Main Role Of Protein In The Body – Proteins are the “workers” of the body and participate in many bodily functions. As we’ve already discussed, proteins come in all sizes and shapes, and each one is specifically structured for its specific function. This page describes some of the important functions of proteins. As you read them, keep in mind that the synthesis of all these different proteins requires adequate amounts of amino acids. As you can imagine, eating a diet that is deficient in protein and essential amino acids can impair many of the body’s functions. (More on this later in the unit.)
The main types and functions of proteins are summarized in the table below, and the following sections of this page give more details about each of them.
Main Role Of Protein In The Body
It breaks down macronutrients into smaller, absorbable monomers. performs steps in metabolic pathways to allow nutrient utilization
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More than a hundred different structural proteins have been discovered in the human body, but the most abundant by far is collagen, which makes up about 6 percent of the total body weight. Collagen makes up 30 percent of bone tissue and includes large amounts of tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein composed primarily of glycine and the amino acids proline. Within its quaternary structure, three protein strands twist around each other like a rope, and then these collagen ropes overlap with others.
This highly ordered structure is even stronger than steel fibers of the same size. Collagen makes bones strong but flexible. The collagen fibers in the dermis of the skin provide it with structure, and the accompanying elastin protein fibers make it flexible. Pinch the skin on your hand and then let go. The collagen and elastin proteins in the skin allow it to return to its original shape. Smooth muscle cells that secrete collagen and elastin proteins surround blood vessels, giving the vessels their structure and ability to stretch back after blood is pumped through them. Another strong, fibrous protein is keratin, an important component of skin, hair and nails.
Enzymes are proteins that carry out specific chemical reactions. An enzyme’s job is to provide a site for a chemical reaction and reduce the amount of energy and time it takes for that chemical reaction to occur (this is known as “catalysis”). On average, more than 100 chemical reactions occur in cells every second, and most of them require enzymes. The liver alone contains over 1,000 enzyme systems. Enzymes are specific and will only use specific substrates that fit their active site, similar to how a lock can only be opened with a specific key. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although it is eventually destroyed and rebuilt. All bodily functions, including breaking down nutrients in the stomach and small intestine, converting nutrients into molecules a cell can use, and building all macromolecules, including protein itself, involve enzymes.
Figure 6.11. Enzymes are proteins. An enzyme’s job is to provide a site for substances to chemically react and form a product, and to reduce the amount of energy and time it takes for this to happen.
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VIDEO: “Enzymes,” by Amoeba Sisters, YouTube (28 Aug 2016), 5:46 min. This video shows the action of enzymes.
Are the chemical messengers produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then carried in the blood to its target cell, where it communicates a message to initiate a specific reaction or cellular process. For example, after you eat a meal, your blood glucose levels rise. In response to elevated blood glucose, the pancreas releases the hormone insulin. Insulin tells the body’s cells that glucose is available and to take it from the blood and store it or use it to make energy or make macromolecules. A major function of hormones is to turn enzymes on and off, so some proteins can even regulate the action of other proteins. While not all hormones are made from proteins, many are.
Adequate protein intake enables the body’s essential biological processes to maintain homeostasis (steady or stable conditions) in a changing environment. One aspect of this is fluid balance, keeping water properly distributed in the various compartments of the body. If too much water suddenly moves from the blood into a tissue, the results are swelling and possibly cell death. Water always flows from an area of high concentration to an area of low concentration. As a result, water moves to areas that have higher concentrations of others
, such as proteins and glucose. To keep water evenly distributed between the blood and cells, proteins are constantly circulating in high concentrations in the blood. The most abundant protein in the blood is the butterfly-shaped protein known as albumin. The presence of albumin in the blood makes the concentration of protein in the blood similar to that in the cells. Therefore, fluid exchange between blood and cells is not at an extreme, but rather is minimized to maintain homeostasis.
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Figure 6.12. The butterfly-shaped protein albumin has many functions in the body, including maintaining fluid and acid-base balance and transporting molecules.
Protein is also necessary to maintain the proper pH balance (the measure of how acidic or basic a substance is) in the blood. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a small change in blood pH can affect body functions. The body has many systems that keep the pH of the blood within the normal range to prevent this from happening. One of them is circulating albumin. Albumin is slightly acidic and because it is negatively charged it balances the many positively charged molecules circulating in the blood, such as hydrogen protons (H
), calcium, potassium and magnesium. Albumin acts as a buffer against sudden changes in the concentrations of these molecules, thereby balancing blood pH and maintaining homeostasis. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.
Proteins also play a vital role in transporting substances throughout the body. For example, albumin chemically binds to hormones, fatty acids, certain vitamins, essential minerals, and drugs and carries them throughout the circulatory system. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and carry it to all tissues of the body. The plasma membrane of a cell is usually not permeable to large polar molecules, so in order to get the necessary nutrients and molecules into the cell, there are many transport proteins in the cell membrane. Some of these proteins are channels that allow specific molecules to move in and out of cells. Others operate as one-way taxis and require energy to operate.
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Figure 6.13. Molecules move in and out of cells via transport proteins, which are either channels or carriers.
VIDEO: “The Sodium-Potassium Pump,” by RicochetScience, YouTube (May 23, 2016), 2:26 min. This tutorial describes how the sodium-potassium pump uses active transport to move sodium ions (Na+) out of a cell and potassium ions (K+) into a cell.
Proteins also play an important role in the body’s immune system. The strong collagen fibers in the skin give it structure and support, but it also serves as a barrier against harmful substances. The immune system’s attack and destruction functions depend on enzymes and antibodies, which are also proteins. For example, an enzyme called lysozyme is secreted into saliva and attacks the walls of bacteria, causing them to rupture. Certain proteins circulating in the blood can be directed to create a molecular knife that stabs the cell membranes of foreign invaders. Antibodies secreted by white blood cells search the entire circulatory system, looking for harmful bacteria and viruses to surround and destroy. Antibodies also activate other factors in the immune system to seek out and destroy unwanted invaders.
VIDEO: “Specific Immunity, Antibodies,” by Carpe Noctum, YouTube (December 11, 2007), 1 min. Watch this video to observe how antibodies protect against foreign invaders.
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Some of the amino acids in proteins can be broken down and used to produce energy. Only about 10 percent of dietary protein is metabolized each day to produce cellular energy. The liver is able to break down amino acids into the carbon skeleton, which can then be fed into the citric acid or Krebs cycle. This is similar to how 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 produce energy, which can compromise new protein synthesis and damage muscle proteins if caloric intake is also low.
Not only can amino acids be used directly for energy, but they can also be used to synthesize glucose through gluconeogenesis. Alternatively, if a person consumes a high-protein diet and eats more calories than their body needs, the extra amino acids will be broken down and turned into fat. Unlike carbohydrates and fat, protein does not have a specialized storage system to be used later for energy.
Proteins that help speed up or facilitate chemical reactions in the body. bring two compounds together to react,
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