2.4 Protein
Learning Objectives
By the end of this section, you will be able to:
- Describe the functions proteins perform in the cell and in tissues
- Discuss the relationship between amino acids and proteins
- Explain the four levels of protein organisation
- Describe the ways in which protein shape and function are linked
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all, however, amino acid polymers arranged in a linear sequence.
Types and Functions of Proteins
Enzymes, which living cells produce, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) upon which it acts. The enzyme may help in breakdown, rearrangement, or synthesis reactions. We call enzymes that break down their substrates catabolic enzymes. Those that build more complex molecules from their substrates are anabolic enzymes, and enzymes that affect the rate of reaction are catalytic enzymes. Note that all enzymes increase the reaction rate and, therefore, are organic catalysts. An example of an enzyme is salivary amylase, which hydrolyses its substrate amylose, a component of starch.
Hormones are chemical-signalling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. For example, insulin is a protein hormone that helps regulate the blood glucose level. Table 2.4.1 lists the primary types and functions of proteins.
Protein Types and Functions
Table 2.4.1. Types of proteins and their functions.
| Type | Examples | Functions |
| Digestive Enzymes | Amylase, lipase, pepsin, trypsin | Help in food by catabolising nutrients into monomeric units |
| Transport | Haemoglobin, albumin | Carry substances in the blood or lymph throughout the body |
| Structural | Actin, tubulin, keratin | Construct different structures, like the cytoskeleton |
| Hormones | Insulin, thyroxine | Coordinate different body systems’ activity |
| Defences | Immunoglobulins | Protect the body from foreign pathogens |
| Contractile | Actin, myosin | Effect muscle contraction |
| Storage | Legume storage proteins, egg white (albumin) | Provide nourishment in early embryo development and the seeding |
Proteins have different shapes and molecular weights. Some proteins are globular in shape; whereas, others are fibrous in nature, for example, haemoglobin is a globular protein, but collagen, located in our skin, is a fibrous protein. Protein sha