A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to

A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 

The electron carriers that participate in the flow of electrons to 02 are a structurally diverse group. Occupying a central position are a series of heme-containing proteins, the cytochromes. Hemes are porphyrins with iron at the center (see chapter 10). Three main types of cytochromes, a, b, and c, exist and are distinguished by different substituents on the periphery of the porphyrin ring and different modes of attachment of the porphyrin to the protein (fig. 14.3). Cytochrome c is a small, water-soluble protein associated loosely with the inner mitochondrial membrane. Another c cytochrome (c,), two b cytochromes (bL and bH), and two a cytochromes (a and a3) are embedded in the membrane as parts of large complexes described below. 

The Fe atoms of the cytochromes undergo oxidation and reduction during respiration, cycling between the ferrous (Fe2+) and ferric (Fe3+) oxidation states. The absorption spectra of the oxidized and reduced forms differ (fig. 14.4). In the 1930s, David Keilin used this property to measure the oxidation-reduction states of cytochromes in living cells. Under anaerobic conditions, the cytochromes rapidly became reduced in the presence of 02, they became oxidized. Certain molecules that inhibited respiration (CO, N3, or CN ) blocked the oxidation other inhibitors (amy-tal, rotenone, and malonate) blocked the reduction. Keilin found that the transfer of electrons from cytochrome c to 02 

Iron protoporphyrin IX (heme) is found in the b-type cytochromes and in hemoglobin and myoglobin. In heme c, cysteine residues of the protein (R) are attached covalently by thioether links to the two vinyl (—CH=CH2) groups of protoporphyrin IX. Heme c is found in the c cytochromes. In heme A, which is found in the a cytochromes, a 15-carbon isoprenoid side chain is attached to one of the vinyls, and a formyl group replaces one of the methyls. 

Optical absorption spectra of a 10 /m solution of cytochrome c in the reduced (blue) and oxidized (red) states. 

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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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