LOV1 protein from Chlamydomonas reinhardtii is a template for the photoadduct formation of FMN and methylmercaptane

Abstract

The adduct of FMN and methylmercaptane was synthesized inside the binding pocket of a light oxygen voltage sensitive (LOV) protein domain in which the cysteine residue of the wild type has been replaced by the photochem. inactive amino acid glycine. In the absence of the protein, the photoreaction between FMN and MM yields the fully reduced hydroquinone form FMNH2 or the anion FMNH. The reaction mechanism of the synthesized FMN and MM adduct is briefly described. The adduct postulated by us is a flavin deriv. with a thioalkyl residue at the C4a-position and a hydrogen atom at the N5-position. It is readily hydrolyzed either by protonation at the sulfur atom or by deprotonation at N5. However, it can be formed and stabilized for many hours in the flavin binding pocket of a LOV protein domain. Thus, the protein not only plays a decisive role in guiding the reaction towards this product, but also in keeping it stable. It is assumed that the reaction proceeds along the same mechanism as the cysteine adduct formation in the wild-type protein, albeit with lower quantum yields and rate consts.