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Proteins are polymers of amino acids. The sequence of amino acids in a protein is its primary structure. Secondary, tertiary, and quaternary structures arise through interactions among the amino acids. A protein’s three-
learning outcomes
You should be able to:
Predict how a protein’s tertiary structure will be affected by a change in an amino acid in its primary structure.
Analyze data to determine why an amino acid change in a protein leads to a change in the protein’s shape.
Predict the effects of various environmental factors on a protein’s structure.
Suppose that, in a given protein, one lysine is replaced by aspartic acid (see Table 3.2). Does this change occur in the primary structure or in the secondary structure? How might it result in a change in tertiary structure? In quaternary structure?
When one amino acid (such as lysine) is replaced by another, the primary structure of the protein is altered. The change could affect tertiary structure if the protein folds differently as a result of electrostatic attractions between charged amino acids (+ to –). In this example, a negatively charged amino acid (aspartic acid) has taken the place of a positively charged one (lysine), and this may prevent correct folding, particularly if a negatively charged amino acid elsewhere in the polypeptide chain is involved in folding (it is attracted to a + amino acid). The same forces might be at work in the interaction of separate chains for quaternary structure.
A single amino acid change in a protein can change its shape. For example, the amino acid glycine may normally reside at a certain position in a protein. If glycine is replaced with either glutamic acid or arginine, the protein shape near that amino acid changes significantly. There are two possible explanations for this:
A small amino acid at that position in the polypeptide is necessary for normal shape.
An uncharged amino acid is necessary for normal shape.
Further amino acid substitutions are done to distinguish between these possibilities. Replacing glycine with serine or alanine results in normal shape, but replacing glycine with valine changes the shape. Which of the two possible explanations is supported by the observations? Explain your answer.
The observations support explanation “a.” Glycine is small and nonpolar. Glutamic acid and arginine are larger and polar (charged). Serine and alanine are small: the protein retains its shape. But serine is polar (it has –OH as its R group), and that does not affect the structure. Valine is larger and nonpolar, and this affects shape. So the issue is size.
Human hair is composed of the protein keratin. At the hair salon, two techniques are used to modify the three-
See Figure 3.10. Heat breaks hydrogen bonds and other weak interactions that maintain protein shape. Disulfide bonds also are required for normal protein shape. Styling and perms partially denature keratin, then renature the protein in a new shape. Your investigation might involve measuring keratin protein structure of hair before and after disrupting hydrogen bonds and disulfide bonds.