Bacterial Terminal Oxidases

Abstract

Oxidases, as such, regardless of their source, represent a diverse and complex series of enzymes. What they have in common is the ability to react with molecular oxygen, activate it chemically (in a manner which is still not understood), and utilize the “activated atoms of oxygen” primarily as electron acceptors. Should the “activated oxygen atoms” be used directly for oxygenating substrate molecules, such as hydrocarbons, then according to the conventions used today, the enzyme is termed an oxygenase rather than an oxidase. The subject of oxygenases is far too complex to be reviewed in any detailed treatment of oxidases. All oxidases serve as electron acceptors for specific dehydrogenation reactions that are carried out by the multitude of dehydrogenases that are found in tissues as well as in bacteria. The major end product that results from the oxidase reaction is either H₂O or H₂O₂. The oxidases can be (1) simple flavoprotein-containing enzymes, such as the glucose oxidase or the D- and L-amino acid oxidases, (2) metalloflavoprotein-containing enzymes, such as the xanthine oxidase which in addition to flavin contains both molybdenum and nonheme iron, (3) the heme-containing oxidases which essentially are free of flavoprotein yet carry out a peroxidase-oxidase type of reaction, and finally (4) the heme-containing and flavoprotein-free cytochrome (or terminal) oxidase molecule which may contain as many as two heme “a” components (multiple heme iron), copper protein, and the active form of which requires an enzyme complex containing some phospholipid.^{1 4 5} It is this latter group, the cytochrome or terminal oxidases, that will be considered in this review, particularly what is known about them in bacterial systems. The cytochrome or terminal oxidases are usually membrane-bound entities, play a major role in electron transport, and are very important in the bioenergetic mechanism of aerobic cells that allow for respiration.