Origins of Circular Dichroism Bands in Bowman--Birk Soybean Trypsin Inhibitor.

摘要

The spectral properties of Bowman-Birk soybean trypsin inhibitor (BBI) were investigated by analyzing difference absorption spectra and difference circular dichroism (CD) spectra and by comparing them with those of tyrosyl model compounds. The O-acetylation of tyrosyl side chains showed that the ultraviolet CD bands of BBI above 225nm originate from disulfide side chains and tyrosyl phenolic groups. A broad negative CD band centered around 280nm in BBI arises mainly from disulfide bonds (epsilon/sub L/ -epsilon/sub R/ = -0.83 M exp -1cm exp -1per disulfide). Each of two tyrosyl residues gives rise to negative CD at this region;together they contribute approximately 10percent of the total CD intensity at 277nm (epsilon/sub L/ -epsilon/sub R/ = -0.36 M exp -1cm exp -1per tyrosyl). Disulfide bonds in BBI also have a broad positive CD band centered around 240nm (epsilon/sub L/ -epsilon/sub R/ = 0.9 M exp -1cm exp -1per disulfide). Tyrosyl side chains give rise to a sharp positive peak at 231nm,overlapping with the positive disulfide CD. Dimerization of monomeric BBI did not alter the CD profile. Fully acetylated BBI has the near ultraviolet disulfide CD and the far ultraviolet polypeptide CD very similar to those of the native inhibitor,indicating that O-acetylation of two tyrosyl side chains did not induce much conformational change in BBI. The near ultraviolet CD of BBI was altered in the presence of 8 M urea or 6 M guanidine hydrochloride,with a greater change brought about by the latter. Dithiothreitol (20mM) completely abolished the tyrosyl and disulfide CD in this region.