Conformational domains and structural transitions of human von Willebrand protein

The conformational states of human von Willebrand protein (vWF) were studied by using ultraviolet (UV) difference, circular dichroism, and fluorescence spectrophotometric techniques in order to gain insight into the forces that maintain its asymmetric, flexible shape. vWF has 24% alpha-helix and 18% beta-pleated sheet structure in the native state. Disulfide bond reduction and carboxamidation reduced the beta-pleated sheet content by 50% without affecting the content of alpha-helix. In addition, the quantum yield of intrinsic (tryptophan/tyrosine) fluorescence decreased by 33% after reduction and alkylation, and the affinity of the hydrophobic fluorescent probes 8-anilino-1-naphthalenesulfonate and 6-(p-toluidino)-2-naphthalenesulfonate for vWF was reduced 2.5-fold. In contrast, intrinsic fluorescence quenching by acrylamide and the UV difference spectrum did not change following reduction. An analysis of changes in the intrinsic fluorescence polarization and the emission maximum shift induced by thermal and guanidine hydrochloride denaturation revealed single, smooth transitions for both native and reduced vWF, suggesting the existence of an ordered structure in both species. This study shows that (1) disulfide reduction and carboxamidation cause significant conformational changes in vWF, (2) vWF may contain discrete, ordered, conformational domains linked by regions of random polypeptide chain, and (3) specific tertiary structural domains within vWF are not affected by disulfide reduction and carboxamidation. This structural model would explain both the asymmetry and flexibility of the molecule.