Change in dimerization mode by removal of a single unsatisfied polar residue located at the interface

The importance of unsatisfied hydrogen bonding potential on protein-protein interaction was studied. Two alternate modes of dimerization (conventional and flipped form) of an immunoglobulin light chain variable domain (V-L) w ere previously identified. In the flipped form, interface residue G1n89 wou ld have an unsatisfied hydrogen bonding potential. Removal of this Gin shou ld render the flipped dimer as the more favorable quaternary form. High res olution crystallographic studies of the Q89A and Q89L mutants show, as we p redicted, that these proteins indeed form flipped dimers with very similar interfaces. A small cavity is present in the Q89A mutant that is reflected in the similar to 100 times lower association constant than found for the Q 89L mutant. The association constant of Q89A and Q89L proteins (4 X 10(6) M -1 and >10(8) M-1) are 10- and 1,000-fold higher than that of the wild-type protein that forms conventional dimers clearly showing the energetic reaso ns for the flipped dimer Formation.