Acetylation of Surface Lysine Groups of a Protein Alters the Organization and Composition of Its Crystal Contacts

This paper uses crystals of bovine carbonic anhydrase (CA) and its acetylated variant to examine (i) how a large negative formal charge can be accommodated in protein–protein interfaces, (ii) why lysine residues are often excluded from them, and (iii) how changes in the surface charge of a protein can alter the structure and organization of protein–protein interfaces. It demonstrates that acetylation of lysine residues on the surface of CA increases the participation of polar residues (particularly acetylated lysine) in protein–protein interfaces, and decreases the participation of nonpolar residues in those interfaces. Negatively charged residues are accommodated in protein–protein interfaces via (i) hydrogen bonds or van der Waals interactions with polar residues or (ii) salt bridges with other charged residues. The participation of acetylated lysine in protein–protein interfaces suggests that unacetylated lysine tends to be excluded from interfaces because of its positive charge, and not because of a loss in conformational entropy. Results also indicate that crystal contacts in acetylated CA become less constrained geometrically and, as a result, more closely packed (i.e., more tightly clustered spatially) than those of native CA. This study demonstrates a physical-organic approach—and a well-defined model system—for studying the role of charges in protein–protein interactions.