Circular dichroism of the guanyloribonuclease Sa and its complex with guanosine 3′-phosphate

Abstract

Extracellular guanyloribonuclease isolated from Streptomyces aureofaciens (RNAase Sa) exhibits a complex negative circular dichroism (CD) around 280 nm with a clear fine structure, and a positive Cotton effect at 235 nm. It contains about 42% of α-helix, 28% of β-structure and 33% of random coil as was determined by computation from the molar ellipticity in the region 240-210 nm. The circular dichroism at 280 nm is mainly due to the immobilization of 1–2 tyrosine residue(s) inside the enzyme molecule. On the other hand, the CD band at 285 nm might be due to an exciton coupling between near ultraviolet transitions of two tyrosine residues. The negative Cotton effect at 290 nm in a circular dichroism difference spectrum of the complex of RNAase Sa with a competitive inhibitor guanosine 3′-monophosphate ( Guo-3′-P ) is probably caused by the immobilization of two other tyrosine residues in the enzyme binding site which interact with the guanine base. A positive Cotton effect at 270 nm in this spectrum was attributed to an exciton interaction of one binding tyrosine residue with the guanine base, and the positive Cotton effect at 255 nm is induced most probably by the second binding tyrosine which might interact by hydrogen bonding between the phenolic hydrogen and O-6 of the pyrimidine ring.