ROLE OF ACTIVE-SITE RESIDUES IN THE CATALYTIC MECHANISM OF RIBONUCLEASE-A

The electronic roles of His-12, Lys-41, Asn-44, His-119, the 119-120 backbone N.sbd.H, and Asp-121 in the transphosphorylation stage of the RNase A mechanism are assigned by combining known experimental facts with electronic structure theory calculations. His-12 is more likely to deprotonate O2'', but in its absence Asn-44 may fulfill this role; His-119 activates the leaving group and facilitates in-line addition of O2''; the 119-120 backbone N.sbd.H increases the electrophilicity of phosphorus; Gln-11.cntdot..cntdot..cntdot.H2O functions in a manner similar to the 119-120 backbone N.sbd.H; Lys-41 increases the phosphorus electrophilicity and stabilizes the trigonal-bipyramidal intermediate; Asp-121 may position the substrate, but does not act as a charge relay with His-119.