Residues Distal from the Active Site that Alter Enzyme Function in M.HhaI DNA Cytosine Methyltransferase |
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Authors: | Vyas Sharma Ben Youngblood Norbert Reich |
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Affiliation: | Department of Chemistry and Biochemistry , University of California , Santa Barbara , CA , 93106 , USA |
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Abstract: | Abstract Ten M.HhaI residues were replaced with alanine to probe the importance of distal protein elements to substrate/cofactor binding21,methyl transfer, and product release. The substitutions, ranging from 6–20 Å from the active site were evaluated by thermodynamic analysis, pre-steady and steady-state kinetics, to obtain Kd AdoMet, Kd DNA, kcat/Km DNA, kcat, and kmethyltransfer values. For the wild-type M.HhaI, product release steps dominate catalytic turnover while the 4-fold faster internal microscopic constant kmethyltransfer presents an upper limit. The methyl transfer reaction has δH? and δS? values of 10.3 kcal/mol and—29.4 cal/(mol K), respectively, consistent with a compressed transition state similar to that observed in the gas phase. Although the ten mutants remained largely unperturbed in methyl transfer, long-range effects influencing substrate/cofactor binding and product release were observed. Positive enhancements were seen in Asp73Ala, which showed a 25fold improvement in AdoMet affinity and in Val282Ala, which showed a 4-fold improvement in catalytic turnover. Based on an analysis of the positional probability within the C5- cytosine DNA methyltransferase family we propose that certain conserved distal residues may be important in mediating long-range effects. |
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