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Cytochromes are electron-transporting protein pigments concerned with cell respiration that contain an iron-containing molecule called heme, allied to that of hemoglobin. When the iron of heme accepts an electron, it changes from the oxidized ferric (Fe III) state to the reduced ferrous (Fe II) state. The oxidation of cytochromes to molecular oxygen and their subsequent reduction by oxidizable substances in the cell is the main way in which atmospheric oxygen enters into the metabolism of the cell. About 90% of all oxygen consumed is mediated by the cytochromes.

Cytochromes make up two of the three large enzyme complexes that together comprise the electron transport or respiratory chain. This chain represents the end of oxidative phosphorylation, the process by which many organisms synthesize the energy-rich molecules of adenosine triphosphate (ATP) needed for life processes.

The source of the electrons to drive the respiratory chain is from the metabolic breakdown (catabolism) of food molecules. Two major pathways of metabolism-glycolysis and the Krebs cycle-break down glucose molecules and provide the electrons for the third pathway, the respiratory chain.

Glycolysis is the preliminary process during which the 6-carbon sugar molecule glucose is split into 3-carbon products, a process that renders only a few electrons for the respiratory chain. The more efficient Krebs cycle, which uses the 2-carbon products of glycolysis as raw materials for a cyclic series of enzymatic reactions, produces many more electrons.

The electron transport chain extends from the initial electron donor, nicotinamide adenine dinucleotide (NADH), to oxygen, the final electron acceptor.

The exchange of electrons begins at the NADH dehydrogenase complex, which passes electrons to ubiquinone (coenzyme Q). Ubiquinone, in turn, passes electrons to the cytochrome b-c1 complex, which is composed of cytochromes and iron-sulfur proteins. The last cytochrome in this complex (cytochrome c) passes electrons to the cytochrome oxidase complex, composed of both cytochromes and copper atoms. Finally, the cytochrome oxidase complex passes electrons to oxygen.

The exchange of electrons along the respiratory chain generates a gradient of protons across the membrane in which the chain is located. When the protons flow back across the membrane, they activate the enzyme ATP synthetase, which produces ATP from adenosine diphosphate (ADP).

Cells that use the respiratory chain produce most of the supply of high-energy molecules of ATP needed for life. Many bacteria do not use oxygen (i.e., they are anaerobic), and consequently lack respiratory chain enzymes. These bacteria must rely on the less efficient glycolysis to produce ATP.

Cytochromes occur in organisms as varied as bacteria, yeast, humans, and insects. Indeed, beginning in 1925, researcher David Keilin made the first observations of cytochrome activity by studying the change in the wavelengths of light absorbed by cytochromes of flight muscles of living insects as the cytochromes underwent oxidation and reduction. He correctly postulated that these pigments underwent reduction and oxidation as they accepted electrons and then transferred them along the chain to the final electron acceptor, oxygen.

The heme group of cytochromes consists of a carbon-based ring called porphyrin, in which the iron atom is tightly bound by nitrogen atoms at each corner of a square. Related porphyrin molecules include hemoglobin, the oxygen-carrying molecule in blood, and chlorophyll, the green pigment of photosynthetic plants.

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