Contributions of Oxyanion Holes to Catalysis

Bond type Calculated strength in the gas phase (k)/mol) Experimental strength in the gas phase (k)/mol) Calculated distance dn- B ( ) Calculation method/notes 

H2OH+ 3OH2 (first solvation shell for H3O ) -290.22 [10] -287.7 [11] N/A Eigen cation MP2 

The contributions of hydrogen bond donors to catalysis can be estimated by site-directed mutagenesis studies in cases where the hydrogen bond donor is located in the amino acid side chain. Deletion of the main chain NH is only possible by substituting the amino acid with a proUne. In all cases, the effects of the substitution to key enzyme kinetic parameters, and K, should be checked. Typically, the oxyanion hole residues contribute only Uttle to the binding of substrate [19-21]. This is reflected in the values, which typically remain very similar 

Mutated or deleted hydrogen bond donor site AAC (kj/mol) 

The conversion of the kinetic data into AAG -values (Table 4.2) assumes that the rate-limiting step is the same in wild type and variant. It also assumes that the mutation does not cause structural rearrangements. Only in very few cases have the kinetic studies on the transition state stabilization by the oxyanion hole contributions been complemented by protein crystallographic studies of the liganded wild-type and mutated variant. One such example, discussed in more detail below, concerns the studies on the Ser42Ala variant of cutinase, in which case it was found that the structural changes are minimal [19]. 

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