7.5: Protein’s Functions in the Body

Diverse Functions of Proteins

Proteins are the “workhorses” of the body and participate in many bodily functions. They are responsible for doing most of the work in your body, from breaking down molecules during chemical digestion to providing structure. Just as proteins have diverse functions, they come in all sizes and shapes (Figure \(\PageIndex{1}\)). For example, some hormones can be made from fewer than 10 amino acids, whereas the largest human protein has 27,000 amino acids. Each protein has a specific structure that allows it to perform a specific function in the body. Table \(\PageIndex{1}\) summarizes the functions of proteins with examples of proteins found in the human body.

Figure \(\PageIndex{1}\): Diversity in proteins. Small proteins, such as hormones, have fewer amino acids than larger proteins, such as enzymes or antibodies. Source: ”Enzyme, antibody, and hormone” from An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0.

Table \(\PageIndex{1}\): Functions and Examples of Proteins in the Human Body.

Protein functions with examples of protein types

Protein Functions	Examples
Structure: Provide strength and structure to tissues.	Collagen, Elastin, Keratin
Motion: Provide movement for cells and the body.	Actin, Myosin
Enzymes: Speed up chemical reactions, including digestion and synthesis of biological molecules	Pepsin, Salivary amylase
Hormones: Chemical signals that help regulate body functions	Insulin, Glucagon
Fluid and pH Balance: Maintain the balance of pH and fluid content in cellular and body compartments.	Albumin
Transport: Move molecules in and out of cells and throughout the body.	Albumin, Hemoglobin
Immunity: Provide defense against foreign pathogens, such as viruses	Lysozyme, Antibodies

Structure

More than 100 different structural proteins have been discovered in the human body, but the most abundant by far is collagen, which makes up about 6% of total body weight. Collagen makes up 30% of bone tissue and comprises large amounts of tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein consisting mostly of glycine and proline amino acids. In its functional form, collagen comprises three protein strands that twist around each other like a rope, and then these collagen ropes overlap with others (Figure \(\PageIndex{2}\)). This highly ordered structure is even stronger than steel fibers of the same size. Collagen makes bones strong but flexible. Collagen fibers in the skin’s dermis provide structure, and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; the collagen and elastin proteins in skin allow it to return to its original shape. Smooth-muscle cells that secrete collagen and elastin proteins surround blood vessels, providing the vessels with structure and the ability to stretch back after blood is pumped through them. Keratin is another strong, fibrous protein that makes up skin, hair, and nails.

Figure \(\PageIndex{2}\): Collagen. Collagen is a strong protein made of intertwined chains of amino acids. Each chain is simply shown as a line in this diagram. Source: "1K6F Crystal Structure Of The Collagen Triple Helix Model Pro- Pro-Gly103 03" by Nevit Dilmen is licensed under CC BY-SA 3.0.

Motion

The closely packed collagen fibrils in tendons and ligaments allow for synchronous mechanical movements of bones and muscle and the ability of these tissues to spring back after a movement is complete. Move your fingers and watch the synchrony of your knuckle movements. To move, muscles must contract. The contractile parts of muscles are the proteins actin and myosin. When a nerve impulse stimulates these proteins, they slide across each other, causing a shortening of the muscle cell. Upon stimulation, multiple muscle cells shorten simultaneously, resulting in muscle contraction.

Enzymes

Although proteins are found in the greatest amounts in connective tissues such as bone, their most extraordinary function is as enzymes. Enzymes are proteins that conduct specific chemical reactions. Enzymes are protein catalysts, which lower the amount of energy and time it takes for a specific chemical reaction to occur. 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 1000 enzyme systems. Enzymes are specific and will use only particular substrates or starting materials that fit into their active site, similar to the way a lock can be opened only with a specific key (Figure \(\PageIndex{3}\)). Nearly every chemical reaction requires a specific enzyme. Fortunately, an enzyme can fulfill its role as a catalyst repeatedly, 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 the building of all macromolecules, including protein itself, involve enzymes.

Figure \(\PageIndex{3}\): Enzymes catalyze chemical reactions. Enzymes are proteins that bind specific starting substances, or substrates, shown as blue and red squares (A and B). In the active site, the enzyme speeds up a chemical reaction between the substrates to form the product (AB), shown as a purple rectangle. “Enzyme activity” from An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0.

Video \(\PageIndex{1}\): Introduction to Enzymes

Watch this video to learn more about how enzymes function.¹ Source: https://youtu.be/qgVFkRn8f10?si=idsFXYZNw_7bcP1C

Hormones

Proteins are responsible for hormone synthesis. Hormones are the chemical messages produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then transported in the blood to its target cell, where 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 body's cells that glucose is available and instructs them 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 are.

Fluid and Acid-Base Balance

Proper protein intake enables the body's basic biological processes 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 low concentration. As a result, water moves toward areas that have higher concentrations of other solutes, such as proteins and glucose. To keep the water evenly distributed between blood and cells, proteins continuously circulate at high concentrations in the blood. The most abundant protein in blood is the butterfly-shaped protein known as albumin (Figure \(\PageIndex{4}\)). 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 extreme but rather minimized to preserve the status quo.

Protein is also essential in maintaining proper pH balance in the blood. The pH scale is a measure of acidity or alkalinity of a solution. A pH greater than 7 indicates a solution is basic, and a pH less than 7 indicates a solution is acidic. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a slight change in blood pH can affect body functions. Acidic conditions can cause proteins to change shape or misfold, which is also known as denaturation. When proteins are denatured, they can no longer function properly. The body has several systems that hold the blood pH within the normal range to prevent this from happening. One of these is the circulating albumin. Albumin is slightly acidic, and because it is negatively charged, it balances the many positively charged molecules, such as hydrogen protons (H⁺) and calcium, potassium, and magnesium, which are also circulating in the blood. Albumin acts as a buffer against abrupt changes in the concentrations of these molecules, thereby balancing blood pH and maintaining the status quo. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.

Figure \(\PageIndex{4}\): Albumin. The butterfly-shaped protein, albumin, has many functions in the body including maintaining fluid and acid-base balance and transporting molecules. Cartoon representation of the molecular structure of protein registered with 1ao6 code. "PDB 1ao6 EBI" by Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute is in the Public Domain.

 

Transport

Albumin and hemoglobin also play a role in molecular transport. Albumin chemically binds to hormones, fatty acids, some vitamins, essential minerals, and drugs and transports them throughout the circulatory system. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the tissues in the body. A cell’s plasma membrane is usually not permeable to large polar molecules, so many transport proteins are present in the cell membrane to get the required nutrients and molecules into the cell. Some of these proteins are channels that allow particular molecules to move in and out of cells, whereas others act as one-way taxis and require energy to function (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Cell Transport. Molecules move in and out of cells through transport proteins, which are either channels or carriers. Facilitated diffusion in cell membrane, showing ion channels (left) and carrier proteins (three on the right). "Scheme facilitated diffusion in cell membrane-en" by Lady of Hats is in the Public Domain.

Immunity

Earlier, we discussed how the strong collagen fibers in the skin provide it with structure and support. The skin’s dense network of collagen fibers also serves as a barricade against harmful substances. The immune system’s attack and destroy functions are dependent on enzymes and antibodies, which are also proteins. An enzyme called lysozyme is secreted in the saliva and attacks the walls of bacteria, causing them to rupture. Certain proteins circulating in the blood can be directed to build a molecular knife that stabs the cellular membranes of foreign invaders. The antibodies secreted by the white blood cells survey the entire circulatory system, looking for harmful bacteria and viruses to surround and destroy. Antibodies also trigger other factors in the immune system to seek and destroy unwanted intruders.

Figure \(\PageIndex{6}\): Antibodies. Proteins play a role in protecting the body against unwanted intruders. Antibodies surround and attack an influenza virus via "antigens" on their surface. Each antibody binds to a specific antigen; an interaction similar to a lock and key. "Schematic diagram of an antibody and antigens" by Fvasconcellos is in the Public Domain.

Energy Production

Some of the amino acids in proteins can be disassembled and used to make energy. Only about 10% of dietary proteins are catabolized daily to make cellular energy. The liver can break down amino acids to the carbon skeleton, which can then be fed into the citric acid cycle. This is similar to the way that glucose is used to make ATP. If a person’s diet does not contain enough carbohydrates and fats, their body will use more amino acids to make energy, which compromises the synthesis of new proteins and destroys muscle proteins. Alternatively, if a person’s diet contains more protein than the body needs, the extra amino acids will be broken down and transformed into fat.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Figures were updated, and the flow of information was slightly changed. Several sections in the original source were moved to other sections in this text.

References

1. Amoeba Sisters. Enzymes (Updated). [Video]. YouTube. https://youtu.be/qgVFkRn8f10?si=B4D8fRXrvxscWxwo. Published August 28, 2016. Accessed August 3, 2023.