Application to the staphylococcal nuclease reaction

Staphylococcal nuclease (SNase) is a single peptide chain enzyme consisting of 149 amino acid residues. It catalyses the hydrolysis of both DNA and RNA at the 5 position of the phosphodiester bond, yielding a free 5 -hydroxyl group and a 3 -phosphate monoester [13] 

Based on this inhibited structure, a reaction mechanism for the enzyme has been postulated [15b] (i) General base catalysis by Glu 43 which accepts a proton from a (crystallographically observed) water molecule in the second ligand sphere of the Ca ion, yielding a free hydroxide ion. (ii) Nucleophilic attack by the OH ion on the phosphorous atom in line with the 5 -0-P ester 

The overall catalytic rate constant of SNase is k = 95s at T = 297K corresponding to a total free energy barrier of (= 14.9 kcal/mol. This should be compared to the pseudo-first-order rate constant for non-enzymatic hydrolysis of phosphodiester (with a water molecule as the attacking nucleophile) which is 2 X 10 s S corresponding to = 36 kcal/mol [16]. 

The rate acceleration accomplished by the enzyme is thus 10 -10, which is quite impressive. 

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