Electron Transfer in Nitrogenase Analyzed by Marcus Theory: Evidence for Gating by MgATP
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- 作者：William N. Lanzilotta ; Vernon D. Parker ; and Lance C. Seefeldt
- 刊名：Biochemistry
- 出版年：1998
- 出版时间：January 6, 1998
- 年：1998
- 卷：37
- 期：1
- 页码：399 - 407
- 全文大小：202K
- 年卷期：v.37,no.1(January 6, 1998)
- ISSN：1520-4995
文摘
Nitrogenase-catalyzed substrate reduction reactions requireelectron transfer between twocomponent proteins, the iron (Fe) protein and the molybdenum-iron(MoFe) protein, in a reaction thatis coupled to the hydrolysis of MgATP. In the present work,electron transfer (Marcus) theory has beenapplied to nitrogenase electron transfer reactions to gain insightsinto possible roles for MgATP in thisreaction. Evidence is presented indicating that an eventassociated with either MgATP binding or hydrolysisacts to gate electron transfer between the two component proteins.In addition, evidence is presented thatthe reaction mechanism can be fundamentally changed such that electrontransfer becomes rate-limitingby the alteration of a single amino acid within the nitrogenase Feprotein (deletion of Leu 127, L127
).These studies utilized the temperature dependence ofintercomponent electron transfer within two differentnitrogenase complexes: the wild-type nitrogenase complex thatrequires MgATP for electron transferand the L127 Fe protein-MoFe protein complex that does not requireMgATP for electron transfer. Itwas found that the wild-type nitrogenase electron transfer reaction didnot conform to Marcus theory,suggesting that an adiabatic event associated with MgATP interactionprecedes electron transfer and israte-limiting. Application of transition state theory providedactivation parameters for this rate-limitingstep. In contrast, electron transfer from the L127 Fe proteinvariant to the MoFe protein (which doesnot require MgATP hydrolysis) was found to be described by Marcustheory, indicating that electrontransfer was rate-limiting. Marcus parameters were determined forthis reaction with a reorganizationenergy (
) of 2.4 eV, a coupling constant(HAB) of 0.9 cm-1, afree energy change (G'
) of -22.0kJ/mol, and a donor-acceptor distance (r) of 14 Å.These values are consistent with parameters deducedfor electron transfer reactions in other protein-protein systemswhere electron transfer is rate-limiting.Finally, the electron transfer reaction within the L127 Feprotein-MoFe protein complex was found tobe reversible. These results are discussed in the context ofmodels for how MgATP interactions mightbe coupled to electron transfer in nitrogenase.
).These studies utilized the temperature dependence ofintercomponent electron transfer within two differentnitrogenase complexes: the wild-type nitrogenase complex thatrequires MgATP for electron transferand the L127 Fe protein-MoFe protein complex that does not requireMgATP for electron transfer. Itwas found that the wild-type nitrogenase electron transfer reaction didnot conform to Marcus theory,suggesting that an adiabatic event associated with MgATP interactionprecedes electron transfer and israte-limiting. Application of transition state theory providedactivation parameters for this rate-limitingstep. In contrast, electron transfer from the L127 Fe proteinvariant to the MoFe protein (which doesnot require MgATP hydrolysis) was found to be described by Marcustheory, indicating that electrontransfer was rate-limiting. Marcus parameters were determined forthis reaction with a reorganizationenergy () of 2.4 eV, a coupling constant(HAB) of 0.9 cm-1, afree energy change (G') of -22.0kJ/mol, and a donor-acceptor distance (r) of 14 Å.These values are consistent with parameters deducedfor electron transfer reactions in other protein-protein systemswhere electron transfer is rate-limiting.Finally, the electron transfer reaction within the L127 Feprotein-MoFe protein complex was found tobe reversible. These results are discussed in the context ofmodels for how MgATP interactions mightbe coupled to electron transfer in nitrogenase.