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

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Abstract
Selective condensation of the unprotected fragments of .alpha.-globin.sbd.namely, a1-30 and .alpha.31-141.sbd.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.degree. 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.sbd.Arg-10 peptide bond. RNase S peptide is in an .alpha.-helical conformation in RNase S, and 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.