. Biochemistry of plants and animals, an introduction. Biochemistry. ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 173. The term cytochrome denotes a complex protein with a prosthetic group belonging to the class called porphyrins and in the present case contains iron. The iron undergoes reversible oxidation-reduction in the presence of enzymes called cytochrome reductase and cyto- chrome oxidase. The reactions all show negative free-energy changes totaling about 50,000 cal. Of this chemical energy, about 31,000 cal. is known to be transferred when three molecules of ATP are formed. The fate of t
. Biochemistry of plants and animals, an introduction. Biochemistry. ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 173. The term cytochrome denotes a complex protein with a prosthetic group belonging to the class called porphyrins and in the present case contains iron. The iron undergoes reversible oxidation-reduction in the presence of enzymes called cytochrome reductase and cyto- chrome oxidase. The reactions all show negative free-energy changes totaling about 50,000 cal. Of this chemical energy, about 31,000 cal. is known to be transferred when three molecules of ATP are formed. The fate of the remaining energy is unknown, and the particular reaction stages yielding the ATP are still not known. Moreover, there may be more than one kind of cytochrome serving as electron carriers in series. But in spite of prolonged investigation little has been learned of the sequence of events in cells. Terminal oxidations. The last step of the above series is the only one mentioned so far that involves molecular oxygen as the oxidizing agent. This reaction and others utilizing oxygen are the terminal oxidations of respiration. When the last cytochrome of the sequence is oxidized by oxygen in the presence of hydrogen ions and an enzyme, cytochrome oxidase, water is formed. The free-energy decrease is thought to be about 25,000 cal., an unusually large amount for a single biological reaction. There must certainly be some mechanism for transferring at least part of this energy to useful compounds, and there may be intervening steps allowing the transfer of energy in smaller quantities. Other terminal oxidations are known in plants and animals, but the extent to which these reactions function in living cells is still in doubt. They do play roles in various abnormalities and will be discussed later in connection with the metabolism of the two king- doms concerned. Microorganisms appear to possess considerable diversity in their terminal oxidations, but again most of the details have no
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