Papain cysteine protease, molecular model. Papain is a protease enzyme native to papaya. There are many varieties of papain, found in a wide range of organisms. The catalytic site of papain is a catalytic dyad of cysteine and histidine side chains, shown in ball-and-stick style. The cysteine protease motif is common in biochemistry. Atoms are colour-coded: carbon is black, hydrogen is white, nitrogen is blue, and sulphur is yellow., by GREG WILLIAMS/SCIENCE PHOTO LIBRARY

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