Bioorganic studies on the recogniton mechanism of the methyl groups of the substrate skeleton by squalene and oxidosqualene cyclases

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This research project has gained insights into the cyclization mechanism of squalene and oxidosqualene catalyzed by hopene, tetrahymanol and lanosterol synthases.Investigations from the two approaches of the substrate analogues and the enzymic chemistry have revealed the following new results :1. Overexpression of squalene-hopene cyclase in Escherichia coli Biosci. Biotechnol. Biochem. 62, pp. 407-411(1998).2. Site-directed mutagenesis experiments, targetted for tryptophan residues, revealed that the repetitive conserved alighnments, i.e. QW motifs, which has been presumed to be active sites by some workers, were not active sites and had a specific function against thermal denaturation. Biosci. Biotechnol. Biochem. 63, (1999), in press.3. Point mutations of W489F and W l 69H has led us to propose the new cyclization mechanism, that is, a ring expansion from 5- to 6-membered D-ring occur during the polycyclization. J.Chem. Soc. Chem. Commun. 2617-2618 (1998).4. The substrate analogues showed that a ring expansion reaction ocurs also at the state of C-ring formation in the case of lanosterol synthase.J.Chem. Soc. Chem. Commun. 1591-1592 (1998).5. the terminal methyl groups are critical to the correct folding of squalene substrate both for the formation of the five-membered B-ring and for the initiation of the polycyclization reaction submitted for publication.