1H‐nmr studies on the structure of a new thionin from barley endosperm

Abstract

A new thionin from barley, ω‐hordothionin, has been shown to exist in aqueous solution as a mixture of two different isoforms in a 3:2 ratio, as revealed by a complete analysis of its two‐dimensional ¹H‐nmr spectra. The conformational heterogeneity arises frtm cis–trans isomerism ahout the Phe 12–Pro 13 peptide bond, where the major, form corresponds to the cis conformation. The complete assignment of chemical shifts and nuclear Overhaiiser effects (NOES) of the two isoforms allow a detailed comparative analysis of their conformational properties, even though a complete calculation of their solution structures is not possible because of a somewhat limited number of NOE constraints. Structures for the two isomers could be modeled, however, on the basis of the high structural homology between ω‐hordothionin and related γ‐thionins, and under the conditions of satisfying all observed experimental data. The two isoforms adopt practically identical global folds and the structural changes imposed by cis–trans isomerization are confined to the region proximal to Pro 13. The cis–trans isomerism occurs in a conserved loop connecting the first β‐strand of the triple‐stranded antiparallel β‐sheet and the α‐helix. A comparative analysis of the sequences of this loop in the different thionins suggests that the cis–trans equilibrium about the X‐Pro peptide bond depends on the size of the side chain of X (X = Gly in γ‐thionins and Phe in ω‐thionin). The structural homology of this new thionin with γ‐thionins as well as with some scorpion toxins and insect defensins suggests that these proteins may share a common mode of functional activity. © 1995 John Wiley & Sons, Inc.