TRICHODIENE SYNTHASE - IDENTIFICATION OF ACTIVE-SITE RESIDUES BY SITE-DIRECTED MUTAGENESIS

Derivatization of 5,5'-dithiobis(2-nitrobenzoic acid)-treated trichodi ene synthase with [methyl-C-14]methyl methanethiosulfonate and analysi s of the derived tryptic peptides suggested the presence of two cystei ne residues at the active site. The corresponding C146A and C190A muta nts were constructed by site-directed mutagenesis. The C190A mutant di splayed partial but significantly reduced activity, with a reduction i n k(cat)/K-m of 3000 compared to the wild-type trichodiene synthase, w hile the C146A mutant was essentially inactive. A hybrid trichodiene s ynthase, constructed from amino acids 1-309 of the Fusarium sporotrich ioides enzyme and amino acids 310-383 of the Gibberella pulicaris cycl ase, had steady state kinetic parameters nearly identical to those of the wild-type F. sporotrichioides enzyme. From this parent hybrid, a s eries of mutants was constructed by site-directed mutagenesis in which the amino acids in the base-rich region, 302-306 (DRRYR), were system atically modified. Three of these mutants were overexpressed and purif ied to homogeneity. The importance of Arg304 for catalysis was establi shed by the observation that the R304K mutant showed a more than 25-fo ld increase in K-m, as well as a 200-fold reduction in k(cat). In addi tion, analysis of the incubation products of the R304K mutant by gas c hromatography-mass spectrometry (GC-MS) indicated that farnesyl diphos phate was converted not only to trichodiene but to at least two additi onal C15H24 hydrocarbons, mle 204. Replacement of the Tyr305 residue o f trichodiene synthase with Phe had little effect on k(cat), while inc reasing the K-m by a factor of ca. 7-8. By contrast, the Y305T mutant exhibited an approximately 120-fold reduction in k(cat) as well as an 80-fold increase in K-m. Moreover, the Y305T enzyme converted farnesyl diphosphate to an approximately equal mixture of trichodiene and an u nidentified sesquiterpene hydrocarbon.