Blocked thiols in glutenin and protein quality

It has already been suggested that when glutenin chains are synthesised, two cysteine residues on each chain preferentially form interchain disulphide bonds (SS) resulting in linear arrays of polypeptide chains. Presumably steric factors stop the growth of a molecule eventually, leaving each terminal chain with a free SH group. It is now postulated that the free SH groups later react with low molecular weight (mol. wt) thiols and SS compounds, which also break some interchain bonds by SS interchange so reducing the average mol. wt of the glutenin. Terminal SH groups and the halves of each broken SS become blocked by low mol. wt compounds. Therefore, if the quantity of glutenin‐bound low mol. wt thiols (end‐blockers) bound to glutenin could be measured it would enable the number of glutenin molecules to be calculated and hence a number‐average mol. wt. When six wheats spanning the normal range of baking quality were tested, glutenin‐bound half‐cystine and glutathione residues were eventually found, but no SH groups could be detected. The number‐average number of chains per glutenin molecule (proportional to mol. wt) was correlated with intrinsic viscosity of glutenin and with loaf volume per gram of loaf protein, thus supporting the hypothesis. It is suggested that the main aim of reducing the native thiol content of flour with improvers is to limit degradation of glutenin molecules. If the hypothesis should be confirmed it may be advantageous to breed wheats with low contents of thiols and active disulphides in the grain, since their quantities may largely account for glutenin mol. wt and, in turn, protein quality.