Promotion of oxidative phosphorylation by complex I-anchored carbonic anhydrases?

Recent high-resolution electron cryo-microscopy (cryo-EM) maps revealed detailed insights into the arrangement of the carbonic anhydrase (CA) module next to a ferredoxin subunit and a proton translocation channel of plant, algal, and protozoan mitochondrial complex I. Complex I-anchored CA may promote the provision of protons at the inside of the inner mitochondrial membrane, thereby supporting oxidative phosphorylation (OXPHOS). The cryoEM maps imply that a possible local pH difference between the active site of the CA module and the positive charge of a ferredoxin-bound single iron or iron–sulfur cluster facilitates the migration of protons to the entrance of a proton translocation channel at complex I. In the context of early endosymbiosis with mitochondria lacking cristae invaginations that shield off high proton leakage, complex I-bound CAs might have ensured maintenance of OXPHOS. The mitochondrial NADH-dehydrogenase complex of the respiratory chain, known as complex I, includes a carbonic anhydrase (CA) module attached to its membrane arm on the matrix side in protozoans, algae, and plants. Its physiological role is so far unclear. Recent electron cryo-microscopy (cryo-EM) structures show that the CA module may directly provide protons for translocation across the inner mitochondrial membrane at complex I. CAs can have a central role in adjusting the proton concentration in the mitochondrial matrix. We suggest that CA anchoring in complex I represents the original configuration to secure oxidative phosphorylation (OXPHOS) in the context of early endosymbiosis. After development of 'modern mitochondria' with pronounced cristae structures, this anchoring became dispensable, but has been retained in protozoans, algae, and plants. [ABSTRACT FROM AUTHOR]