| Abstract
| - The SgcC4 l-tyrosine 2,3-aminomutase (SgTAM) catalyzes the formation of (S)-β-tyrosine inthe biosynthetic pathway of the enediyne antitumor antibiotic C-1027. SgTAM is homologous to thehistidine ammonia lyase family of enzymes whose activity is dependent on the methylideneimidazole-5-one (MIO) cofactor. Unlike the lyase enzymes, SgTAM catalyzes additional chemical transformationsresulting in an overall stereospecific 1,2-amino shift in the substrate l-tyrosine to generate (S)-β-tyrosine.Previously, we provided kinetic, spectroscopic, and mutagenesis data supporting the presence of MIO inthe active site of SgTAM [Christenson, S. D.; Wu, W.; Spies, A.; Shen, B.; and Toney, M. D. (2003)Biochemistry 42, 12708−12718]. Here we report the first X-ray crystal structure of an MIO-containingaminomutase, SgTAM, and confirm the structural homology of SgTAM to ammonia lyases. Comparisonof the structure of SgTAM to the l-tyrosine ammonia lyase from Rhodobacter sphaeroides provides insightinto the structural basis for aminomutase activity. The results show that SgTAM has a closed active sitewell suited to retain ammonia and minimize the formation of lyase elimination products. The amino aciddeterminants for substrate recognition and catalysis can be predicted from the structure, setting theframework for detailed mechanistic investigations.
|
