Optical Rotatory Dispersion of Nonhelical Proteins

From the ¹ From The University of Texas M.D. Anderson Hospital and Tumor Institute, Department of Biochemistry, Section of Protein Structure, Houston, Texas 77025

The optical rotatory dispersion of a group of proteins having the Moffitt constant b₀ near zero was studied in the far ultraviolet spectral zone. Sodium poly--l-glutamate, myoglobin, and serum albumin were used as reference substances. It was found that, in addition to the normal type of rotatory dispersion, as displayed, for example, by myoglobin, four other types of curves could be demonstrated. They are as follows.

Type I is represented by phosvitin and the lysine- and proline-rich histone, which are flexible disordered macromolecules. The rotatory dispersion of these proteins is similar to that of disordered polyglutamate. The rotatory power of phosvitin and histone at 198 mµ is near zero.

Type II is exemplified by oxidized serum albumin. These curves have two minima, one at 230 to 233 mµ and another at 210 to 215 mµ. The major conformation of oxidized serum albumin seems to be a disordered chain, with a small amount of helix. Computed curves from data of helical and disordered polyglutamate yielded this type of curve when the helix content was about 25% or less.

Type III is represented by chymotrypsinogen, trypsin, and thyrotropin. These proteins have curves with a minimum at 218 to 225 mµ, and they are weakly dextrorotatory at 195 to 200 mµ. It was concluded that these proteins are devoid of complete -helical strands; instead, they are characterized by some other kind of ordered structures.

Type IV curves are observed with serum -globulin, the myeloma protein, and Bence-Jones protein. These curves have a shallow minimum at 220 to 225 mµ, a positive maximum at 210 mµ, and a minimum at 198 to 200 mµ.

Addition of the disordered polyglutamate or phosvitin to the solutions of the highly -helical myoglobin or serum albumin did not produce a displacement of the negative minimum at 233 mµ, although the amplitude was diminished. If the ratio of the disordered polymer relative to myoglobin was high, type II curves were obtained.

Treatment of chymotrypsinogen with decyl sodium sulfate caused a conformational transition resulting in some helix formation. The type III curve of chymotrypsinogen was thereby transformed into the ordinary curve of the partially -helical proteins; i.e. it then possessed a trough at 233 mµ and a peak at 198 mµ. The detergent had a similar effect on the rotatory dispersion of trypsin, Bence-Jones protein, and -globulin, but not on the highly charged phosvitin.

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