Fibrous Versus Globular Proteins
Thus far we have looked at amino acids: essential, non-essential and conditionally essential. We have also recognized that amino acids differ based on their R side chains and can be polar, non-polar, hydrophilic, hydrophobic, basic, acidic or special. We have learned that amino acids are linked together to form polypeptide chains which eventually form functional proteins. We have also looked at the four levels of structure of proteins. In this topic we address the classification of proteins into two common groups of proteins: fibrous and globular.
The polypeptide chains of Fibrous Proteins are arranged into long strands or sheets. This makes them structurally tough. It is no small wonder that in many animals they are in the fur, quills, scales, nails, feathers, horns, antlers, hooves, and parts of the skin where they have a defensive or protective role. The fibrous proteins collagen and elastin are essential to connective tissues, including tendons, cartilage, bone, and the deeper skin layers. Leather is almost pure collagen.
Earlier we looked at Fibrin which is made from fibrinogen, a fibrous protein that binds platelets together to form blood clots. We also mentioned Actin and myosin, the fibrous proteins that play a major part in skeletal muscle contraction. Another notable fibrous protein that we have not yet mentioned is tubulin. This fibrous protein forms microtubules, tiny hollow tubes inside cells which play a role in cell movement, in the transport of material within nerve cells, and in the maintenance of cellular shape.Â
In contrast to the long strands or sheets of fibrous proteins, Globular Proteins consist of polypeptide chains that are tightly folded into a spherical or globular shape. Many have electrically charged groups of atoms exposed to cytoplasm and bodily fluids and many of them are highly soluble.Â
- The largest category of globular proteins are Antibodies (Immunoglobulins).Â
- Most enzymes are globular proteins.
- Other globular proteins are hormones.
