FOLDING OF APOMINIMYOGLOBIN

The acid unfolding pathway of apominimyoglobin (ape-mini-Mb), a 108-aa fragment (aa 32-139) of horse heart apomyoglobin has been studied by means of circular dichroism, in comparison with the native apoprotein. Similar to sperm whale apomyoglobin [Hughson, F. M., Wright, P. E. an d Baldwin, R. L. (1990) Science 249, 1544-1548], a partly folded inter mediate (alpha-helical content approximate to 35%) is populated at pH 4.2 for horse heart apomyoglobin. For this intermediate, Hughson ct al . proposed a structural model with a compact subdomain involving terti ary interactions between the folded A, G, and H helices, with the,rema inder of the protein essentially unfolded. As described in this paper, a folding intermediate with an alpha-helical content of approximate t o 33% is populated at pH 4.3-5.0 also in ape-mini-Mb. The add unfoldin g path way is similarly affected in both the native and the mini apopr otein by 15% trifluoroethanol, a helix-stabilizing compound. Thus, the folding of the apo-mini-Mb intermediate is similar to that observed f or the native apoprotein, in spite of the absence in the miniprotein o f the A helix and of a large part of the H helix, which are crucial fo r the stability of apo-Mp intermediate. Our results suggest that acqui sition of a folded state in apo-mini-Mb occurs through an alternative pathway, which may or may not be shared also by apo-Mb.