Effect of D-amino acids on the functional activity and conformational stability of ribonuclease-A

Using cytidine 2':3' cyclic monophosphate as a substrate, Km and k(cat) of ribonuclease-A in the presence of different concentrations of D-amino acids (Ala, Ser, Pro and Lys) and their L-isomers were measured at pH 6.0 and 25 degrees C. These kinetic parameters remained unchanged in the presence and absence of D-and L-amino acids. This is the first experimental evidence showing that D-amino acids are compatible with the enzyme function. Values of Tm (midpoint of denaturation), deltaHm (enthalpy change at Tm) and deltaCp (constant-pressure heat capacity change) were also determined from the heat-induced denaturation curves of the protein, measured in the presence and absence of D- and L-isomers of an amino acid at four different pH values. It is shown for the first time that these thermodynamic parameters, within experimental errors, do not depend on the stereospecificity of an amino acid. Estimates of deltaGDo with the help of Gibbs-Helmoltz equation (deltaGDo = deltaHm (1-298.15/Tm)--deltaCp [(Tm-298.15) + 298.15 In (298.15/Tm)]) using known values of Tm, deltaHm and deltaCp suggested that D- and L-amino acids are compatible with protein stability, for deltaGDo remained unchanged in the presence of amino acids.