Figure 3–20 Protein assemblies. (A) A protein with just one binding site can (A) form a dimer with another identical protein. (B) Identical proteins with two free assembled different binding sites often form a long helical filament. (C) If the two binding subunits structures sites are disposed appropriately in relation to each other, the protein subunits may form a closed ring instead of a helix. (For an example of A, see
Figure 3–18; for an example of B, see Figure 3–21; for examples of C, see Figures 5–14 and 14–31.) site site complementary to another region of the surface of the same molecule (Figure 3–20). An actin filament, for example, is a long helical structure produced from many molecules of the protein actin (Figure 3–21). Actin is a globular protein that is very abundant in eukaryotic cells, where it forms one of the major filament systems of the cytoskeleton (discussed in Chapter 16).
We will encounter many helical structures in this book. Why is a helix such a common structure in biology? As we have seen, biological structures are often formed by linking similar subunits into long, repetitive chains. If all the subunits are identical, the neighboring subunits in the chain can often fit together in only one way, adjusting their relative positions to minimize the free energy of the contact between them. As a result, each subunit is positioned in exactly the same way in relation to the next, so that subunit 3 fits onto subunit 2 in the same way that subunit 2 fits onto subunit 1, and so on. Because it is very rare for subunits to join up in a straight line, this arrangement generally results in a helix—a regular structure that resembles a spiral staircase, as illustrated in Figure 3–22. Depending on the twist of the staircase, a helix is said to be either right-handed or left-handed (see Figure 3–22E). Handedness is not affected by turning the helix upside down, but it is reversed if the helix is reflected in the mirror.
The observation that helices occur commonly in biological structures holds true whether the subunits are small molecules linked together by covalent bonds (for example, the amino acids in an α helix) or large protein molecules that are linked by noncovalent forces (for example, the actin molecules in actin filaments). This is not surprising. A helix is an unexceptional structure, and it is generated simply by placing many similar subunits next to each other, each in the same strictly repeated relationship to the one before—that is, with a fixed rotation followed by a fixed translation along the helix axis, as in a spiral staircase.
Many Protein Molecules Have Elongated, Fibrous Shapes
Enzymes tend to be globular proteins: even though many are large and complicated, with multiple subunits, most have an overall rounded shape. In Figure 3–21, we saw that a globular protein can also associate to form long filaments. But there are also functions that require each individual protein molecule to span a large distance. These proteins generally have a relatively simple, elongated three-dimensional structure and are commonly referred to as fibrous proteins.
One large family of intracellular fibrous proteins consists of α-keratin, introduced when we presented the α helix, and its relatives. Keratin filaments are extremely stable and are the main component in long-lived structures such as hair, horn, and nails. An α-keratin molecule is a dimer of two identical subunits, with the long α helices of each subunit forming a coiled-coil (see Figure 3–9). The coiled-coil regions are capped at each end by globular domains containing binding sites. This enables this class of protein to assemble into ropelike intermediate filaments—an important component of the cytoskeleton that creates the cell’s internal structural framework (see Figure 16–67).