Energy coupling in cyanobacterial NDH-1 complexes.

Cyanobacterial NDH-1 complexes use a conserved Type-I energy-transducing core for respiratory electron transfer, photosystem I cyclic electron flow, and, in specialized isoforms, active CO2 uptake. This review examines how the common structural and energetic principles of respiratory Complex I may be adapted to these distinct physiological functions. Building on current mechanistic models, this review frames quinone reduction as producing a distributed, metastable "tense state" in the membrane arm. This state is represented not by a single strained conformation, but by coupled changes in proton affinity, ion-pair geometry, hydration, and local electrostatics. This framework is then applied to the cyanobacterial CO2-uptake complexes NDH-13 and NDH-14, in which the distal NdhF3/NdhF4-CupA/B module appears to repurpose part of the canonical proton-coupling machinery for Zn-dependent CO2 hydration. Bioenergetic estimates indicate that ferredoxin-dependent plastoquinone reduction could, in principle, support three-proton translocation together with CO2 hydration and bicarbonate accumulation. The model connects Complex I energy transduction pathways to cyanobacterial CO2 hydration chemistry and identifies testable predictions regarding coupling at the Zn-site to distant plastoquinone reduction and proton transfer.