Enzyme promiscuity in amino acid oxidases: a tool for sustainable processes.

Enzymatic promiscuity is the ability of enzymes to catalyze additional reactions different from those for which they have evolved. This phenomenon plays a key role in the divergent evolution of novel enzymes. Amino acid oxidases (AAOs) are a group of FAD containing enzymes that catalyze the oxidative deamination of amino acids. AAOs evolved to fulfill very different physiological roles, by reshaping of their functional and structural features. Thus these enzymes represent an ideal model to understand the mechanisms that originated molecular biodiversity in modern enzyme families. In addition, their strict enantioselectivity renders AAOs interesting biocatalysts for the production of optically pure amino acids, valuable compounds widely used in the pharmaceutical and food industries. Through extensive literature and database search we identified two novel L-amino acid oxidases (LAAOs). Detailed structural and functional characterization showed that the first one, the aminoacetone oxidase from S. oligofermentans (SoAAO), is not a canonical LAAO, since it does no possess the typical features of these enzymes, and has only a low promiscuous activity on L-amino acids. Its preferred substrate is aminoacetone that is converted to 2,5-dimethylpyrazine. Thus we propose that SoAAO could act as a scavenger of aminoacetone (a prooxidant metabolite), protecting the cell from oxidative damage. The second one, L-amino acid deaminase from P. myxofaciens (PmaLAAD), resembles more closely the typical LAAOs: it is a membrane associated protein active on large hydrophobic L-amino acids. PmaLAAD does not use molecular oxygen, but a cytochrome b-like protein, as a direct electron acceptor. We propose that PmaLAAD is involved in catabolic utilization of L-amino acids to fuel the electron-transfer chain of Proteus membranes. Comparison of the 3D structure of the two proteins with other LAAOs reveals that SoAAO diverged very early from this group of flavoenzymes, originating a novel structural group, while PmaLAAD has a clear evolutionary link with LAAOs. In conclusion, the characterization of two novel microbial LAAOs allowed us to define their structure/function relationships, to clarify their physiological role and to obtain new insights on the underlying mechanisms of the molecular evolution of AAOs.