Study of RNase A mechanism and folding by means of synthetic 63-residue analogs.

A 63-residue RNase A analog containing residues 26 to 35 then alanine, 41 to 59 and 73 to 84 then glycine, 100 to 110 then glycine, and 117 to 124 was synthesized by the solid phase method. The deletions comprised ordered (an alpha helix, parts of the beta sheet) and less ordered structures including 27 of the 56 residues invariant in mammalian ribonucleases. The synthetic 63-residue analog was cleaved from the insoluble support with liquid HF, reduced-reoxidized, fractionated by gel filtration, and purified further on an affinity column specific for the active site fold of RNase A. It had an activity of 8 to 14 per cent in the transphosphorylation step using poly(C) and poly(U) as substrates. It also had low synthetic and hydrolytic activity (0.2 per cent) and showed RNase A-like specificity toward the substrates tested. This indicated that all residues essential for substrate binding and catalysis were present and that their relative positions in the three-dimensional structure were probably very similar to those in native RNase A. Therefore, structure-function studies with the 63-residue RNase A analog should allow conclusions about the mode of action of the natural enzyme. As a first step in this direction, lysine 41 which is believed to be important for catalysis was replaced in the 63-residue analog by tyrosine or glutamine. The resulting (Tyr-41)- and (Gln 41)-63-residue analogs were also bound by the affinity column and had the same substrate specificity as native RNase A. They differed from each other, from the (Lys 41)-63-residue analog, and the 124-residue natural enzyme only with respect to the relative rates of the catalyzed reactions. Thus, lysine 41 does not seem to be essential for the functioning of RNase A.