Structural basis of a Ni acquisition cycle for [NiFe] hydrogenase by Ni-metallochaperone HypA and its enhancer

The Ni atom at the catalytic center of [NiFe] hydrogenases is incorporated by a Ni-metallochaperone, HypA, and a GTPase/ATPase, HypB. We report the crystal structures of the transient complex formed between HypA and ATPase-type HypB (HypB(AT)) with Ni ions. Transient association between HypA and HypB(AT) is controlled by the ATP hydrolysis cycle of HypB(AT), which is accelerated by HypA. Only the ATP-bound form of HypB(AT) can interact with HypA and induces drastic conformational changes of HypA. Consequently, upon complex formation, a conserved His residue of HypA comes close to the N-terminal conserved motif of HypA and forms a Ni-binding site, to which a Ni ion is bound with a nearly square-planar geometry. The Ni binding site in the HypAB(AT) complex has a nanomolar affinity (K-d = 7 nM), which is in contrast to the micromolar affinity (Kd = 4 mu M) observed with the isolated HypA. The ATP hydrolysis and Ni binding cause conformational changes of HypB(AT), affecting its association with HypA. These findings indicate that HypA and HypB(AT) constitute an ATP-dependent Ni acquisition cycle for [NiFe]-hydrogenase maturation, wherein HypB(AT) functions as a metallochaperone enhancer and considerably increases the Ni-binding affinity of HypA.