Conserved Tyr residues determine functions of Alicyclobacillus acidocaldarius squalene-hopene cyclase

The catalytic cavity of Alicyclobacillus acidocaldarius squalene-hopene cyc lase is mainly lined by aromatic amino acids. In recombinant cyclases, thre e our of four tyrosine residues (Y) have been mutated to phenylalanine resi dues (F). The mutant cyclases Y495F and Y612F had less activity than the wi ld-type cyclase, but a wild-type product pattern. Mutant Y609F had wild-typ e activity but a drastically altered product pattern with hopene and signif icant amounts of bicyclic alpha-polypodatetraene and different tetracyclic triterpenes (dammaradienes and eupha-7,24-diene). The experiments demonstra ted that Y495 and Y612 may be involved in the initiation of the cyclization reaction and Y609 in the stabilization and/or positioning of the intermedi ate carbocations. (C) 2000 Federation of European Microbiological Societies . Published by Elsevier Science B.V. All rights reserved.