Engineering of squalene cyclizing enzymes

The broad substrate tolerance and catalytic potential of squalene cyclizing enzymes of bacterial and plant origin are remarkable; the enzymes readily accept variety of non-physiological substrate analogues and efficiently perform sequential ring-forming reactions to produce a series of unnatural cyclic triterpenes. By utilizing the catalytic plasticity of the enzymes, it is possible to generate unnatural novel cyclic polyprenoids by enzymatic conversion of chemically synthesized substrate analogues. Here we present recent examples including (a) enzymatic formation of a "supra-natural" hexacyclic polyprenoid as well as heteroaromatic ring containing cyclic polyprenoids by bacterial squalene: hopene cyclase from Alicyclobacillus acidocaldarius, and (b) enzymatic cyclization of 22,23-dihydro-2,3-oxidosqualene and 24,30-bisnor-2,3-oxidosqualene by plant oxidosqualene: beta-amyrin cyclase from Pisum sativum.