Structural Basis of Redox-Coupled CO ₂ hydration by the Cyanobacterial NDH-1MS’ complex

The NAD(P)H dehydrogenase-like complex, NDH-1MS’, is a constitutively expressed CO ₂ -concentrating mechanism critical for sustaining CO ₂ fixation in cyanobacteria. This complex has been proposed to function as a vectorial carbonic anhydrase and accumulates intracellular bicarbonate against chemical equilibrium by coupling CO ₂ 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 CO ₂ 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 CO ₂ 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 CO ₂ hydration coupled to proton transfer events in the antiporter-like subunits across the thylakoid membrane.