Abstract
Protein IL-1β plays a key role in the innate immunity and development of the host reaction to microbial invasion and tissue injury. In structure, it has been reported to contain a hydrophobic cavity in protein interior, however the importance has not been clarified. We intend to realize how protein stability correlates with hydrophobicity on inside of cavity. The structures of chicken IL-1β and the variant, Y157F, have been reported to be nearly identical. The replacement of phenylalanine at residue 157 makes no structural difference in its side chain orientation. Two structures own highly similar hydrophobic cavities in both size and shape. A visible difference is an existing hydrogen bond between tyrosine side chain hydroxyl group and I133 carbonyl group in native IL-1β, where mutant Y157F lacks the hydrogen bonding. Surprisingly, melting temperature of Y157F is 10-degree higher than native IL-1β, reflected by a circular dichroism measurement. To resolve the contradiction of that hydrogen bond usually stabilizes macromolecular folding, we probe protein backbone dynamics by NMR. NMR H/D exchange experiment reveals that contact between sheets β11 and β13 is weakened in the presence of hydrogen bond between I133 CO and Y157 OH, meanwhile the corresponding region is relatively stable in Y157F. We purpose that Y157 OH group introduces an unfavorable energy in the hydrophobic cavity because of its polarity, thus hydroxyl sequesters itself on forming a hydrogen bond with the proximate residue, I133. The structural element β11, where I133 is located at, therefore exerts a force to move toward to the cavity (or Y157) and away from the paired □-sheet, β13. The effect might destabilize the pair-wise □ sheets and further perturbs the overall protein stability.