Download MendeleyA Single Mutation in the Castor {Delta}9-18:0- Desaturase Changes Reaction Partitioning from Desaturation to Oxidase Chemistry.
Guy, J.E., Abreu, I.A., Moche, M., Lindqvist, Y., Whittle, E., Shanklin, J.(2006) Proc Natl Acad Sci U S A 103: 17220
- PubMed: 17088542
- DOI: https://doi.org/10.1073/pnas.0607165103
- Primary Citation of Related Structures:
2J2F - PubMed Abstract:
Sequence analysis of the diiron cluster-containing soluble desaturases suggests they are unrelated to other diiron enzymes; however, structural alignment of the core four-helix bundle of desaturases to other diiron enzymes reveals a conserved iron binding motif with similar spacing in all enzymes of this structural class, implying a common evolutionary ancestry. Detailed structural comparison of the castor desaturase with that of a peroxidase, rubrerythrin, shows remarkable conservation of both identity and geometry of residues surrounding the diiron center, with the exception of residue 199. Position 199 is occupied by a threonine in the castor desaturase, but the equivalent position in rubrerythrin contains a glutamic acid. We previously hypothesized that a carboxylate in this location facilitates oxidase chemistry in rubrerythrin by the close apposition of a residue capable of facilitating proton transfer to the activated oxygen (in a hydrophobic cavity adjacent to the diiron center based on the crystal structure of the oxygen-binding mimic azide). Here we report that desaturase mutant T199D binds substrate but its desaturase activity decreases by approximately 2 x 10(3)-fold. However, it shows a >31-fold increase in peroxide-dependent oxidase activity with respect to WT desaturase, as monitored by single-turnover stopped-flow spectrometry. A 2.65-A crystal structure of T199D reveals active-site geometry remarkably similar to that of rubrerythrin, consistent with its enhanced function as an oxidase enzyme. That a single amino acid substitution can switch reactivity from desaturation to oxidation provides experimental support for the hypothesis that the desaturase evolved from an ancestral oxidase enzyme.
Organizational Affiliation:
Department of Medical Biochemistry and Biophysics, Division of Molecular Structural Biology, Karolinska Institutet, Tomtebodavägen 6, S-171 77 Stockholm, Sweden.
