Comparative investigations of gluten proteins from different wheat species

Abstract

Flours of different wheat species (common wheats including winter wheat, spring wheat, and wheat rye hybrid, spelt, durum wheat, emmer, and einkorn) were successively extracted with a salt solution and 60% (v/v) aqueous ethanol. The alcohol extracts (gliadins) were separated by reversed-phase HPLC. Six to nine different ω-gliadins were obtained for each wheat sample and were characterized by their relative amounts, the amino acid compositions, the N-terminal amino acid sequences, and the molecular masses. The wheats investigated showed typical differences in the qualitative and quantitative HPLC patterns. The amino acid compositions of all ω-type gliadins revealed significantly higher proportions of glutamine, proline, and phenylalanine compared with other gluten proteins. These three amino acids accounted for 70 to 86% of the total composition. Typical differences in amino acid compositions, N-terminal sequences, and molecular masses allowed a clear differentiation of the proteins into ω5- and ω1,2-type gliadins; the gliadin fractions of emmer and einkorn contained only the ω5-type, but not the ω1,2-type. ω5-Gliadins were characterized by extremely high proportions of glutamine (52–57 mol %) and relatively high proportions of proline (18–21 mol %) and phenylalanine (9–10 mol %). ω1,2-Gliadins had less glutamine (39–45 mol %) and phenylalanine (6–8 mol %), but much more proline (22–31 mol %). ω5-Gliadins of all wheats except emmer could be assigned to the N-terminal sequence variants SRQLSP or SRLLSP; the typical sequence of emmer ω5-gliadins was SMELQT. The N-terminal sequences of ω1,2-gliadins were characterized by two basic variants beginning with KELQSP or ARQLNP. The wheat rye hybrid had additionally components with the sequence RQLNPS known from ω-secalins of rye. The determination of molecular masses by MALDI-TOF mass spectrometry revealed a range of 44,000–55,000 for the ω5-type and a range of 36,000–44,000 for the ω1,2-type. Thus, the actual masses were by far lower than the values derived from SDS-PAGE mobility.