The rate of evolution is given as the number of mutations per 100 amino acid residues per 100 million years.

The rate of evolution is given as the number of mutations per 100 amino acid residues per 100 million years.

Table 20.05 gives the evolutionary rates for an assortment of proteins. Fibrinopeptides are involved in the blood clotting process. They need an arginine at the end and must be mildly acidic overall. Apart from this they can vary widely as there are so few constraints on what is needed. In contrast, histones bind to DNA and are responsible for its correct folding. Almost all changes to a histone would be lethal for the cell, so they evolve extremely slowly.

Cytochrome c is an enzyme whose function depends most critically on a few amino acid residues at the active site, which bind to its heme cofactor. Consequently, these active site residues rarely vary, even though amino acids around them change. Of 104 residues, only Cys-17, His-18 and Met-80 are totally invariant. In other places variation is low; large, nonpolar, amino acid residues always fill positions 35 and 36. Several cytochrome c molecules have been examined by X-ray crystallography and all have the same 3-D structure. Although cytochrome c molecules may vary by as many as 88% of their residues, they retain the same 3-D conformation. Thus, little variation is seen with the amino acids that are essential to the function or structure of cytochrome c.

Insulin is a hormone that evolves at much the same rate as cytochrome c. Insulin consists of two protein chains (A and B) encoded by a single insulin gene. During protein synthesis, a long pro-insulin molecule is made. This has the middle, the C-peptide, cut out and discarded. Disulfide bonds hold the A and B chains together. Since the C chain is not part of the final hormone, it is free to evolve much faster and it changes at almost 10 times the rate of the A and B chains. Notice that all these proteins maintain the critical residues throughout evolution. It is important to note that mutations are random. A mutation is just as likely to occur in the A, B, and C portion of the insulin gene. The mutations that do occur in the A and B portions may be detrimental to the organism, therefore, these mutations never get passed to a new generation. On the other hand mutations in the C portion occur, do not harm the function of the protein, and get passed onto the progeny.

Rapidly evolving sequences reveal the relationships between closely related organisms. Slowly changing sequences are needed to compare distantly related organisms.

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