Gas Phase Stability of Protein-Protein Complexes

ESI-MS has been used extensively over the last decade to study protein-protein interactions [1]. Serine proteases and their inhibitors provide a valuable model for the study of protein complexes as a great deal is known about solution binding and about X-ray crystal structures of such systems. It is generally accepted that hydrophobic interactions are weakened in the gas phase and that hydrophilic interactions are strengthened. We have recently shown that a series of complexes of chymotrypsin and eglin c mutants containing variation in hydrophobic residues showed no difference in collision induced dissociation curves even though solution binding constants ranged from weak (μM) to strong (pM). Solution binding constants could only be correlated with results in MS, and not MS/MS, mode [2]. Here, we further that study by examining the binding of a select group of these mutants to the serine protease subtilisin Carlsberg. Eglin c has similar inhibition constants for subtilisin Carlsberg and chymotrypsin, but shows vastly different dissociation in the gas phase.