Proteins act as enzymes, hormones, and antibodies. They maintain fluid balance and acid and base balance. They also transport substances such as oxygen, vitamins, and minerals to target cells throughout the body. Structural proteins, such as collagen and keratin, are responsible for the formation of bones, teeth, hair, and the outer layer of skin, and they help maintain the structure of blood vessels and other tissues. In contrast, motor proteins use energy and convert it into some form of mechanical work (e.g., dividing cells, contracting muscle). Enzymes are proteins that facilitate chemical reactions without being changed in the process.
The inactive form of an enzyme is called a proenzyme. Hormones (chemical messengers) are proteins that travel to one or more specific target tissues or organs, and many have important regulatory functions. Insulin, for example, plays a key role in regulating the amount of glucose in the blood. The body manufactures antibodies (giant protein molecules), which combat invading antigens. Antigens are usually foreign substances such as bacteria and viruses that have entered the body and could potentially be harmful. Immuno-proteins, also called immuno-globulins or antibodies, defend the body from possible attack by these invaders by binding to the antigens and inactivating them.
Proteins help to maintain the body’s fluid and electrolyte balance. This means that proteins ensure that the proper types and amounts of fluid and minerals are present in each of the body’s three fluid compartments. These fluid compartments are intracellular (contained within cells), extracellular (existing outside the cell), and intravascular (in the blood). Without this balance, the body cannot function properly.Proteins also help to maintain balance between acids and bases within body fluids. The lower a fluid’s pH, the more acidic it is. Conversely, the higher the pH, the less acidic the fluid is. The body works hard to keep the pH of the blood near 7.4 (neutral). Proteins also act as carriers, transporting many important substances in the bloodstream for delivery throughout the body. For example, a lipoprotein transports fat and cholesterol in the blood.