Glycerol-enhanced detection of a preferential structure latent in unstructured 1SS-variants of lysozyme

Four species of 1SS-varinats of lysozyme were almost unstructured in water, judged from their near-UV CD and (1) H-(15) N-HSQC spectra. Some preferential structure might exist in such a disordered state, but the population of molecules in such a conformation must have been too small to be detected by spectroscopic methods. Indeed, our previous study showed that the addition of 30% glycerol induced the unstructured 2SS-variant of lysozyme to form a native-like structure. To extend this method to more disordered proteins, we attempted to detect some preferential structure latent in unstructured 1SS-variants by the glycerol-enhanced detection. Only in one molecular species of the four 1SS-variants, 1SS[6-127] containing a single disulfide bridge of Cys6-Cys127, a preferential structure was found in the presence of 50% glycerol. It was detected by near-UV CD measurements and the H/D exchange method combined with the NMR spectroscopy. The glycerol-induced structure in 1SS[6-127] was not localized only in the vicinity of Cys6-Cys127, and largely protected regions distributed themselves among A-, B-, and C-helices and Ile55 and Leu56. It was similar to the glycerol-induced structure in 2SS[6-127, 64-80] containing two disulfide bridges of Cys6-Cys127 and Cys64-Cys80, although the former was less rigid than the latter. The role of A-helix (residues 4-15) is proposed as an origin of excellent potential of Cys6-Cys127 for inducing a tertiary structure in the α-domain.