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26.4 Amino Acid Sequences of Cytochrome c The two computer graphics show how similar the three-dimensional structures of tuna and rice cytochrome c are.The amino acid sequences shown in the table were obtained from analyses of cytochromes c from 33 species of plants, fungi, and animals.

view that most such nonsynonymous mutations are disadvantageous and are eliminated from the population by natural selection. As a result, the more essential a molecule is for cell functioning, the slower the rate of its evolution. These functional constraints provide part of the answer to the question posed above.

A molecule that illustrates this principle is the enzyme cytochrome c, a component of the respiratory chain of mitochondria. Together with other enzymes of the citric acid cycle and respiratory chain, cytochrome c is found in all eu-karyotes and is essential for the life of the eukaryotic cell. The amino acid sequences of cytochrome c are known for more than a hundred species of organisms, including protists, plants, fungi, and mammals. Within these cytochromes c are regions that have accumulated changes relatively quickly; for example, amino acid positions 44, 89, and 100 differ among many of the organisms compared (Figure 26.4). There are also invariant amino acid positions, such as 14, 17, 18, and 80. This particular set of invariant residues is known to interact with the iron-containing heme group, which is essential for enzyme functioning. Because any mutations that changed these amino acids would have diminished the functioning of cyto-chrome c, they would have been removed by natural selection when they arose.

Changes in macromolecules can serve as molecular clocks

Earlier in this chapter, we mentioned that the rate of evolution of some macromolecules might in effect be a molecular clock. To function as a molecular clock, a particular macro-molecule would need to evolve at an approximately constant rate in all evolutionary lineages that possess it. But do macro-molecules actually behave in this way?

Often they do. For example, if we use the fossil record to determine the time since the divergence of certain organisms, and then plot this time against the number of amino acids by which the nucleotide sequence of the organisms' cytochrome

Multiple amino acids at a position indicate a great deal of change and that the position is probably less significant.

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