Gluten is the seed storage protein in mature wheat seeds (and in the seeds of closely related species). It is the sticky substance in bread wheat which allows dough to rise and retain its shape during baking. The same, or very similar, proteins are also found in related grasses within the tribe Triticeae. Seed glutens of some non-Triticeae plants have similar properties, but none can perform on a par with those of the Triticeae taxa, particularly the Triticum species (bread wheat, durum wheat, etc.). What distinguishes bread wheat from these other grass seeds is the quantity of these proteins and the level of subcomponents, with bread wheat having the highest protein content and a complex mixture of proteins derived from three grass species (Aegilops speltoides, Aegilops tauschii strangulata, and Triticum monococcum).
Triticeae seed proteins fall into four groups: albumins – soluble in hypotonic solutions and are coagulated by heat globulins – soluble on 'isotonic' solutions prolamins – soluble in aqueous alcohol glutelins – are soluble in dilute acids or bases, detergents, chaotropic or reducing agents.
Of these proteins the last two, prolamin (in wheat – gliadin) and glutelin (in wheat – glutenin) form the classically defined gluten components in wheat.
Triticeae glutens are primarily important to a developing definition 'gluten-free' in dietary treatments for gluten sensitivity which are intended to exclude pathogenic proteins from the diet of susceptible individuals (namely coeliac disease). The poisonous motifs appear to be spread widely in Triticeae, but not other taxa, for most coeliacs. However all four proteins are involved in wheat allergies, and proteins from non-wheats may not be involved in certain gluten allergies, or in idiopathic sensitivities.
Prolamins and glutelins
Proteins of the Triticeae endosperm that are generally rich in arginine, proline, glutamine, and/or asparagine. Prolamins Triticum (true wheats) – gliadins Hordeum (food barleys) – hordeins (B-hordein is homologous to LMW-glutenin) Secale (food ryes) – secalins glutelins Triticum – glutenin Hordeum – barley glutelin Secale – rye glutelin
Genetics of prolamins and glutelins
Because of the usefulness of wheat glutens, genetic studies have largely focused on wheat genetics. Wheat has three genomes (AABBDD) and it can encode for many variations of the same protein, even in the gliadin subcategories many types of gliadin per cultivar, X = genome (A, B, or D genome chromosomes (1 to 7)). The A and B genomes are derived from wild emmers wheat which in turn is a natural digenomic species that contains a Triticum monococcum- and Aegilops speltoides-like genome. The D genome is derived from the extant species Aegilops tauschii strangulatum.
The genetic studies indicate that in wheat, each protein type can be encoded by several loci and several different alleles for each loci can be found in different genomes, allowing a great number of uniquely encoded isoforms.
Biochemistry of Triticeae prolamins and glutelins
Chemical behavior
Gliadins, an example of the prolamins in Triticeae, are separated on the basis of electrophoretic mobility and isoelectric focusing α-/β-gliadins – soluble in low percentage alcohols γ-gliadins – ancestral form of cysteine rich gliadin with only intrachain disulfide ω-gliadins – soluble in higher percentages of alcohol and acidic acetonitrile Cultivar glutelins in Triticeae Glutenin is 35-40% of wheat (Triticum aestivum) protein. Glutenin in wheat forms long covalantly interlinked polymers of two repeating subunits High molecular weight (HMW) – proline-less (Glu-1 locus) Low molecular weight (LMW) – α-gliadin-like polypeptide (Glu-3 locus) Barley (Hordeum) has two glutelins, soluble at high pH, precipitates at low pH α-glutelin (major component, HMW) – cuts at 1 to 3% rel. saturation ammonium sulfate β-glutelin (minor component) – cuts at 18% rel. saturation ammonium sulfate Rye (Secalin) has one glutelin HMW – (equivalent of Barley α-glutelin) LMW – subspecies sylvestre has (Glu-R3) glutenin-like (Ssy1, Ssy2 and Ssy3 loci)
