Role of arginine 59 in the gamma-class carbonic anhydrases.
The functional role of the highly conserved active site Arg 59 in the prototype of the gamma-class carbonic anhydrase Cam (carbonic anhydrase from Methanosarcina thermophila) was investigated. Variants (R59A, -C, -E, -H, -K, -M, and -Q) were prepared by site-directed mutagenesis and characterized by size exclusion chromatography (SEC), circular dichroism (CD) spectroscopy, and stopped-flow kinetic analyses. CD spectra indicated similar secondary structures for the wild type and the R59A and -K variants, independent of nondenaturing concentrations of guanidine hydrochloride (GdnHCl). SEC indicated that all variants purified as homotrimers like the wild type. SEC also revealed that the R59A and -K variants unfolded at > or = 1.5 M GdnHCl, compared to 3.0 M GdnHCl for the wild type. These results indicate that Arg 59 contributes to the thermodynamic stability of the Cam trimer. The R59K variant had k(cat) and k(cat)/K(m) values that were 8 and 5% of the wild-type values, respectively, while all other variants had k(cat) and k(cat)/K(m) values 10-100-fold lower than those of the wild type. The R59A, -C, -E, -M, and -Q variants exhibited 4-63-fold increases in k(cat) and 9-120-fold increases in k(cat)/K(m) upon addition of 100 mM GdnHCl, with the largest increases observed for the R59A variant, which was comparable to the R59K variant. The kinetic results indicate that a positive charge at position 59 is essential for the CO(2) hydration step of the overall catalytic mechanism.
Tripp
Brian C
BC
Department of Biochemistry and Molecular Biology, Eberly College of Science, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Tu
Chingkuang
C
Ferry
James G
JG
D000595:Amino Acid Sequence; D001120:Arginine / Q000737:chemistry*; D001665:Binding Sites; D002256:Carbonic Anhydrases / Q000737:chemistry*; D002384:Catalysis; D002845:Chromatography; D002942:Circular Dichroism; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D019791:Guanidine / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010957:Plasmids / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013816:Thermodynamics
D001120:Arginine; D002256:Carbonic Anhydrases; D019791:Guanidine