Members of the papain family are wide-spread in nature, having been found in baculovirus, eubacteria, yeast, plants and animals. Despite structural similarities regarding the residues surrounding the catalytic site, the cysteine proteases have only low levels of overall sequence similarity . Papain-like cysteine proteinases are synthesised as inactive proenzymes with N-terminal propeptide regions. The propeptid plays important roles as inhibitor of enzymatic activity and for the correct folding of the newly synthesized protein . The mature enzymes are generally 25-28 kDa in size.
A number of allergens have been identified belonging to the papain family including inhalant allergens such as the major dust mite allergen Der p 1  and food allergens such as actinidin, the major allergen from kiwifruit, bromelain from pineapple, ficin from fig and papain from papaya . Gly m Bk30K (P34), a major allergen from soybean seed storage vacuoles, shows sequence similarity to papain-like proteases but lacks their enzymatic activity.
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