Pseudomonad Cyclopentadecanone Monooxygenase Displaying an Uncommon Spectrum of Baeyer-Villiger Oxidations of Cyclic Ketones |
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Authors: | Hiroaki Iwaki Shaozhao Wang Stephan Grosse Hlne Bergeron Ayako Nagahashi Jittiwud Lertvorachon Jianzhong Yang Yasuo Konishi Yoshie Hasegawa and Peter C K Lau |
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Institution: | Hiroaki Iwaki, Shaozhao Wang, Stephan Grosse, Hélène Bergeron, Ayako Nagahashi, Jittiwud Lertvorachon, Jianzhong Yang, Yasuo Konishi, Yoshie Hasegawa, and Peter C. K. Lau |
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Abstract: | Baeyer-Villiger monooxygenases (BVMOs) are biocatalysts that offer the prospect of high chemo-, regio-, and enantioselectivity in the organic synthesis of lactones or esters from a variety of ketones. In this study, we have cloned, sequenced, and overexpressed in Escherichia coli a new BVMO, cyclopentadecanone monooxygenase (CpdB or CPDMO), originally derived from Pseudomonas sp. strain HI-70. The 601-residue primary structure of CpdB revealed only 29% to 50% sequence identity to those of known BVMOs. A new sequence motif, characterized by a cluster of charged residues, was identified in a subset of BVMO sequences that contain an N-terminal extension of ~60 to 147 amino acids. The 64-kDa CPDMO enzyme was purified to apparent homogeneity, providing a specific activity of 3.94 μmol/min/mg protein and a 20% yield. CPDMO is monomeric and NADPH dependent and contains ~1 mol flavin adenine dinucleotide per mole of protein. A deletion mutant suggested the importance of the N-terminal 54 amino acids to CPDMO activity. In addition, a Ser261Ala substitution in a Rossmann fold motif resulted in an improved stability and increased affinity of the enzyme towards NADPH compared to the wild-type enzyme (Km = 8 μM versus Km = 24 μM). Substrate profiling indicated that CPDMO is unusual among known BVMOs in being able to accommodate and oxidize both large and small ring substrates that include C11 to C15 ketones, methyl-substituted C5 and C6 ketones, and bicyclic ketones, such as decalone and β-tetralone. CPDMO has the highest affinity (Km = 5.8 μM) and the highest catalytic efficiency (kcat/Km ratio of 7.2 × 105 M−1 s−1) toward cyclopentadecanone, hence the Cpd designation. A number of whole-cell biotransformations were carried out, and as a result, CPDMO was found to have an excellent enantioselectivity (E > 200) as well as 99% S-selectivity toward 2-methylcyclohexanone for the production of 7-methyl-2-oxepanone, a potentially valuable chiral building block. Although showing a modest selectivity (E = 5.8), macrolactone formation of 15-hexadecanolide from the kinetic resolution of 2-methylcyclopentadecanone using CPDMO was also demonstrated. |
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