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Functional evaluation of invariant arginines situated in the mobile lid domain of phosphoribulokinase.

Rhodobacter sphaeroides phosphoribulokinase contains four invariant arginines (R49, R168, R173, and R187). The high-resolution structure of this enzyme [Harrison, D. H. T., Runquist, J. A., Holub, A., and Miziorko, H. M. (1998) Biochemistry (submitted for publication)] reveals that it folds in a manner similar to that of adenylate kinase. Three invariant arginines (R168, R173, and R187) as well as arginine-186, which is conserved in prokaryotic phosphoribulokinases, have not been previously functionally evaluated. These arginine residues map within the mobile lid domain that is a distinctive feature of the adenylate kinase family of proteins. Precedent for the significant function of arginines in phosphotransferase reactions prompted substitution of glutamine for each of these three invariant arginines. Solution state characterization of the isolated mutant proteins indicated that they retained a high degree of structural integrity, as indicated by their stoichiometric binding of an alternative nucleotide substrate (trinitrophenyl-ATP) as well as the allosteric effector (NADH). Kinetic characterization indicated > 10(4)-fold diminution in V/KRu5P for R168Q, attributable to a > 300-fold decrease in catalytic efficiency and an increase (approximately 50-fold) in Km Ru5P. For R173Q, a 15-fold diminution in Vmax and a 100-fold increase in Km Ru5P were observed. These observations implicate new components of the ribulose 5-phosphate binding site. Additionally, they confirm assignment of the mobile lid domain as part of the phosphoribulokinase active site, even though this region is well separated from other active site elements in the structure of the open form of the protein. Characterization of R186Q and R187Q mutants suggests that they influence the cooperativity of substrate binding.