Branched chains disulphides

Unlike polysaccharides, proteins do not have branched chains, but several chains may be linked together via disulphide bridges rather than peptide bonds. The primary structure of ox insulin is shown in Fig. S.A2. The protein consists of two peptide chains which are linked via the formation of the disulphide bridges. Disulphide bridges are formed by the condensation of the thiol groups of two cysteine residues. 

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. 

The mannan is in the form of a mannoprotein, with large blocks of branching mannan attached by V-glycosidic links to asparagine and smaller chains linked by the mannosyl-serine bond, which seems peculiar to fungi (see Chapters 1 and 4 for details of the structure and its biosynthesis). ITie polypeptide part of the mannoprotein is cross-linked in the cell wall by way of disulphide bridges, while the mannan blocks interlink by way of phosphate bridges. In the model of Kidby and Davies (1970) these cross-links lead to the... 

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