X-ray structure of bovine pancreatic RNase S. A nonhydrolyzable substrate analog, the dinucleotide phosphonate UpcA (orange), is bound in the active site. RNase A is a catalytically active form of RNase A in which the peptide bond between residues 20 and 21 has been hydrolyzed. The active site Lys and His side chains are shown in white and disulfide linkages are shown in yellow. [Based on an X-ray structure by Frederic Richards, Yale University.]