A new class IV cytochrome P450 enzyme: An evolutionary tale and an in silico analysis

A new member of class IV cytochrome P450 (P450) (Roberts et al., 2002) was identified in Acinetobacter radioresistens S13. As such this is expected to be a self-sufficient enzyme consisting of a N-terminal catalytic P450 domain fused to a C-terminal reductase domain. Interestingly phylogenetic analysis places the sequence of the P450 domain close to that of class I P450 from Rhodococcus jostii. The reductase domain is close to a R. jostii oxidoreductase containing FMN, NADH and 2Fe2S centre. R. jostii genes coding for class I P450 (cypx116) and the oxidoreductase (oxred116) are located in a plasmid in adjacent position. R. jostii does not possess genes coding for class IV P450 proteins and A. radioresistens does not have class I P450 proteins. Therefore, it can be speculated that A. radioresistens cypx gene is the result of: (1) a horizontal gene transfer of the R. jostii DNA fragment harbouring cypx116 and oxred116, (2) gene fusion, and (3) integration of the fused new cypx gene in the A. radioresistens chromosome. In the absence of any published crystal structure for this class of P450 enzymes, an in silico 3D model was generated based on homology modeling with class I P450 from Bacillus subtilis (3EJB). The P450 enzyme from the latter bacterium is known to catalyze the oxidation of fatty acids (Cryle et al. 2003). In silico docking experiments showed, on the basis of the binding energies, that stearic and palmitic acid could be putative substrates of this enzyme. Moreover, the expression of S13 cypx was induced by the presence of these molecules in the bacterial growth medium. Therefore, both in silico and in vitro experiments support turnover of these fatty acids by this novel class IV P450 enzyme.