Globular Proteins

The two major classes of proteins are the fibrous proteins and the globular proteins. Fibrous proteins are structural proteins. They are very abundant in cells. Globular proteins, on the other hand, far outnumber fibrous proteins. Globular proteins perform most of the chemical "work" of the cell - synthesis, transport, and catabolism. They are folded into compact structures very unlike the extended, filamentous forms of the fibrous proteins.

X-ray diffraction technology has enabled scientists to determine atomic positions in globular proteins to within 0.3 nm, a resolution sufficient to identify individual amino acid residues. The general structure of one globular protein, myoglobin, is shown in Figure 6.1.

Globular proteins illustrate the concept of tertiary structure - which arises from folding of the polypeptide chain upon itself. Unlike secondary structure, which is caused by interactions between amino acids close to each other, tertiary structures are seen to be stabilized by interactions between amino acids that are often far apart. Tertiary structures have little regularity. Because they arise from folding of secondary structures, which are dependent on the primary amino acid sequence, tertiary structures are specific for each protein sequence. Similarities in tertiary structure can usually be seen, however, in proteins with similar amino acid sequences.

Figure 6.16 schematically depicts differences in tertiary structure between six different proteins. In this figure, color coding is used to identify the helical (blue) and sheet (orange) secondary structures. Notice that the secondary structures, which are common to all of the proteins are folded in such a way that none of the overall molecular shapes looks the same. Despite the apparent chaos of tertiary folding, there are several general rules that have been observed:

> 1. Folds favor orientation of amino acid residues in specific ways that pack hydrophobic amino acid residues on the inside of the protein (away from water) and hydrophilic amino acid residues on the outside of the protein.

> 2. sheets are usually twisted, or wrapped into barrel structures (see immunoglobulin and prealbumin in Figure 6.16).

> 3. Turns (interruptions between secondary structures) can occur in a number of ways. They usually occur at the surface of proteins. Figure 6.18 depicts two examples of turns, which completely reverse the direction of the polypeptide chain in only four amino acid residues. This occurs via hydrogen bonding between resides 1 and 4. A tighter turn, called the turn, is shown in Figure 6.19. It occurs in only 3 residues.

> 4. Not all parts of globular proteins can be classified as helix, sheets, or turns. These other regions depicted in green in Figure 6.16 are sometimes described as irregularly structured regions and random coils. An irregular region is a non-regular structure that is stable enough to be seen by x-ray crystallographic analysis. A random coil, by contrast, has so many possible structures as to be random at any given time. It does not crystallize readily so its structure cannot be determined by x-ray diffraction. Be aware, though, that the term random coil is often used inaccurately to also describe irregularly structured regions.

See also: Fibrous Proteins, Secondary Structure (General), -Helix, -Sheets, -Turns