Disulphide bonds formation from cysteine

Figure 6.1. The role of HMW subunits in gluten structure and functionality. Amino acid sequences derived from direct analysis of purified proteins and the isolation and sequencing of corresponding genes show that the proteins have highly conserved structures, with repetitive domains flanked by shorter nonrepetitive domains containing cysteine residues (SH) available for formation of interchain disulphide bonds. Molecular modelling indicates that the individual repetitive domains form a loose spiral structure (bottom right) while SPM shows that they interact by noncovalent forces to form fibrils (centre right). Includes figures from Parchment et al. (2001) and Humphries et al. (2000).

Little is known about the precise organisation of the HMW and LMW subunits in glutenin polymers, but most researchers consider that the HMW subunits come from a disulphide-bonded network, with the LMW subunits acting as branches, possibly also providing some cross-links. The formation of such a network by the HMW subunits would be facilitated by the distribution of cysteine residues, which are predominantly located in the N-terminal (3 or 5 cysteines) and C-terminal (1 cysteine) domains (Figure 13.12). It would be possible for the HMW subunits to form polymers via head-to-tail disulphide bonds, with some cross-links (either directly or via LMW subunits) and branches. In addition, one or two intra-chain disulphide bonds could be formed within the N-terminal domains. The number and distribution of crosslinks within these glutenin polymers would undoubtedly contribute to their elastic properties. 

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