The P34 protein is the main allergen for soybean sensitive humans. Soybean protein P34， a thiol protease belonging to the papain family， is a monomeric allergen having an N-terminal amino acid sequence and amino acid composition identical to that of the seed 34kDa protein. It is an insoluble glycoprotein having a pI of 4.5 and a calculated mass of 28.643 Dalton， representing 2–3% of total soybean protein. Upon glycosylation， the mass will be somewhat larger， resulting in a ~32kDa band in non-reduced SDS PAGE gels. It exhibits no enzymatic function due to an absence of the catalytic cysteine. P34 is stored in storage vacuoles of soybean cotyledons.
