Remodeling of the binding site of nucleoside diphosphate kinase revealed by X-Ray structure and H/D exchange.

In order to be fully active and participate in the metabolism of phosphorylated nucleotides, most nucleoside diphosphate kinases (NDPK) have to assemble into stable hexamers. Here we studied the role played by six inter-subunit salt bridges R80-D93 in the stability of NDPK from the pathogen Mycobacterium tuberculosis ( Mt). Mutating R80 into Ala or Asn abolished the salt bridges. Unexpectedly, compensatory stabilizing mechanisms appeared for R80A and R80N mutants and we studied them by biochemical and structural methods. R80A mutant crystallized into I222 space group unusual for NDPK and its hexameric structure revealed occurrence at the trimer interface of a stabilizing hydrophobic patch around the mutation. Functionally relevant, a trimer of the R80A hexamer showed a remodeling of the binding site. In this conformation, the cleft of active site is more open, and then active His117 is more accessible to substrates. HDX-MS analysis of WT, R80A and R80N mutants showed that the remodeled region of the protein is highly solvent accessible indicating that equilibrium between open and closed conformation is possible. We propose that such equilibrium occurs in vivo and explains how bulky substrates access the catalytic His117.