By the end of this lesson and the next few, you should 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 organization
- Describe the link between protein shape and function
What are proteins?
Proteins are one of the most abundant organic molecules in living systems. In fact, they have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. In addition, 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. However, their structures, like their functions, vary greatly. Nevertheless, they are all polymers of amino acids, arranged in a linear sequence.
Types and Functions of Proteins
Enzymes are usually complex or conjugated proteins. Basically, living cells produce enzymes to act as catalysts in biochemical reactions (like digestion). Each enzyme is specific for the substrate (a reactant that binds to an enzyme) it acts on. The enzyme may help in breakdown, rearrangement, or synthesis reactions.
Catabolic enzymes are enzymes that break down their substrates. Conversely, anabolic enzymes are enzymes that build more complex molecules from their substrates. However, catalytic enzymes are enzymes that affect the rate of reaction. Note that all enzymes increase the rate of reaction and, therefore, are organic catalysts. An example of an enzyme is salivary amylase, which hydrolyzes its substrate amylose, a component of starch.
Hormones are chemical-signaling molecules, usually small proteins or steroids that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. Endocrine cells secrete hormones. For example, insulin is a protein hormone that helps to regulate the blood glucose level.
Table Showing Protein Types and Functions
The table below shows the primary types and functions of proteins.
|Protein Types and Functions|
|Digestive Enzymes||Amylase, lipase, pepsin, trypsin||Help in digestion of food by catabolizing nutrients into monomeric units|
|Transport||Hemoglobin, 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 the activity of different body systems|
|Defense||Immunoglobulins||Protect the body from foreign pathogens|
|Contractile||Actin, myosin||Effect muscle contraction|
|Storage||Legume storage proteins, egg white (albumin)||Provide nourishment in early development of the embryo and the seedling|
Proteins have different shapes and molecular weights. Some proteins are globular in shape whereas others are fibrous in nature. For example, hemoglobin is a globular protein, but collagen, found in our skin, is a fibrous protein. Protein shape is critical to its function. Furthermore, many different types of chemical bonds maintain this shape.
Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the shape of the protein. As a result, this could lead to loss of function, known as denaturation. Generally, all proteins consist of different arrangements of the same 20 types of amino acids.