Contribution of structural peculiarities of onconase to its high stability and folding kinetics

Onconase (ONC) from Rana pipiens is the smallest member of the ribonuclease A (RNase A) superfamily. Despite a tertiary structure similar to RNase A, ONC is distinguished by an extremely high thermodynamic stability. In the present paper we have probed the significance of three structural regions, which exhibit structural peculiarities in comparison to RNase A, for the stability of ONC to temperature and guanidine hydrochloride induced denaturation: (i) the N-terminal pyroglutamate residue, (ii) the hydrophobic cluster between helix I and the first beta-sheet, and (iii) the C-terminal disulfide bond. For this purpose, the enzyme variants