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CMP kinases fromBacillus subtilisand fromEscherichia coliare encoded by thecmkgene (formerly known asjofCinB. subtilisand asmssAinE. coli). Similar in their primary structure (43% identity and 67% similarity in aminoacid sequence), the two proteins exhibit significant differences in nucleotide binding and catalysis. ATP, dATP, and GTP are equally effective as phosphate donors withE. coliCMP kinase whereas GTP is a poor substrate withB. subtilisCMP kinase. While CMP and dCMP are the best phosphate acceptors of both CMP kinases, the specific activity with these substrates and ATP as donor are 7- to 10-fold higher in theE. colienzyme; the relativeV_mvalues with UMP and CMP are 0.1 for theB. subtilisCMP kinase and 0.01 for theE. colienzyme. CMP increased the affinity ofE. coliCMP kinase for ATP or for the fluorescent analog 3′-anthraniloyl dATP by one order of magnitude but had no effect on theB …