Mechanistic insights into tyrosinase-mediated crosslinking of soy glycinin derived peptides.

Tyrosinase from Bacillus megaterium (TyrBm) was previously used to modulate soy glycinin-based emulsions and gels. To study the crosslinking mechanism, TyrBm oxidation of three tyrosine-containing octapeptides derived from glycinin was analyzed by oxygen consumption measurements, absorbance and mass spectrometry. A significant lag period and lower activity were measured when tyrosine was located in the middle of the peptide chain. Mass spectrometry analysis showed that these peptides are crosslinked via the oxidative quinone ring of the tyrosine residue by aryl-alkylamine addition or aryloxy radical coupling to form di-DOPA (3,4-dihydroxyphenylalanine). In contrast, peptides containing tyrosine in the N- or C-terminus, were rapidly oxidized forming multimer units within thirty minutes. When small amino acids were adjacent to the terminus tyrosine, formation of di-tyrosine was observed. This work confirms that protein crosslinking by TyrBm occurs by several chemical mechanisms and may assist in designing peptide-based inhibitors for the food and cosmetic applications.