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The disulphide linkages of the 16 half-cystine residues in the sweet-tasting protein thaumatin have been investigated by enzymatic hydrolysis of the intact molecule. The peptides obtained after proteolytic cleavage with trypsin and pepsin, and in one case with chymotrypsin have been purified by gel filtration, high-performance liquid chromatography and peptide mapping by paper high-voltage electrophoresis in one direction and paper chromatography in the second dimension. Disulphide bonds appeared to be formed by cysteine residues in positions 9-204, 56-66, 71-77, 121-193, 126-177, 134-149, 145-158 and 159-164. The labile disulphide bond responsible for the enzymatic properties of the sweet tasting protein thaumatin appeared to be between Cys-145 and Cys-158.

After partial reduction of disulfide bonds in the thaumatins, the sweet-tasting proteins from the fruits of Thaumatococcus danielii Benth, a rapid autodigestion was demonstrated. In the presence of suitable substrates, the reduced thaumatins showed protease, amidase and esterase activity. Thiol-blocking reagents like mercury(II) chloride inhibited the enzymatic activity. Of the thaumatins b, c, I, II and III (with increasing isoelectric points), thaumatin I showed the lowest enzymatic activity. In this series, the enzymatic activity increased from thaumatin I to thaumatin III as well as from thaumatin I to thaumatin b. Acetylation of the epsilon-amino group of lysine residues in the thaumatins by acetic anhydride, causing a decrease in basicity, led to an increase in enzymatic activity, which is correlated with the number of acetyl groups introduced. Comparison of the amino acid sequence of thaumatin I with that of cysteine proteases of plant origin showed no similarities. Moreover, the thaumatins lack histidine, one of the amino acids in the active site of the cysteine proteases. Monellin, the sweet-tasting protein from the fruits of Dioscoreophyllum cumminsii Diels, is not enzymatically active. However, when monellin with acetylated epsilon-amino groups of lysine residues was brought into a reducing environment it appeared to be enzymatically active. The similarities in properties of the thaumatins and monellin suggest a structural relationship between these proteins.

The primary structure of the sweet-tasting protein thaumatin has been elucidated. The protein consists of a single polypeptide chain of 207 residues. The sequence of the N-terminal part of the chain was determined by sequenator analysis. As the protein contains only one methionine residue, it was possible to deduce the N-terminal sequence of the C-terminal cyanogen bromide fragment by automatic sequencing of the cyanogen-bromide-cleaved, succinylated protein. To arrive at the sequence of the whole protein tryptic and Staphylococcus protease peptides, together with chymotryptic peptides and a 2-(2-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine (BNPS-skatole) fragment were also sequenced. Comparing the amino acid sequence of thaumatin with that of the other sweet-tasting protein, monellin, we have located five sets of identical tripeptides. Since immunological cross-reactivity of thaumatin antibodies with monellin has recently been described, one or more of these tripeptides might be part of a common antibody recombination site and possibly be involved in the interaction with the sweet-taste receptor.