4.2 Functional Domains and Symmetry

Large, globular proteins are generally made up of two or more recognizable distinct structures called domains. A module is a compact, folded protein structure that is usually stable by itself in an aqueous solution.

Figure 4.5: A Protein Domain

Domains (see figure 4.5) are usually made up of a single, continuous portion of the polypeptide or an entire chain (in cases where multiple polypeptides assemble together).

Figure 4.6: Examples of Protein Domains

Some examples of protein domains are shown in figure 4.6. Completely new proteins tend to have evolved from gene duplications, insertions, deletions, inversions, or translocations: an efficient way of creating potential for novel function (i.e., genetic shuffling).

4.2.1 Symmetry

Figure 4.7: Symmetry in Proteins

Figure 4.7 shows several protein symmetries. A C2 symmetry is quite common among proteins (especially a homodimer like glycogenin).

Figure 4.8: Symmetries in Viral Capsids

Figure 4.8 displays icosohedral symmetry in a poliovirus (i.e., (a)) and helical symmetry in a tobacco mosaic virus (i.e., (b)).