Structural Basis of Redox-Coupled CO2 hydration by the Cyanobacterial NDH-1MS' complex
The NAD(P)H dehydrogenase-like complex, NDH-1MS, is a constitutively expressed CO2-concentrating mechanism critical for sustaining CO2 fixation in cyanobacteria. This complex has been proposed to function as a vectorial carbonic anhydrase and accumulates intracellular bicarbonate against chemical equilibrium by coupling CO2 hydration to the photosynthetic cyclic electron flow. Using cryo-electron microscopy, we determined the structure of NDH-1MS from Thermosynechococcus vestitus. Contrary to its inducible counterpart NDH-1MS, our data suggest that a candidate CO2 hydration site may reside in the carbon-concentrating subunit CupB instead of the CupB-NdhF4 interface previously proposed. The active-site region partially resembles metal ion-independent iota-carbonic anhydrases and may bind CO2 or bicarbonate. We further observe features consistent with a Grotthuss-type proton transfer network connecting the active-site region to the antiporter-like subunits NdhF4 and NdhD4; however, unlike complex I, the proton export channel in NdhF4 appears to be blocked by bulky hydrophobic amino acids. Structural comparison between the oxygenic photosynthesis-specific subunit NdhV-bound state and dissociated state further reveals a correlation between plastoquinone stability and NdhV association. Taken together, we propose a revised working model in which NDH-1MS may function as a metal ion-independent vectorial carbonic anhydrase, with CO2 hydration coupled to proton transfer events in the antiporter-like subunits across the thylakoid membrane. SignificancePhotosynthetic complex I-like CO2-uptake systems are central to cyanobacterial carbon fixation, yet the molecular basis for coupling CO2 hydration to electron transfer has remained unresolved. Here, structures of NDH-1MS in NdhV-bound and NdhV-free states revise the architectural framework for catalysis and coupling in the constitutive CO2-uptake module. The data identify a CupB-centered candidate active-site region, reveal a non-canonical terminal antiporter-like subunit with an apparently blocked P-side exit, and link NdhV association to altered plastoquinone stability. Together, these findings refine current models for vectorial carbonic anhydrase activity in cyanobacterial NDH-1 and provide a structural basis for understanding how photosynthetic redox chemistry may be coupled to inorganic carbon concentration.