Molecular interplay of an assembly machinery for nitrous oxide reductase.

Emissions of the critical ozone-depleting and greenhouse gas nitrous oxide (N2O) from soils and industrial processes have increased considerably over the last decades1-3. As the final step of bacterial denitrification, N2O is reduced to chemically inert N2 (refs. 1,4) in a reaction that is catalysed by the copper-dependent nitrous oxide reductase (N2OR) (ref. 5). The assembly of its unique [4Cu:2S] active site cluster CuZ requires both the ATP-binding-cassette (ABC) complex NosDFY and the membrane-anchored copper chaperone NosL (refs. 4,6). Here we report cryo-electron microscopy structures of Pseudomonas stutzeri NosDFY and its complexes with NosL and N2OR, respectively. We find that the periplasmic NosD protein contains a binding site for a Cu+ ion and interacts specifically with NosL in its nucleotide-free state, whereas its binding to N2OR requires a conformational change that is triggered by ATP binding. Mutually exclusive structures of NosDFY in complex with NosL and with N2OR reveal a sequential metal-trafficking and assembly pathway for a highly complex copper site. Within this pathway, NosDFY acts as a mechanical energy transducer rather than as a transporter. It links ATP hydrolysis in the cytoplasm to a conformational transition of the NosD subunit in the periplasm, which is required for NosDFY to switch its interaction partner so that copper ions are handed over from the chaperone NosL to the enzyme N2OR.

Functional assembly of nitrous oxide reductase provides insights into copper site maturation.

The multicopper enzyme nitrous oxide reductase reduces the greenhouse gas N2O to uncritical N2 as the final step of bacterial denitrification. Its two metal centers require an elaborate assembly machinery that so far has precluded heterologous production as a prerequisite for bioremediatory applications in agriculture and wastewater treatment. Here, we report on the production of active holoenzyme in Escherichia coli using a two-plasmid system to produce the entire biosynthetic machinery as well as the structural gene for the enzyme. Using this recombinant system to probe the role of individual maturation factors, we find that the ABC transporter NosFY and the accessory NosD protein are essential for the formation of the [4Cu:2S] site CuZ, but not the electron transfer site CuA Depending on source organism, the heterologous host E. coli can, in some cases, compensate for the lack of the Cu chaperone NosL, while in others this protein is strictly required, underlining the case for designing a recombinant system to be entirely self-contained.

The Copper Chaperone NosL Forms a Heterometal Site for Cu Delivery to Nitrous Oxide Reductase.

The final step of denitrification is the reduction of nitrous oxide (N2 O) to N2 , mediated by Cu-dependent nitrous oxide reductase (N2 OR). Its metal centers, CuA and CuZ , are assembled through sequential provision of twelve CuI ions by a metallochaperone that forms part of a nos gene cluster encoding the enzyme and its accessory factors. The chaperone is the nosL gene product, an 18 kDa lipoprotein predicted to reside in the outer membrane of Gram-negative bacteria. In order to better understand the assembly of N2 OR, we have produced NosL from Shewanella denitrificans and determined the structure of the metal-loaded chaperone by X-ray crystallography. The protein assembled a heterodinuclear metal site consisting of ZnII and CuI , as evidenced by anomalous X-ray scattering. While only CuI is delivered to the enzyme, the stabilizing presence of ZnII is essential for the functionality and structural integrity of the chaperone.