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Medicine LibreTexts

6.3: Functions of Protein

  • Page ID
    21138
  • Learning Objectives

    • Describe the various functions of protein in the body.

    Proteins build muscle and all body tissues. It is a crucial role of the proteins to provide the structure of virtually all body tissues. Some proteins have other "jobs" to do. In other words, they are not part of the body structure, but have specific tasks to perform such as acting as transporter, antibodies, enzymes and more. You have already learned about some of these important proteins. When you learned about diabetes you studied the hormones insulin and glucagon. These are examples of two protein molecules that act as hormones with very specific and important "jobs" relative to carbohydrate metabolism.

    Enzymes

    Some proteins function as enzymes. Enzymes are proteins that conduct specific chemical reactions. An enzyme’s job is to provide a site for a chemical reaction and to lower the amount of energy and time it takes for that chemical reaction to happen (Figure \(\PageIndex{1}\)). This is why enzymes are sometimes called catalysts. On average, more than one hundred chemical reactions occur in cells every single second and most of them require enzymes. The liver alone contains over one thousand enzyme systems. Enzymes are specific and will use only particular substrates (or substances in Figure \(\PageIndex{1}\)) that fit into their active site, similar to the way a lock can be opened only with a specific key. Nearly every chemical reaction requires a specific enzyme. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although eventually it is destroyed and rebuilt. All bodily functions, including the breakdown of nutrients in the stomach and small intestine, the transformation of nutrients into molecules a cell can use, and building all macromolecules, including protein itself, involve enzymes.

    Process of 2 substances attaching to an enzyme which allows substances to combine into 1 product.
    Figure \(\PageIndex{1}\): Enzymes are proteins. An enzyme’s job is to provide a site for substances to chemically react and form a product and decrease the amount of energy and time it takes for this to happen.

    Hormones

    Proteins are responsible for hormone synthesis. Hormones are chemical messengers produced in one part of the body and then transported in the blood to a different part of the body. When the hormone gets to the target tissue/part of the body, it communicates a message to initiate a specific reaction or cellular process. For instance, after you eat a meal, your blood glucose levels rise. In response to the increased blood glucose, the pancreas releases the hormone insulin. Insulin tells the cells of the body that glucose is available and to take it up from the blood and store it or use it for making energy or building macromolecules. A major function of hormones is to turn enzymes on and off, so some proteins can even regulate the actions of other proteins. While not all hormones are made from proteins, many of them are. Other examples of hormones made from proteins include glucagon, melatonin, and thyroid hormone.

    Fluid and Electrolyte Balance

    Proper protein intake enables the basic biological processes of the body to maintain the status quo in a changing environment. Fluid balance refers to maintaining the distribution of water in the body. If too much water in the blood suddenly moves into a tissue, the results are swelling and, potentially, cell death. Water always flows from an area of high concentration to one of a low concentration. As a result, water moves toward areas that have higher concentrations of other solutes, such as proteins and glucose. Proteins attract fluid, so to keep the water evenly distributed between blood and cells, proteins continuously circulate at high concentrations in the blood. When protein intake is deficient it can cause edema (swelling). The most abundant protein in the blood is albumin. Albumin’s presence in the blood makes the protein concentration in the blood similar to that in cells. Therefore, fluid exchange between the blood and cells is not in the extreme, but rather is minimized to preserve the status quo. Transport proteins (discussed below) in the cell membrane help to maintain the proper balance of electrolytes (like sodium and potassium) inside and outside the cell.

    Acid-Base (pH) Balance

    Protein is also essential in maintaining proper pH balance (the measure of how acidic or basic a substance is) in the blood. The pH scale ranges from 0 (strongly acidic) to 14 (strongly basic/alkaline). Blood pH is maintained between 7.35 and 7.45, which is slightly basic. If the blood becomes too acidic (a condition known as acidosis) it means that the level of hydrogen (H+) in the blood is excessive. If the blood becomes too basic/alkaline (a condition known as alkalosis) it means that the level of H+ in the blood is deficient. Even a slight change in blood pH can affect body functions. Two examples of this include:

    • When proteins are exposed to acids or bases the proteins change shape and stop functioning as intended. This process of proteins uncoiling and losing their shape and function is known as denaturation. Denaturation of proteins also occurs with exposure to heat, heavy metals, alcohol, and other damaging substances.
    • You learned in Chapter 4 that acidic blood (from ketoacidosis) can lead to coma and/or death in extreme cases.

    The body has several systems that hold the blood pH within the normal range to prevent issues. Some proteins act as buffers and release hydrogen (H+) into the blood if it gets too basic. Proteins can also take hydrogen from the blood if it gets too acidic. By releasing and taking hydrogen when needed, proteins maintain acid-base balance and keep blood pH within a normal range.

    Transport

    Proteins also play a role in nutrient transport. A cell’s membrane is usually not permeable to large molecules. To get the required nutrients and molecules into the cell, many transport proteins exist in the cell membrane. Some of these proteins act as channels that allow particular molecules to move in and out of cells. Others act as one-way taxis and require energy to function (Figure \(\PageIndex{2}\)).

    Drawing of the cell membrane with a protein channel and carrier proteins which allow molecules to pass through the membrane.
    Figure \(\PageIndex{2}\): Molecules move in and out of cells through transport proteins, which are either channels or carriers. Facilitated diffusion in the cell membrane, showing ion channels (left) and carrier proteins (three on the right). (CC0; by LadyofHats via Wikimedia Commons)
    .

    Antibodies

    Our immune system is designed to attack and destroy foreign substances. When a foreign substance attacks the body, the immune system produces antibodies (Figure \(\PageIndex{3}\)) to defend against it. Antibodies are special proteins that recognize a unique molecule on harmful bacteria and viruses known as an antigen. Antibodies bind to the antigen and destroy it. Antibodies also trigger other factors in the immune system to seek and destroy unwanted intruders.

    Drawing of an antibody (Y-shaped figure) that attracts antigens that will bind at the edges of the antibody and then be destroyed.
    Figure \(\PageIndex{3}\): Antibodies are proteins that surround and attack foreign substances by attaching to antigens on the surface of the foreign substance. Each antibody binds to a specific antigen; an interaction similar to a lock and key. (public domain; by Fvasconcellos via Wikimedia Commons)

    Wound Healing, Tissue Regeneration, and Nerve Function

    Proteins are involved in all aspects of wound healing, a process that takes place in three phases: inflammation, proliferation, and remodeling. For example, if you get a small cut, your skin will turn red and become inflamed. The healing process begins with proteins, such as bradykinin, which dilate blood vessels at the site of injury. An additional protein called fibrin helps to secure platelets that form a clot to stop the bleeding. Next, in the proliferation phase, cells move in and mend the injured tissue by installing newly made collagen (protein) fibers. The collagen fibers help pull the wound edges together. In the remodeling phase, more collagen is deposited, forming a scar. Scar tissue is only about 80 percent as functional as normal uninjured tissue. If a diet is insufficient in protein, the process of wound healing is markedly slowed.

    While wound healing takes place only after an injury is sustained, a different process called tissue regeneration is ongoing in the body. During tissue regeneration, an exact structural and functional copy of the old tissue is created. Ultimately, the old tissue is replaced with brand new, fully functional tissue. Cells are constantly being broken down, repaired, and replaced. When proteins in the cells are broken down, the amino acids are recycled into new proteins. Some cells (such as skin, hair, nails, and intestinal cells) have a very high rate of regeneration, while others, (such as heart-muscle cells and nerve cells) do not regenerate at any appreciable levels. Tissue regeneration is the creation of new cells (cell division), which requires many different proteins including enzymes, transport proteins, hormones, and collagen. The cells lining the intestine regenerate every three to five days. Protein-inadequate diets impair tissue regeneration, causing many health problems including impairment of nutrient digestion and absorption.

    Amino acids can be used to make neurotransmitters (e.g., epinephrine) that transmit messages from one nerve cell to another.

    Energy Source

    Some of the amino acids in proteins can be disassembled and used to make energy. In healthy people, protein contributes little to energy needs. If a person’s diet does not contain enough carbohydrates and fats their body will use amino acids to make energy. When proteins are needed for energy, they are taken form the blood and body tissues (e.g., muscle). To use proteins for energy, deamination is required. Deamination is a process where the amine group is removed from the amino acid and the nitrogen is transported to the kidney for excretion. The remaining components are metabolized for energy. To protect our body tissues from being broken down for energy, it is important to eat an adequate amount of fat and carbohydrate. It's also important to note that our body cannot store excess protein. Excess protein intake results in nitrogen excretion; the remaining components are used for energy or converted to fat for later use. 

    Key Takeaways

    • Proteins have multiple functions, including: acting as enzymes and hormones, maintaining proper fluid and acid-base balance, providing nutrient transport, making antibodies, enabling wound healing and tissue regeneration, and providing energy when carbohydrate and fat intake is inadequate.
    • Without adequate intake of protein containing all the essential amino acids, protein functions will be impaired.
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