Tyrosine-containing model peptides were oxidized by horseradish peroxidase (HRP). This led to a peptide polymerization via condensation of the aromatic rings. Dimers, trimers, and tetramers (depending on the peptide length and on the position of the tyrosine in the sequence) were identified by electron spray mass spectroscopy. The second-order rate constants of the second step of the HRP reduction (CII --> E) was decreased by the presence of a positively charged amino group in the vicinity of the aromatic ring as determined by stopped flow measurements [k3 = 19 398 M-1 s-1 and k3 = 1016 M-1 s-1 for N-acetyltyrosine (NAT) and l-Tyr oxidations, respectively]. High-performance liquid chromatography was used to follow the kinetics of polymerization of some model peptides after their enzymatic oxidation. The first polymerization products exhibited a strong inhibitory effect toward further oxidation by HRP. This effect was not observed when using manganese-dependent peroxidase (MnP) which does not bind directly to the tyrosine residue but rather acts as a "distant catalyst". Saturation of the HRP was achieved with Pro-Gln-Gln-Pro-Tyr (kcat = 58 s-1, = 2.1 mM), NAT (kcat = 94 s-1, = 5.6 mM), and Gly-Tyr (kcat = 175 s-1, = 10.8 mM). Analysis of steady state kinetics of the reaction showed that the dimers formed initially behaved like competitive inhibitors. The value of the dissociation constant between HRP and dimers was 20 microM. A simplified model which accounts for these observations, including the formation of a Michaelis-Menten-like complex involving the donor and enzyme, is proposed and discussed.

Wheat gliadins, that are glutamine rich and lysine poor proteins, are good substrates for transglutaminases reactions. This study was conducted to determine the efficiency with which guinea pig liver transglutaminase catalyzes transfer and hydrolysis reactions of native and acylated gliadins. In all reactions, 35% of the total glutaminyl residues were modified. Neutral pH simultaneously enhanced glutaminyl residue hydrolysis and protein cross‐linking, while acidic pH reduced the cross‐linking reaction. Functional properties of two enzymatically modified gliadins and a chemically deamidated one were tested at neutral pH. A deamidation level of 25–27% appeared to be an optimum for the emulsification properties. Enzymatically modified gliadin showed better resistance to coalescence than the chemically deamidated one; a result that probably is related to the presence of high molecular weight polymers.