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From Proteopedia

Papain Protein

Introduction

Papain, also known as papaya proteinase I, is a cystaine protease enzyme in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis)[wikipedia]. Papain is present in the leaves, latex, roots and fruit of the papaya plant [Britannica]. The latex of Carica papaya is a rich source of four cysteine endopeptidases including papain, chymopapain, glycyl endopeptidase, and caricain. The proteins are synthesized as inactive precursors that become active within two minutes of the plant being wounded and the latex expelled. Papain is a minor constituent, but it has been more widely studies because it is more easily purified [Worthington].

It is a sulfhydryl protease from the latex of the papaya fruit. Its molecules consist of one polypeptide chain with 212 amino acid residues. The chain is folded into with the in a groove between the domains.