Denatured states of tick anticoagulant peptide. Compositional analysis of unfolded scrambled isomers

In the presence of denaturant and thiol catalyst, a disulfide-containing protein denatures and converts to a mixture of scrambled isomers, which can be purified and structurally characterized. Scrambled isomers adopt a different conformation and a varied extent of unfolding. Their relative concentration (composition) signals the state of unfolding of the denatured protein and is determined by the denaturing condition. In this report, tick anticoagulant peptide (TAP) (60 amino acids and 3 disulfides) has been denatured in the presence of urea, guanidine hydrochloride, guanidine thiocyanate, organic solvents, and at elevated temperature. The recoveries of scrambled TAP were analyzed. The results demonstrate that each denaturing condition generates a unique structure (composition of scrambled species) of denatured TAP. Among various species of scrambled TAP, the beads-form species contains the smallest disulfide loop and appears to represent the most extensively unfolded state. The yield of the beads-form species as a fraction of the total denatured TAP is invariably determined by the strength of the denaturing condition.