Meta-Biol

• Pathways

Complex IV (COX, cytochrome c oxidase) contains the hemeprotein cytochrome a and a3. It also contains copper atoms which undergo a transition from Cu+ to Cu2+ during the transfer of electrons through the complex to molecular oxygen. A bimetallic centre containing a copper atom and a heme-linked iron protein binds oxygen after 4 electrons have been picked up. Water, the final product of oxygen reduction, is then released. Oxygen is the final electron acceptor in the respiratory chain. The overall reaction can be summed as

4Cyt c (red.) + 12H+ (in) + O2 = 4Cyt c (ox.) + 2H2O + 8H+ (out)

Four protons are taken up from the matrix side of the membrane to form the water (scalar protons). Wikstrom (1977) suggests 4 protons are additionally transferred out from the matrix to the intermembrane space.

COX ancillary proteins mediate membrane insertion, catalytic core processing, copper transport and insertion into core subunits and heme A biosynthesis (Stilburek et al. 2006, Fontanesi et al. 2006, Soto et al. 2012). To date, all Mendelian disorders presenting COX deficiency have been assigned to mutations in ancillary factors, with the exception of an infantile encephalomyopathy caused by a defective COX6B1 and an exocrine pancreatic insufficiency caused by a defective COX4I2 gene (Soto et al. 2012). Balsa et al have shown that NDUFA4, formerly considered to be a constituent of NADH dehydrogenase (Complex I), is instead a component of the cytochrome c oxidase (CIV) (Balsa et al. 2012). Patients with NDUFA4 mutations display COX deficiencies (Pitceathly et al. 2013).

Carbon monoxide (CO) readily inhibits oxygen consumption by mitochondrial cytochrome oxidase. This inhibition is responsible for much of its toxicity when it is applied externally to the body. However, CO has been implicated in normal cellular signalling, especially in anti-inflammatory effects. The addition of antioxidants or inhibition of complex III of the electron transport chain by antimycin A attenuates the inhibitory effects of CO on lipopolysaccharide (LPS)-induced NLRP3 formation and TNF-alpha secretion, and blocked CO-induced p38 MAPK phosphorylation. These effects may be mediated via inhibition of cytochrome c oxidase and its generation of mitochondrial reactive oxygen species (Alonso et al, 2003; Zuckerbraun et al, 2007; Cooper and Brown, 2008; Jung et al, 2014; Ishigami et al, 2017).