Conformational studies of alpha-globin in 1-propanol: propensity of the alcohol to limit the sites of proteolytic cleavage.

Selective condensation of the unprotected fragments of alpha-globin--namely, alpha 1-30 and alpha 31-141--is catalyzed by Staphylococcus aureus V8 protease in the presence of 25% 1-propanol. The propensity of 1-propanol to induce the alpha-helical conformation and to generate a "native-like" topology for the polypeptide chain has been now investigated in an attempt to understand the molecular basis of this enzyme-catalyzed stereospecific condensation. Removal of heme from the alpha-chain decreases the overall alpha-helical conformation of the protein considerably. A significant amount of the alpha-helical conformation is restored in the presence of 25% 1-propanol and the digestion of alpha-globin by V8 protease becomes more selective concomitant with the increase in helicity. V8 protease digestion of alpha-globin at pH 6.0 and 4 degrees C occurs at Glu-30, Asp-47, Glu-27, and Glu-23 in the absence of 1-propanol. In the presence of 25% 1-propanol, the digestion is selective to the peptide bond of Glu-30. This selectivity appears to be a characteristic feature of the native conformation of alpha-chain (polypeptide chain with bound heme). 1-Propanol induces the alpha-helical conformation into RNase S peptide also. However, this increased helical conformation did not protect the RNase S peptide from V8 protease digestion at the Glu-9-Arg-10 peptide bond. RNase S peptide is an alpha-helical conformation in RNase S, an interacting fragment-complementing system of S protein and S peptide. S peptide is resistant to V8 protease hydrolysis in this conformation. Thus, the resistance of a peptide bond in a segment of a protein to protease digestion appears to be a consequence of the secondary structure as well as the tertiary interactions of this segment with the rest of the molecule. The results suggest that the 1-propanol induces alpha-helical conformation into segments of alpha-globin as well as packing of these helices in a native-like topology.