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 t o 2.4 kbar, has been investigated by steady-state fluorescence and fre quency domain fluorometry. In particular, we have examined the intrins ic tryptophanyl emission and that of the extrinsic probe 1-anilino-8-n aphthalenesulfonate (ANS) bound to apomyoglobin at neutral pH, as well as at strongly acidic high-salt conditions. Apomyoglobin at neutral p H undergoes a pressure-induced structural transition, which causes the disorganization of the heme binding region with a consequent ANS diss ociation; a concomitant increase in solvent accessibility to the N-ter minus of the macromolecule in which tryptophans are located is also ob served. At 2.4 kbar, the tryptophanyl emission is not coincident with that of a fully solvent exposed residue, thus suggesting that the N-te rminal region of the apomyoglobin molecule retains elements of organiz ed structure. The spectroscopic properties of the structural state att ained at 2.4 kbar and neutral pH are different from those of the acidi c compact state. The acidic compact state of apomyoglobin undergoes a pressure-induced structural change that brings the tryptophanyl residu es in contact with the solvent, but does not affect the ability to bin d ANS.