1.6 Fibrous proteins
Most of the proteins described so far have been globular proteins. There are, however, some distinctive features that characterise fibrous proteins and we present here a general overview of these. Elongated fibrous proteins frequently play a structural role in the cell. They do not readily crystallise but tend to aggregate along their long axis to form fibres. X-ray diffraction studies of these fibres, in contrast to analysis of protein crystals, provides only very limited information on the structure of the protein.
The overall shape and structure of fibrous proteins are determined principally by their secondary structure. α-keratin is an intracellular globular protein comprising two long subunits, the α-helical portions of which form a coiled-coil structure as already described (). Through interactions between globular domains at either end of the helical region, this protein can form long filaments. These structures are termed intermediate filaments and, along with microtubules and microfilaments, comprise a cell's cytoskeleton. Notice that the basic building blocks of microtubules and microfilaments are globular proteins, whilst intermediate filaments are composed of fibrous units.
The extracellular matrix (connective tissue), which can be thought of as the glue that holds cells together in a tissue, contains many fibrous proteins. Principal among these is collagen, which consists of a triple helix of three polypeptide chains (Figure 22). A single collagen molecule is 300 nm long and these units can assemble into many long overlapping strands to form strong cable-like structures that support and protect cells. The extracellular matrix derives much of its strength from collagen. In contrast, other extracellular matrix components such as elastin contribute elastic qualities. Elastin's polypeptide constituents are covalently cross-linked, creating a non-rigid net-like structure, which permits a degree of distortion or deformation in the tissue.