The roles of an aspartate and an arginine, which are completely conserved in the active sites of β-class carbonic anhydrases, were investigated by steady-state kinetic analyses of replacement variants of the β-class enzyme (Cab) from the archaeon Methanobacterium thermoautotrophicum. Previous kinetic analyses of wild-type Cab indicated a two-step zinc-hydroxide mechanism of catalysis in which the kcat/Km value depends only on the rate constants for the CO2 hydration step, whereas kcat also depends on rate constants from the proton transfer step (K. S. Smith, N. J. Cosper, C. Stalhandske, R. A. Scott, and J. G. Ferry, J. Bacteriol. 182:6605-6613, 2000). The recently solved crystal structure of Cab shows the presence of a buffer molecule within hydrogen bonding distance of Asp-34, implying a role for this residue in the proton transport step (P. Strop, K. S. Smith, T. M. Iverson, J. G. Ferry, and D. C. Rees, J. Biol. Chem. 276:10299-10305, 2001). The kcat/Km values of Asp-34 variants were decreased relative to those of the wild type, although not to an extent which supports an essential role for this residue in the CO2 hydration step. Parallel decreases in kcat and kcat/Km values for the variants precluded any conclusions regarding a role for Asp-34 in the proton transfer step; however, the kcat of the D34A variant was chemically rescued by replacement of 2-(N-morpholino)propanesulfonic acid buffer with imidazole at pH 7.2, supporting a role for the conserved aspartate in the proton transfer step. The crystal structure of Cab also shows Arg-36 with two hydrogen bonds to Asp-34. Arg-36 variants had both kcat and kcat/Km values that were decreased at least 250-fold relative to those of the wild type, establishing an essential function for this residue. Imidazole was unable to rescue the kcat of the R36A variant; however, partial rescue of the kinetic parameter was obtained with guanidine-HCl indicating that the guanido group of this residue is important.
All Science Journal Classification (ASJC) codes
- Molecular Biology