Biochemical and spectroscopic studies of (S)-2-hydroxypropylphosphonic acid epoxidase in fosfomycin biosynthesis
Fosfomycin, produced by a few strains of Streptomyces, is a clinically useful antibiotic against both gram-positive and gram-negative bacteria. The unique structure of fosfomycin, characterized by a carbon-phosphorous bond and an epoxide ring, has attracted many attentions to its biosynthetic pathway. (S)-2- hydroxylpropanylphosphonic acid epoxidase (HppE) catalyzes the last step in the pathway, conversion of a secondary alcohol, (S)-2-hydroxylpropanylphosphonic acid ((S)-HPP), to the final epoxide product. This is a completely new enzymatic reaction in contrast to many other biological epoxidation reactions that usually involve olefin oxidation. It is worth noting that the oxygen atom in the hydroxyl group of (S)-HPP is retained during the ring closure. The previous studies of HppE have led to the discovery of a novel biological epoxidation system, (S)-HPP + NADH + O2 + H+ → fosfomycin + NAD+ + 2H2O. The further biochemical and EPR spectral analyses suggested that HppE is a new type of mononuclear non-heme iron-dependent enzyme carrying a 2-His-1-carboxylate triad iron-binding motif. In addition to three amino acid ligands, both phosphonate and hydroxyl groups of (S)-HPP also coordinate to the iron center in the enzyme-substrate complex. The bidentate substrate-binding mode is presumably responsible for the strict regiospecificity and stereospecificity in the HppE-catalyzed epoxidation. A catecholate-to-FeIII charge transfer complex was identified in the HppE active site, by UV-visible absorption and resonance Raman spectroscopies. The catechol is derived from a hydroxylated amino acid residue, Tyr105, as a product of the self-catalytic hydroxylation. The oxidation of Try105 to DOPA is a side reaction that occurs only in the absence of substrate, and the presence of DOPA has no apparent effects on the epoxidase activity of HppE. Mechanistic information of HppE has been continuously garnered by EPR spectral analysis. Using NO as a dioxygen analogue and a probe for ferrous center, the first step of the catalytic cycle has been revealed to be the reduction of substrate-bound iron center from its ferric state to ferrous state by FMN/NADH. Besides, the presence of a spin-coupled adduct of protein-centered radical and ferric center has been supported by experimental evidence. However, it is still not clear whether the “hidden” radical plays any roles in HppE catalysis.