Remarkable thermal stability of doubly intramolecularly cross-linked hen lysozyme

In order to examine how a protein can be effectively stabilized, two intramolecular cross-links, Glu35–Trp108 and Lys1–His15, which have few unfavorable interactions in the folded state, were simultaneously introduced into hen lysozyme. Both of the intramolecularly cross-linked lysozymes, 35–108 CL and 1–15 CL, containing cross-links Glu35–Trp108 and Lys1–His15, respectively, showed increases in thermal stability of 13.9 and 5.2°C, respectively, over that of wild type, at pH 2.7. On the other hand, a doubly cross-linked lysozyme showed an increase in thermal stability of 20.8°C over that of wild type, under identical conditions. Since the sum of the differences in denaturation temperature between wild type and each of the cross-linked lysozymes was nearly equal to that between wild type and the doubly cross-linked lysozyme, we suggest that the efficient stabilization of the lysozyme molecule was the direct result of the double intramolecular cross-links.