Abstract:

    The reversible phosphoryl transfers between phosphagen and ADP are important means to reserve or release energy in cell energy metabolism. Both directions of the transfer are catalyzed by phosphagen kinase (PK) family members, including creatine kinases (CKs) and arginine kinases (AKs). In invertebrates, cell employees AKs and arginines, while in vertebrates, CKs and creatines perform the similar functions. Usually, AK is in the context of monomer, but CK functions as dimer, tetramer, or even octamer. Why CK need to oligomerize while AK needn`t? Is there any advantage for those “twins” or just an adaption for different substrate? The answers to these questions remain unclear, though a dozen of high-resolution structures of PKs have been reported. Our studies focused on a bridge between those two enzymes, an AK in dimeric form. The 1.75 Å-resolution crystal structure of this dimeric AK (dAK) reveals intriguing conformational differences between the two protomers of a dAK dimer. Based on further structural analysis, we identified a novel conformational state of dAK protomers, which is different from the traditional “open” and “closed” states of an enzyme. And the novel state of one protomer is tightly related to the allostery of the adjacent protomer through the dimerization interface. Mutations that weaken the inter-protomer connections dramatically reduced the enzymatic activities of dAK, indicating the importance of the allosteric propagation mediated by the homodimer interface. These results suggest a reciprocating mechanism of dimeric PK, which is shared by other ATP related oligomeric enzymes, e.g. ATP synthase. The advantages brought by the reciprocating mechanism could be the reason why PK evolves from monomers to dimers.