Functional basis of electron transport within photosynthetic complex I

Katherine H Richardson,Ana M Esteves,Gemma Mcguire,William K Myers,John J Wright,Jacqueline Thiemann,Michael Hippler,John J L Morton,Marc M Nowaczyk,Maxie M Roessler,Guy T Hanke,Mantas Šimėnas

doi:10.1038/s41467-021-25527-1

Abstract

Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS-CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes. However, little is known about the PS-CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability. Here, we overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme – the reduction potentials of its cofactors, in this case the iron-sulphur clusters of PS-CI (N0, N1 and N2), and unambiguously assign them to the structure using double electron-electron resonance. We have thus determined the bioenergetics of the electron transfer relay and provide insight into the mechanism of PS-CI, laying the foundations for understanding of how this important bioenergetic complex functions.

Highlights

Photosynthesis and respiration rely upon a proton gradient to produce ATP
Our results are contrary to what is observed in respiratory complex I (R-CI), where with the exception of the E. coli enzyme cluster N2 has the most positive reduction potential of the Electron paramagnetic resonance (EPR)-visible Fe-S clusters[18], with the adjacent cluster remaining oxidised upon NADH reduction[21]
The two adjacent isopotential clusters in Photosynthetic Complex I (PS-CI) challenges the mechanistic principle of alternating highand low-potential clusters in electron transfer relays[21,53,54,55]

Summary

Introduction

Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS-CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes. Little is known about the PS-CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability We overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme – the reduction potentials of its cofactors, in this case the iron-sulphur clusters of PS-CI (N0, N1 and N2), and unambiguously assign them to the structure using double electron-electron resonance. ATP demands are high, such as when performing C4 type photosynthesis[7], or sustaining growth under low light or other stresses[8] In this way, PS-CI is critical to yield in some crops[9]. Understanding the intricate electron transfer process that provides the free energy for proton translocation is of fundamental interest but important because it can fine-tune the redox state of the compartment or cell under stress[10]

Methods

Results

Conclusion