Respiratory complex I belongs to the complex I superfamily together with the photosynthetic complex I, and other redox-driven ions pumps. It is hypothesized that complex I evolved most likely from Mrp (Multiple resistance and pH adaptation) antiporters and hydrogenases . Recent high resolution structural data on several of the evolutionarily related enzymes such as FHL (formate hydrogenlyase) , MBH (membrane bound hydrogenase) and MBS (membrane bound sulfane sulfur reductase) have provided interesting insights on complex I function.