Staphylococcus aureus V8-protease catalyzed segment exchange reaction of alpha-chain of hemoglobin S: a semisynthetic approach for the preparation of variants of alpha-chain.

The Glu(30)-Arg(31) peptide bond of alpha-chain of hemoglobin is readily and quantitatively hydrolyzed by Staphylococcus Aureus V8-protease at pH 4.0 and 37 degrees C. This region of the alpha-chain represents a 'permissible discontinuity region' of the chain within its tertiary interactions. Protease catalyzed reformation of peptide bonds in such permissible discontinuity regions of fragment complementing systems of proteins appears to be a useful procedure for the preparation of variants of the protein. Therefore, attempts have been now made to 'restitch' the segments alpha 1-30 and alpha 31-141 using V8-protease. The alpha-amino group of alpha 1-30 was selectively dihydroxypropylated (DHP) using [14C]-glyceraldehyde to follow the protease catalyzed reformation of globin. V8-protease catalyzed the condensation of N alpha-DHP-alpha 1-30 and alpha 31-141 to generate N alpha-DHP-alpha 1 141 in the presence of 30% propanol at pH 6.0 and 4 degrees C. The synthetic yield of N alpha-DHP-alpha 1-141 is about 55% in 48 hrs and remained nearly the same even after seven days. Under the same conditions alpha-globin was digested at the Glu(30)-Arg(31) peptide bond to nearly 40%. Thus, the amount of Glu(30)-Arg(31) peptide bond formed in 48 hrs appears to be the result of an equilibrium in the V8-protease catalyzed hydrolytic and synthetic reactions. The reformation of Glu(30)-Arg(31) bond appears to be very selective. V8-protease did not catalyze the peptide bond formation in an equimolar mixture of alpha 31-141 with either alpha 1-23, alpha 1-27, or alpha 28-30 all of which have glutamic acid as the carboxy terminal residue just as in alpha 1-30. The high selectivity in the protease catalyzed formation of Glu(30)-Arg(31) peptide bond suggests that the segment alpha 1-30 of the intact alpha-globin could be exchanged with synthetic analogs of alpha 1-30 using V8-protease without actually isolating the fragment alpha 31-141. Incubation of [14C]-N alpha-DHP-alpha 1-30 with alpha-globin at pH 6.0 and 4 degrees C along with V8-protease in 30% n-propanol indeed resulted in the exchange of the segment alpha 1-30 of alpha-globin with [14C]-N alpha-DHP-alpha 1-30. This V8-protease catalyzed 'segment exchange reaction' should facilitate the preparation of alpha-chains with double mutations from the large number of naturally occurring alpha-chains with single mutation.(ABSTRACT TRUNCATED AT 400 WORDS)