Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo–electron microscopy structure of photosynthetic complex I from the cyanobacterium Thermosynechococcus elongatus. The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process.

Photosynthetic organisms capture light energy to drive their energy metabolism, and employ the chemical reducing power to convert carbon dioxide (CO 2 ) into organic molecules. Photorespiration, however, significantly reduces the photosynthetic yields. To survive under low CO 2 concentrations, cyanobacteria evolved unique carbon-concentration mechanisms that enhance the efficiency of photosynthetic CO 2 fixation, for which the molecular principles have remained unknown. We show here how modular adaptations enabled the cyanobacterial photosynthetic complex I to concentrate CO 2 using a redox-driven proton-pumping machinery. Our cryo-electron microscopy structure at 3.2 Å resolution shows a catalytic carbonic anhydrase module that harbours a Zn 2+ active site, with connectivity to protonpumping subunits that are activated by electron transfer from photosystem I. Our findings illustrate molecular principles in the photosynthetic complex I machinery that enabled cyanobacteria to survive in drastically changing CO 2 conditions.

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.

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, and is fundamental to bioenergetics in photosynthetic bacteria and some higher plant cell types. 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, and unambiguously assign them to the structure using double electron electron resonance (DEER). 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.

No abstract