STUDIES OF LOCATION OF TYROSYL AND TRYPTOPHYL RESIDUES IN PROTEIN .2. APPLICATIONS OF MODEL DATA TO SOLVENT PERTURBATION STUDIES OF PROTEINS RICH IN BOTH TYROSINE AND TRYPTOPHAN

The uses and applications of model compound data of the accompanying paper (Herskovits, T. T., and Sorensen, M. (1968), Biochemistry 7, 2533 (this issue ; preceding paper) are described and as a means of illustration applied to the simultaneous study of the location of tyrosyl and tryptophyl residues in pepsin, rabbit muscle aldolase, and bovine serum albumin. From the study on pepsin with six perturbants, it is concluded that in the native enzyme 2-3 of the 5 tryptophyls and 10-12 of the 17 tyrosyls are exposed, with perhaps 1 or 2 of these residues being partly buried. In native aldolase, a much larger fraction of the tyrosyls and tryptophyls are found to be buried. Of the 47 tyrosyls 34-40% appear to be exposed, while of the 11-12 tryptophyls only about 15-20% are exposed; it is suggested that some of the observed perturbations may be due to a combination of exposed and partly exposed chromophores. In serum albumin, the two tryptophyl residues are found to be nearly fully exposed to perturbants with less than 4.4-Å diameter (deuterium oxide and ethylene glycol), while of the order of 50-70% exposure is observed with perturbants having greater than 5.2-Å diameters (glycerol and hexaethylene glycol). It is suggested that the two tryptophyls are probably located at the periphery of some of the conformational subunits of serum albumin postulated by Foster, or situated at other areas of polypeptide fold which must be largely accessible to solvent penetration. Previous studies on the location of the tyrosyls in bovine and human serum albumin (Herskovits, T. T., and Laskowski, M., Jr. (1962), J. Biol. Chem. 237, 2481) have suggested that about 70% of the 18-21 tyrosyls appear to be buried in the native folds of these proteins, with about 20% of 3-5 of these groups being located at areas of subunit contact. The latter key tyrosyls are not accessible to perturbants with molecular diameters greater than 5.2 Å. © 1968, American Chemical Society. All rights reserved.