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 active site of papain is a catalytic dyad of cysteine (yellow) and histidine (pink) side chains. Papain is a commercial product, best know as a meat tenderizer., by GREG WILLIAMS/SCIENCE PHOTO LIBRARY

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