There are various functions of proteins in the body that are described below.
Proteins as Structural Elements
Proteins, such as collagen, serve as the scaffolding of the body, and thus are important for the structure of tissues.
Figure 2.241 Triple-helix structure of collagen1
Proteins as Enzymes
We will discuss a number of enzymes throughout this class, and the vast majority are proteins. An enzyme catalyzes (enhances the rate) of a chemical reaction. The key part of an enzyme is its "active site". The active site is where a compound to be acted on, known as a substrate, enters. Enzymes are specific for their substrates; they do not catalyze reactions on any random compounds floating by. You might have heard the "lock and key" analogy used for enzymes and substrates, respectively. After the substrate enters the active site and binds, the enzyme slightly changes shape (conformation). The enzyme then catalyzes a reaction that, in the example below, splits the substrate into two parts. The products of this reaction are released and the enzyme returns to its native or original shape. It is then ready to catalyze another reaction. The figure and video below nicely illustrate the function of an enzyme.
Figure 2.242 The function of enzymes2
Enzymes’ names commonly end in -ase, and many are named for their substrate. For example the enzyme amylase cleaves bonds found in amylose and amylopectin.
Proteins as Hormones
Many hormones are proteins. A hormone is a compound that is produced in one tissue, released into circulation, then has an effect on a different organ. Most hormones are produced from several organs, collectively known as endocrine organs. Insulin is an an example of a hormone that is a protein.
Proteins maintain Fluid Balance
Proteins help to maintain the balance between fluids in the plasma and the interstitial fluid. Interstitial fluid is the fluid that surrounds cells. Interstitial fluid and plasma (fluid part of blood) are the two components of extracellular fluid, or the fluid outside of cells. The following figure illustrates the exchange of fluid between interstitial fluid and plasma.
Figure 2.243 Interstitial Fluid and plasma3
Proteins maintain Acid-Base Balance
Proteins serve as buffers, meaning that they help to prevent the pH of the body from getting too high or too low.
Proteins as Transport
Transport proteins move molecules through circulation or across cell membranes. One example is hemoglobin that transports oxygen through the body. We will see a number of other examples as we move through class.
Proteins as Immune Function
Antibodies are proteins that recognize antigens (foreign substances that generate antibody or inflammatory response) and bind to and inactivate them. Antibodies are important in our ability to ward off disease.
Proteins can also serve as neurotransmitters and can be used for energy by forming glucose through gluconeogenesis.