Energy Transduction in Nitrogenase

Journal Article · Fri Aug 10 00:00:00 EDT 2018 · Accounts of Chemical Research

DOI:https://doi.org/10.1021/acs.accounts.8b00112· OSTI ID:1827182

Seefeldt, Lance C. ^[1]; Hoffman, Brian M. ^[2]; Peters, John W. ^[3]; Raugei, Simone ^[3]; Beratan, David N. ^[4]; Antony, Edwin ^[5]; Dean, Dennis R. ^[6]

---

1. Utah State Univ., Logan, UT (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Saint Louis University
2. Northwestern Univ., Evanston, IL (United States)
3. Washington State Univ., Pullman, WA (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
4. Duke Univ., Durham, NC (United States)
5. Marquette Univ., Milwaukee, WI (United States)
6. Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)

Nitrogenase is a complicated two-component enzyme system that uses ATP binding and hydrolysis energy to achieve one of the most difficult chemical reactions in nature, the reduction of N₂ to NH₃. One component of the Mo-based nitrogenase system, Fe protein, delivers electrons one at a time to the second component, the catalytic MoFe protein. This process occurs through a series of synchronized events collectively called the “Fe protein cycle”. Elucidating details of the events associated with this cycle has constituted an important challenge in understanding the nitrogenase mechanism. Electron delivery is a multistep process involving three metal clusters with intra- and interprotein events. It is proposed that the first electron transfer event is a gated intraprotein transfer of one electron from the MoFe protein P-cluster to the FeMo cofactor. Measurement of the effect of osmotic pressure on the rate of this electron transfer process revealed that it is gated by protein conformational changes. This first electron transfer is activated by binding of the Fe protein containing two bound ATP molecules. The mechanism of how this protein–protein association triggers electron transfer remains unknown. The second electron transfer event is proposed to be a rapid interprotein “backfill” with transfer of one electron from the reduced Fe protein 4Fe–4S cluster to the oxidized P-cluster. In this way, electron delivery can be viewed as a case of “deficit spending”. Such a deficit-spending electron transfer process can be envisioned as a way to achie flow. Hydrolysis of two ATP molecules associated with the Fe protein occurs after the electron transfer and therefore is not used to directly drive the electron transfer. Rather, ATP hydrolysis is proposed to contribute to relaxation of the “activated” conformational state associated with the ATP form of the complex, with the free energy from ATP hydrolysis being used to pay back energy associated with component protein association and electron transfer. Release of inorganic phosphate (Pi) and protein–protein dissociation follow electron transfer and ATP hydrolysis. Furthermore, the rate-limiting step for the Fe protein cycle is not dissociation of the two proteins, as previously believed, but rather is release of Pi after ATP hydrolysis, which is then followed by rapid protein– protein complex dissociation. Nitrogenase is composed of two catalytic halves that do not function independently but rather exhibit anticooperative nuclear motion in which electron transfer in onehalf of the complex partially inhibits electron transfer and ATP hydrolysis in the other half. Calculations indicated the existence of anticooperative interactions across the entire nitrogenase complex, suggesting a mechanism for the control of events on opposite ends of this large complex. The mechanistic necessity for this anticooperative process remains unknown. This Account presents a working model for how all of these processes work together in the nitrogenase “machine” to transduce the energy from ATP binding and hydrolysis to drive N₂ reductionve one-direction electron flow, limiting the potential for back electron.

• Research Organization: Marquette Univ., Milwaukee, WI (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division
• Grant/Contract Number: SC0017866
• OSTI ID: 1827182
• Alternate ID(s): OSTI ID: 1545069
• Journal Information: Accounts of Chemical Research, Journal Name: Accounts of Chemical Research Journal Issue: 9 Vol. 51; ISSN 0001-4842
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Charge transfer
Electron transfer
Hydrolysis
Kinetic parameters
Metal organic frameworks
Nitrogenase
Peptides and proteins