Conformation prediction and spectral studies on adrenodoxin. The accessibility of the tyrosine residue at position 82 in the polypeptide.

The nature and accessibility of Tyr 82 were studied by fluorimetry and ultraviolet spectroscopy. A strongly emitting residue was indicated by the fluorescence yield measurement. Lowering of fluorescence yield in the presence of guanidine may be suggestive of the location of the tyrosine residue in a helical environment. The addition of external quenchers, Cs+, I-, and phosphate, to adrenodoxin lowered its fluorescence intensity showing the accessibility of Tyr 82. For solvent perturbation studies using ethylene glycol and polyethylene glycol as perturbants, spectral changes were induced in adrenodoxin which gave rise to difference spectra attributable to the tyrosine residue. In addition, the spectrophotometric titration of tyrosine in adrenodoxin showed a normal titration curve with an estimated pK of 10.4. These results can be explained in terms of an accessible tyrosine residue that is partially blocked from solute and solvent perturbation. The secondary structure of adrenodoxin was studied by the method of Chou and Fasman (Chou, P. Y., and Fasman, G. D. (1978) Annu. Rev. Biochem. 47, 251-276) for the empirical predictions of protein conformations. The prediction shows a protein with moderately high amount of ordered structures (25% alpha-helix and 37% beta-sheet) and the tyrosine residue at position 82 in a helical environment.