Electron transfer oxidases, classification

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

This classification is perfectly valid for the metalloenzymes, even though most of them fall in group 1, such as the several dioxygenases, (mono)oxygen-ases, and oxidases (dehydrogenase). Many examples will be discussed in the later chapters of the book. The enzymes known and classified [7] so far very often involve a metal, either directly at the active center or indirectly at another place, for instance, in the electron transfer process [8],... 

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