Gliadins from wheat grain: an overview, from primary structure to nanostructures of aggregates

Abstract

Gliadins are well-known wheat grain proteins, particularly important in food science. They were studied as early as the 1700s. Despite their long history, it has been difficult to identify their higher-order structure as they aggregate in aqueous solution. Consequently, most studies have been performed by extracting the proteins in 70% ethanol or dilute acidic solutions. The carboxy-terminal half of α- and γ-gliadins have α-helix-rich secondary structures stabilized with intramolecular disulfide bonds, which are present in either aqueous ethanol or pure water. The amino-terminal-repeat region of α- and γ-gliadins has poly-L-proline II and β-reverse-turn structures. ω-Gliadins also have poly-L-proline II and β-reverse-turn structures, but no α-helix structure. The size and shape of gliadin molecules have been determined by assessing a variety of parameters: their sedimentation velocity in the analytical ultracentrifuge, intrinsic viscosity, small-angle X-ray scattering profile, and images of the proteins from scanning probe microscopes such as a tunneling electron microscope and atomic force microscope. Models for gliadins are either rods or prolate ellipsoids whether in aqueous ethanol, dilute acid, or pure water. Recently, gliadins have been shown to be soluble in pure water, and a novel extraction method into pure water has been established. This has made it possible to analyze gliadins in pure water at neutral pH, and permitted the characterization of hydrated gliadins. They formed hierarchical nanoscale structures with internal density fluctuations at high protein concentrations.

Keywords: Gliadin; Nanostructure; Protein aggregate; SAXS; Wheat protein.

Fig. 1

Traditional (top) and molecular classification (bottom) of wheat grain proteins. (Adapted from Shewry et al. 1986)

Fig. 2

Schematic representation of α-gliadins, γ-gliadins, and ω-gliadins. SIG signal peptide; N N-terminal region; R repetitive domain (small boxes indicate repeat motifs). C1, CII, CIII and C represent C-terminal subregions. Black boxes in α- gliadins represent polyglutamine peptides; numbers indicate cysteine residues; and lines connecting numeric characters indicate disulfide bonds

Fig. 3

SAXS profiles of gliadins in distilled water. a Solution components at low concentrations; b gel-like solid samples at high concentrations. The curves are vertically shifted for clarity. (From Sato et al. 2015)

Fig. 4

Schematic illustrations of the nanostructures of gliadin assemblies in distilled water over a wide range of concentrations. a Gliadins are present as water-soluble isolated monomers and a few dimers at 0.025–0.5 wt%. b Gliadin molecules self-assemble to form small aggregates (dashed line circles) at 0.5–10 wt%. The distance between the aggregates is estimated to be ∼40 nm. c Gliadins begin to form continuous networks at 15–20 wt%. d Gliadin molecules fill the space, but condensed regions due to density fluctuation (dashed line circles) appear above 30 wt%. The distance between dense domains is estimated to be 14 nm at 40 wt%. e Above 50 wt%, the density fluctuation almost vanishes, but large aggregates over 100 nm in size are formed