An engineered outer membrane pore enables an efficient oxygenation of aromatics and terpenes

Abstract

Biocatalysis with cytochrome P450 enzymes are important for the industrial production of fine chemicals, pharmaceuticals, fragrance and flavor compounds since chemoselective hydroxylation of aromatics and terpenes are chemically difficult to achieve. A few P450 based industrial processes have been developed based on whole cell catalysis. However, the outer membrane of microbial cells forms an effective barrier, which reduces the uptake of hydrophobic substrates. The coexpression of outer membrane proteins in E. coli such as the ferric hydroxamate uptake protein (FhuA) can provide alternative solutions to chemical or physical methods for increasing compound flux through the outer membrane of E. coli and thereby to boost productivities. In this study we employed an engineered FhuA Δ1-160 variant in which the “cork domain” was removed (first 160 residues are deleted); FhuA Δ1-160 has a cross-section of 39–46 Å with a “free” inner diameter of about 14 Å that serves as passive diffusion channel. FhuA WT and Δ1-160 were coexpressed on a bicistronic system with two P450 BM3 variants for regiospecific hydroxylation of aromatic compounds toluene and anisole as well as for oxidation of two terpenes (α)-pinene and (R)-(+)-limonene. The presence of FhuA Δ1‐160 resulted in a doubled product concentration for toluene (35 μ to 50 μM), anisole (25 μM to 45 μM), pinene (12 μM to 20 μM) and limonene (12 μM to 25 μM) and five times higher for the coumarin derivative BCCE. In order to characterizes and compensate for expression variations a quantification method based on Chromeo546-labled StrepTactinII was developed to quantify the number of FhuA Δ1-160 in the outer E. coli membrane (∼44000 of FhuA Δ1-160 per cell). Morphology studies showed that a 6% E. coli surface coverage can be achieved with FhuA Δ1‐160 without significantly influencing the E. coli rod shape. In summary, FhuA Δ1-160 efficiently increases uptake of hydrophobic aromatics and terpenes for whole-cell biotransformations and can likely be used for other enzymes and/or substrates.