable interest as shown in recent reviews [15,28] and works of Krondorfer et al [39], Sygmund et al [40], Leferink et al [41], and others [42][43][44].…”

“…The resulting flavin semiquinone is unstable but could be detected by for glycolate oxidase [27]. The environment around the C4a-N5 locus, which is directly involved in the reaction with oxygen, is discussed to be the crucial area affecting oxidase activity [28]. A positive charge around the flavin reaction site, which can be provided by amino acid residues [15,26,[29][30][31][32] or bound substrate and/or product [33][34][35][36], can influence this.…”