Investigation of the disulphide-bond network of hordeins in barley

In wheat, gluten proteins mainly consist of glutenins and gliadins. The glutenins are held together via a network of disulphide-bonds, consisting of a backbone of S-S linked X and Y-type high-molecular weight glutenin subunits (HMW-GS) with further connection to low molecular weight glutenin subunits (LMW-GS) scattered among the backbone, while the gliadins primarily participate in the network via hydrophobic interactions.<br/>Barley shows some advantages as cereal crop compared to wheat including more resilience and high nutritional value but lacks the good gluten-related baking qualities of wheat. The barley gluten homologs are hordeins, with D- and B hordeins being homologues to HMW- and LMW-GS, respectively. It is, however, unknown if a comparable disulphide-bonded network exists between them and, furthermore, whether the barley gliadin homologues, the C- and γ-hordeins, participate in the network.<br/>In this study the Hordeum vulgare barley varieties PS3 and Golden Promise, a baking and a brewing variety, respectively, were chosen for further investigation of the relationship between disulphide network and functional properties. The proteome network of non-reduced, hordein-enriched samples was mapped using bottom-up LC-MS analysis. The peptides were identified and mapped to a barley database using Sequest HT and the cysteine bridges were found and validated by MS Annika. The peptides contain interconnected cysteine bridges were further manually validated using predesigned criteria.<br/>Multiple cysteine bridges belonging to several different seed protein were identified and the first barley dulsulphite network model was created. The barley disulphide network suggested similarities to the network in wheat. The results suggest that D-hordein creates a backbone structure similar to the X/Y HMW-GS, but with B- and γ-hordeins interspersed along the network, despite D-hordein only having type and γ-hordein being the homolog to gliadins and not the LMW-GS. This is surprising as the current wheat disulphide model only include HMW-GS and LMW-GS as S-S linked, indicating an underlying difference between the wheat and barley network. <br/>Models of the disulphide network may help accelerate the development of barley varieties with better baking qualities closing the gap in baking quality and thereby encouraging more people to consume barley and benefit from its health effects. <br/>