Pectinesterase. The primary structure of the tomato enzyme

The first primary structure of a pectinesterase has been determined by analysis of the main form of this plant enzyme from tomatoes. The analysis was untraditional in the sense that few data were obtained by frequently used methods. Thus, digestion with trypsin, Glu-specific protease and several other enzymes gave limited cleavages and highly insoluble products. Instead, the determination was to a large extent based on peptides derived from chemical cleavages with CNBr at Met, N-chlorosuccinimide at Trp and hydroxylamine at Asn-Gly, plus an enzymatic cleavage at modified cysteine residues after chemical derivatizations. The structure shows the protein chain to be 305 residues long, with four half-cystine residues and five tryptophan residues. The N-terminus has a free alpha-amino group (from isoleucine). Two types of residue, tyrosine and histidine, have been functionally implied in the catalytic activity of the enzyme. Tyrosine is common in the protein (at 18 positions). However, only two histidine residues are found, and both are close to tyrosine residues in the primary structure (His-128 separated by one intervening residue from Tyr-126, and His-269 adjacent to Tyr-268), defining segment(s) of possible interest in relation to the active site.