Onconase(®) (ONC) is an amphibian member of the
pancreatic ribonuclease superfamily that is selectively toxic to
tumor cells. It is a much less efficient
enzyme than the archetypal
ribonuclease A and, in an attempt to gain further insight, we report the first atomic resolution crystal structure of ONC, determined in complex with
sulfate ions at 100 K. The electron density map is of a quality sufficient to reveal significant nonplanarity in several
peptide bonds. The majority of active site residues are very well defined, with the exceptions being Lys31 from the catalytic triad and Lys33 from the B(1) subsite, which are relatively mobile but rigidify upon
nucleotide binding. Cryocooling causes a compaction of the unit cell and the
protein contained within. This is principally the result of an inward movement of one of the lobes of the
enzyme (lobe 2), which also narrows the active site cleft. Binding a
nucleotide in place of
sulfate is associated with an approximately perpendicular movement of lobe 2 and has little further effect on the cleft width. Aspects of this deformation are present in the principal axes of anisotropy extracted from C(α) atomic displacement parameters, indicating its intrinsic nature. The three lowest-frequency modes of ONC motion predicted by an anisotropic network model are compaction/expansion variations in which lobe 2 is the prime mover. Two of these have high similarity to the cryocooling response and imply that the essential 'breathing' motion of
ribonuclease A is conserved in ONC. Instead, shifts in conformational equilibria may contribute to the reduced ribonucleolytic activity of ONC.