Conformation of aspergillopeptidase A in aqueous solution: II. Ultraviolet optical rotatory dispersion of aspergillopeptidase A

Abstract

The optical rotatory dispersion of acid proteinase of Aspergillus sailoi (aspergillopeptidase A, EC 3.4.4.17) was investigated between 195 to 250 mμ as a function of pH, temperature, and solvent composition. The optical rotatory dispersion curves for these states differ considerably. In the native state, aspergillopeptidase A had a minimum at 220 mμ with [ m′] = −2400 and a maximum at 203 mμ with [ m′] = 900, respectively. The optical rotatory properties of aspergillopeptidase A did not change at pH's between 2.7 to 5.7. It was concluded that aspergillopeptidase A is devoid of a complete α-helical strand, as judged from the ultraviolet optical rotatory dispersion curve at 233 and 198 mμ spectral zone. On weak alkali (above pH 7.0) denaturation and heat denaturation at 55°, the flat maximum at 203 mμ vanished, and the optical rotatory dispersion spectra were altered in such a way as to suggest the presence of a randomly coiled form in the denaturated proteins. The conformation of aspergillopeptidase A was apparently converted by the anionic detergent, sodium lauryl sulfate, into a partially α-helical conformation, as judged from the optical rotatory dispersion curve in the ultraviolet region. The [ m′] 233 and [ m′] 198 values were found to be −3400 and 11600, respectively.