Molten globule versus variety of intermediates: influence of anions on pH-denatured apomyoglobin

The molten globule state was shown to be the third thermodynamic state of p rotein molecules in addition to their native and unfolded states. On the ot her hand, it was reported that optical and hydrodynamic properties of pH-de natured apomyoglobin depend on the nature of anions added to the protein so lution, This observation was used to conclude that there are many 'partly f olded' intermediates between the native and unfolded states rather than one distinct molten globule state, However, little is known on the structures of pH-denatured apomyoglobin in the presence of different anions, Two tyros ine residues in horse apomyoglobin have been successively modified by the r eaction with tetranitromethane, This approach was employed to measure the d istances between tryptophans and modified tyrosines in different states of apomyoglobin by the method of direct energy transfer. Experimental data sho w that the distance between the middle of the A-helix and the beginning of the G-helix and/or the end of the H-helix in 'anion-induced' states are ver y close to those in the native holo- and apomyoglobins. This suggests that the AGH helical complex, being the most structured part of apomyoglobin in the molten globule state, exists also in pH-denatured apomyoglobin in the p resence of different anions, Consequently, all non-native forms of apomyogl obin studied so far share the common important feature of its native struct ure, (C) 1999 Federation of European Biochemical Societies.