Wheat seed proteins exhibit a complex mechanism of protein elasticity

Elastomeric proteins are found in a number of animal tissues (elastin, abdu ctin and resilin), where they have evolved to fulfil a range of biological functions. All exhibit rubber-like elasticity, undergoing deformation witho ut rupture, storing the energy involved in deformation, and then recovering to their initial state when the stress is removed. The second part of the process is passive, entropy decreasing when the proteins are deformed, with the higher entropy of the relaxed state providing the driving force for re coil. In plants there is only one well-documented elastomeric protein syste m, the alcohol-soluble seed storage proteins (gluten) of wheat. The elastic properties of these proteins have no known biological role, the proteins a cting as a store for the germinating seed. Here we show that the modulus of elasticity of a group of wheat gluten subunits, when cross-linked by gamma -radiation, is similar to that of the cross-linked polypentapeptide of ela stin. However, thermoelasticity studies indicate that the mechanism of elas tic recoil is different from elastin and other characterized protein elasto mers. Elastomeric force, f, has two components, an internal energy componen t, f(e), and an entropic component, f(s). The ratio f(e)/f can be determine d experimentally; if this ratio is less than 0.5 the elastomeric force is p redominantly entropic in origin. The ratio was determined as 5.6 for the cr oss-linked high M-r subunits of wheat glutenin and near zero for the cross- linked polypentapeptide of elastin. Tensile stress must be entropic or ener getic in origin, the results would suggest that elastic recoil in the wheat gluten subunits, in part, may be associated with extensive hydrogen bondin g within and between subunits and that entropic and energetic mechanisms co ntribute to the observed elasticity. (C) 2001 Elsevier Science BN. All righ ts reserved.