Conformational comparison between alpha-lactalbumin and lysozyme.

Comparisons of the conformation behavior between LA and LZ (especially BLA and HEL) are described, which have contributed to the development and understanding of the molten globule folding intermediate of protein. BLA (and other LAs) is clearly a very good model protein for fundamental studies of molten globule protein conformation and the mechanism of protein unfolding/folding in vitro. Our work for about 20 years, on the conformational characterizations of proteins in the LA-LZ family, primarily based on macroscopic observations, led to three important findings: 1) equilibrium intermediate of unfolding/folding of LA is in the molten globule state, as assumed later for many proteins; 2) kinetic (transient) folding intermediates of BLA and HEL are the same as the equilibrium intermediate of BLA, and they include the framework remaining in the N state; and 3) the bound Ca2+, which is the main factor explaining the apparent difference in the conformational behavior between BLA and HEL, is found in some LZs. The conformational characterization of two Ca(2+)-binding LZs (ELZ and PLZ) by CD, calorimetry, X-ray, and NMR structural analysis is shown in detail and the mechanism of molecular evolution is discussed for the LA-LZ family. However, first concept of the molten globule has gradually changed with the definition of the collapsed form, the N-like or the U-like molten globule and so on, based on the results of measurements at residual resolutions including X-ray, NMR, and isotope exchange of some proteins. Our recent data on such microscopic measurements of LAs in the molten globule state are therefore compared with those of various proteins by other researchers. The roles of the disulfide bonds to stabilize the molten globule state are also discussed with our conformational research results on disulfide-reduced BLA. It is hoped that the multidimensional-NMR can be used for the 3D structural analysis of proteins in the LA-LZ family in the U, I, and N states in the solutions, and that our conformational work on the proteins done in vitro can be applied to understand the conformational events of proteins in vivo.