Phosphorescence anisotropy of liver alcohol dehydrogenase in the crystalline state. Apparent glasslike rigidity of the coenzyme-binding domain

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Abstract
Phosphorescence anisotropy from internal tryptophan (Trp) residues in proteins which are in the crystalline state may provide an experimental approach suitable to study the flexibility of rather rigid segments of protein structure. The phosphorescence anisotropy of Trp-314 in liver alcohol dehydrogenase, which is enclosed within the .beta.-sheet forming the coenzyme-binding domain, was measured with the protein free in solution and in the crystalline state. In contrast to the free protein, where the rotational correlation time reflects the tumbling rate of the whole macromolecule, there is effectively no loss in anisotropy in the crystalline state. At room temperature, the triplet lifetime of 0.5 s implies that the rotational correlation time of the indole side chain must be larger than 1 s. Anisotropy data show that fluctuations of the indole ring about the average position can only be of limited amplitude (cone of semiangle less than 15.degree.) and that the resistance opposed by the .beta.-sheet to out-of-plane rotational motions is equivalent to a vicosity larger than 2.5 .times. 108 P, a value which confirms the particular rigidity anticipated for such an assembly of secondary structure.