S. Odani et al., THE PRIMARY STRUCTURE AND CHARACTERIZATION OF CARBOHYDRATE CHAINS OF THE EXTRACELLULAR GLYCOPROTEIN PROTEINASE-INHIBITOR FROM LATEX OF CARICA-PAPAYA, European journal of biochemistry, 241(1), 1996, pp. 77-82
A secretory proteinase inhibitor was isolated from the latex of green
fruits of papaya (Carica papaya). The protein exhibited stoichiometric
inhibition of bovine trypsin and alpha-chymotrypsin by the same site
or overlapping binding sites. The complete covalent structure consisti
ng of 184 amino acids and two disulfide bonds was determined by protei
n analysis. During the structural analysis, a procedure was establishe
d to separate very hydrophilic peptides by reverse-phase HPLC. The res
ult revealed that the latex protein belongs to an extensively diverse
plant protein family that includes inhibitors of serine, cysteine and
aspartic proteases, a taste-modifying protein, wound responsive protei
ns, storage proteins, amylase inhibitors and even an oxidoreductase. I
n this superfamily, the latex proteinase inhibitor is most similar to
the curious protein, miraculin, which makes sour food taste sweet. Two
carbohydrate chains, each probably composed of (mannose)(5), (xylose)
(1), (fucose)(0-2), and (N-acetylglucosamine)(2) residues, were attach
ed to asparagine 84 and 90. Mass-spectrometric and compositional analy
sis suggested that they may represent a new class of plant xylose-cont
aining carbohydrate chains with five mannose residues.