The Sequence of Electron Carriers Was Deduced from Kinetic Measurements

The Sequence of Electron Carriers Was Deduced from Kinetic Measurements 

With the help of spectrophotometric techniques, Britton Chance and others showed that any of the normal respiratory substrates can reduce all of the cytochromes in the membrane. Either succinate or NADH-linked substrates also can reduce all of the UQ. The flavins, however, are reduced only partially by succinate but almost completely by a combination of succinate and malate. These observations fit the idea that all of the cytochromes and UQ are part of a common network to which various flavoprotein dehy-drogenases can feed electrons. 

Chance and others also measured the rates at which the different electron carriers became oxidized, following addition of 02 to anaerobic mitochondria, or became reduced after addition of a suitable substrate. When 02 was added, cytochrome a3 became oxidized first, followed by cytochrome a, the c cytochromes, the b cytochromes, and flavins, in that order. Later work showed that UQ became oxidized at about the same rate as the b cytochromes. Observations of this sort led to the proposal that the electron carriers are arranged in the following sequence  

This scheme was supported and refined by examining the effects of specific inhibitors of individual steps in the electron-transport chain. If CO or CN was added in the presence of a reducing substrate and 02, all of the electron carriers became more reduced. This fits the idea that these inhibitors act at the end of the respiratory chain, preventing the transfer of electrons from cytochrome to 02. If amytal (a barbiturate) or rotenone (a plant toxin long used as a fish poison) was added instead, NAD+ and the flavin in NADH dehydrogenase were reduced, but the carriers downstream became oxidized. The antibiotic antimycin caused NAD+, flavins, and the b cytochromes to become more reduced, but cytochromes c, cx, a, and a3 all became more oxidized. The situation here is analogous to the construction of a dam across a stream When the gates are closed, the water level rises upstream from the dam, and falls downstream. The observation that antimycin did not inhibit reduction of UQ showed that the quinone fits into the chain upstream of cytochromes c, t i, a, and a3. 

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A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 The Sequence of Electron Carriers Was Deduced from Kinetic Measurements Redox Potentials Give a Measure of Oxidizing and Reducing Strengths... 

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