Structural and mechanistic insight into alkane hydroxylation by Pseudomonas putida AlkB

Pseudomonas putida GPo1 alkane hydroxylase (AlkB) is an integral membrane protein that catalyses the hydroxylation of medium-chain alkanes (C₃–C₁₂). 1-Octyne irreversibly inhibits this non-haem di-iron mono-oxygenase under turnover conditions, suggesting that it acts as a mechanism-based inactivator. Upon binding to the active site, 1-octyne is postulated to be oxidized to an oxirene that rapidly rearranges to a reactive ketene which covalently acylates nearby residues, resulting in enzyme inactivation. In analysis of inactivated AlkB by LC–MS/MS, several residues exhibited a mass increase of 126.1 Da, corresponding to the octanoyl moiety derived from oxidative activation of 1-octyne. Mutagenesis studies of conserved acylated residues showed that Lys¹⁸ plays a critical role in enzyme function, as a single-point mutation of Lys¹⁸ to alanine (K18A) completely abolished enzymatic activity. Finally, we present a computational 3D model structure of the transmembrane domain of AlkB, which revealed the overall packing arrangement of the transmembrane helices within the lipid bilayer and the location of the active site mapped by the 1-octyne modifications.