Thermotoga maritima catalytic activity

The three-dimensional structure of the PMT active site is not known. The protein structure of a Thermotoga maritima spermidine synthase obtained by protein crystallisation (229) suggested that those amino acids of the SPDS active site that were continuously different between PMTs and SPDS were responsible for the differences in co-substrate acceptance. Mutagenesis of those two amino acids in the D. stramonium PMT with the idea of changing PMT activity into spermidine synthase, however, resulted in a complete loss of catalytic activity (88). A refined protein model of PMT constructed as a consequence of the result discouraged the concept of comparable substrate and co-substrate binding in PMT and SPDS. Rather, the substrate putrescine binds differentially at the active sites of both enzymes. 

Figure 9 Calystegine B2 in the active site of a Thermotoga maritima P-glucosidase. A p-D-glucoside is shown in an equivalent orientation to the inhibitor. Interactions between calystegine B2 and the catalytic residues in the active site of p-glucosidase are indicated by hydrogen bonds in broken lines protonation of calystegine B2 is not shown.

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