Heme/Cu terminal oxidases

The appearance of photosynthesis and the subsequent increase in the concentration of O2 in the Earth s atmosphere 2 billion years ago provided the environmental pressure for the evolution of enzymes to catalyze the 4e/4H+ reduction of O2 to H2O as an energy source. All known enzymes that perform this task, called terminal oxidases ", are membrane-bound proteins. Two distinct classes of terminal oxidases are known. The better-studied superfamily of heme/Cu terminal oxidases are distributed throughout all three domains of life (Bacteria, Archaea, and Eukarya) and contain a binuclear por-phyrinatoiron(heme)-Cu site at which O2 is reduced. The much less studied and smaller group of cytochrome bd terminal oxidases is widely distributed in Gram-negative heterotrophs onlyH. Cytochromes bd contain a binuclear heme-... 

To provide a model for nitrite reductases72 Karlin and co-workers characterized a nitrite-bound complex (226) (r = 0.05)214 In an endeavor to model nitrite reductase activity, Tanaka and co-workers prepared a few mononuclear complexes (227) (r = 0.74)215 (228) (r = 0.82),216 (229) (r = 0.97),217 (230) (r = 0.16),217 (231) (r = 0.07),217 and (232) (r = 0.43 and r = 0.53)217 and studied the electrochemical reduction of N02A As a part of their activity on modeling heme-copper terminal oxidases, Holm and co-workers prepared complex (233) (r = 0.96).218 Using a sterically hindered tris(pyridylmethyl)amine, Canary et al. prepared a complex (234) (r=1.00), studied its redox behavior, and discussed various factors that may contribute to the difference (higher potential for the new complex) in the redox potential of a Cu Cu1 couple between substituted and unsubstituted ligands.2 9... 

Very likely CO2, CO, and N2 were among the main components by the time of the origin of life within the Earth s early atmosphere. In addition, NO, H2, H2O, and sulfuric gases could also have been present, and probably trace amounts of O2 [23]. Thus, it was not too surprising that several molecular links do exist between the evolution of denitrification enzymes and cytochrome c oxidase, the terminal oxidase of aerobic respiration [24]. Perhaps the most important is the homology between nitric oxide reductase and heme/copper cytochrome oxidases, and the presence of the mixed-valence [Cu " (Scys)2Cu " CuA electron-transfer center in nitrous oxide reductases, the quinol-dependent NO reductase from the gram-positive Bacillus azotoformans, and heme/copper cytochrome oxidases [24-27]. 

Cytochrome c Oxidase. This is a ubiquitous enzyme that catalyzes the terminal step in the four-electron reduction of 02 to 2H20. It contains both heme iron and copper and there are two kinds of copper sites, CuA and CuB, in each of which there is an association of copper with heme iron. The CuB site seems to have one copper atom linked to a heme unit, but details are still obscure.37 From modeling studies and EXAFS data, opinion presently favors some sort of Fe—X—Cu bridge... 

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