What is gluten-why is it special?(1)

This extraction of gluten and related proteins is often performed sequentially with the four Osborne fractions being called albumins (soluble in water), globulins (soluble in dilute saline), prolamins (soluble in 60–70% alcohol), and glutelins (insoluble in the other solvents but may be extracted in alkali).

The gluten protein fraction comprises a complex mixture of components which can be separated into groups by electrophoresis. Electrophoresis of the gliadins at low pH separates four groups of bands, called (in terms of decreasing mobility) α-gliadins, β-gliadins, γ -gliadins, and ω-gliadins. The glutenin polymers are too big to be separated by conventional electrophoresis, but reduction of the inter-chain disulphide bonds that stabilize the polymers allows the subunits to be separated by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) into two groups of bands, called the HMW and LMW subunits. Comparisons of amino acid sequences of these groups of gluten protein components clarifies their relationships, showing that the HMW subunits and ω-gliadins form discrete groups, with the α-gliadins, γ-gliadins, and B- type LMW subunits forming a third group. The minor groups of C-type and D-type LMW subunits appear to be modified forms of gliadins in which mutations to form cysteine residues allow their incorporation into glutenin polymers, with the C-type LMW subunits being modified α-gliadins or γ-gliadins and the D-type modified ω- gliadins.

The most widely studied repetitive sequences are those present in the HMW subunits of glutenin. These comprise repeats based on three motifs: the hexapeptide PGQGQQ, the nonapeptide GYYPTSPQQ or GYYPTSLQQ, and in x-type subunits only, a tripeptide GQQ (P, proline; G, glycine; Q, glutamine, Y, tyrosine; P, proline; T, threonine, S, serine; L, leucine). The repetitive sequences also play a crucial role in triggering coeliac disease. All of the 31 “celiac disease relevant T-cell epitopes” are listed.

The LMW gliadins/purinins have masses of about 17–19 kDa and are more closely related to the γ-gliadins in sequence.

1. P. Shewry, What is gluten-why is it special? Front. Nutr. 6, 101 (2019).