Pressure-induced perturbation of apomyoglobin structure: fluorescence studies on native and acidic compact forms

The nature of the structural changes that apomyoglobin undergoes when subjected to hydrostatic pressure, ranging from atmospheric pressure to 2.4 kbar, has been investigated by steady-state fluorescence and frequency domain fluorometry. In particular, we have examined the intrinsic tryptophanyl emission and that of the extrinsic probe 1-anilino-8-naphthalenesulfonate (ANS) bound to apomyoglobin at neutral pH, as well as at strongly acidic high-salt conditions. Apomyoglobin at neutral pH undergoes a pressure-induced structural transition, which causes the disorganization of the heme binding region with a consequent ANS dissociation; a concomitant increase in solvent accessibility to the N-terminus of the macromolecule in which tryptophans are located is also observed. At 2.4 kbar, the tryptophanyl emission is not coincident with that of a fully solvent exposed residue, thus suggesting that the N-terminal region of the apomyoglobin molecule retains elements of organized structure. The spectroscopic properties of the structural state attained at 2.4 kbar and neutral pH are different from those of the acidic compact state. The acidic compact state of apomyoglobin undergoes a pressure-induced structural change that brings the tryptophanyl residues in contact with the solvent, but does not affect the ability to bind ANS.