Substrate specificity of a novel squalene-hopene cyclase from Zymomonas mobilis

Squalene-hopene cyclases (SHC; EC 5.4.99.17) catalyze the cyclization of triterpenoids via cationic intermediates in one of the most complex reactions known in biochemistry. In this study, we report the functional expression of a novel SHC from the ethanol producing bacterium Zymomonas mobilis (ZmoSHC1; YP_163283.1). Biochemical characterization of ZmoSHC1 uncovered unique substrate activity patterns compared to the previously reported AacSHC from Alicyclobacillus acidocaldarius and ZmoSHC2, the second squalene-hopene cyclase from Z. mobilis. ZmoSHC1 showed cyclization of the non-natural substrates homofarnesol (C16) and citronellal (C10) in addition to hopene formation from squalene (C30). Moreover, ZmoSHC1 turned out to reveal high biocatalytic stability during long-term incubations. Remarkably, ZmoSHC1 exhibited a shift of activity towards substrates of shorter chain lengths, displaying over 50-fold higher conversion of homofarnesol and more than 2-fold higher conversion of citronellal in comparison to squalene conversion.