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Proteins and the Human Body written by Miriam Chun, Laura Deede, Xum Giang, Brad Johnson, Kyle Volker

Contents 1 What is Protein? 2 Protein in depth: Soybean Protein and Nutrition 3 A Popular High-Protein Diet 4 Proteins and Disease 4.1 Protein Related Genetic Disease 5 Footnotes

What is Protein?

Protein is one of the essential chemical families in living organisms. Proteins are found in almost every tissue of our body, and nearly 75% of the human body’s dry weight comes from them.^[1] Proteins are complex chains of molecules called amino acids, which are composed primarily of carbon, hydrogen, oxygen, and nitrogen (two of the twenty amino acids have sulfur). A single protein contains about 500 or more amino acids that are linked together by covalent bonds, also known as peptide bonds. Figure.1 is a picture of a molecule (polypeptide) made up of four amino acids that are covalently linked together. This figure portrays how different amino acids are connected in a simple chain. Most proteins, however, are much more complex. Due to the large number of amino acids, many fold and contain various types of bonds within their structure. Therefore, each protein has a different function and structure depending on its sequence of amino acids.^{[2] [3]} Proteins make us who we are, and are a crucial part of our existence. For example, hemoglobin, a component of our blood, is made up of proteins, and it carries oxygen to different parts of the body. Enzymes are a type of protein that work by increasing the rate of chemical reactions, and are vital for every biological process in our body. They assist in processes such as DNA replication, cell division, and the breakdown of food into usable energy, as well as constructing muscles, bones and teeth.^[4] Without enzymes, we wouldn’t be able to function. Unfortunately, our body does not store amino acids, as it does fats or carbohydrates, so a continuous supply is necessary to make new proteins to keep us healthy and functioning. And since the body cannot synthesize all twenty amino acids necessary for protein production, nine amino acids have to be consumed from food; these are called essential amino acids.^[5]

Protein in depth: Soybean Protein and Nutrition

Everyday we consume two different kinds of proteins: complete and incomplete. The incomplete proteins are the proteins that our bodies cannot fully digest; they can be found in foods like vegetables, beans, and nuts. Complete proteins are, in general, found in animal proteins, such as beef, cheese, fish, and eggs. These proteins can be fully digested in our system. The major difference between incomplete and complete proteins is that the incomplete proteins contain very little or no amino acids, whereas the complete proteins can provide large amounts of amino acids. In other words, if the protein can contribute enough of the nine essential amino acids that we lack, it is called complete protein. Food sources that do not supply all the essential amino acids are called incomplete protein.^[6] As a result, vegetarians who only take non-animal products in their diet may have a problem getting all of the essential amino acids that they need. Soy foods, like tofu and soy milk, are an excellent source of protein for vegetarians. Unlike many other vegetables, soybeans are considered a complete protein because they contain eight essential amino acids. Soy beans also have a high concentration of isoflavones which may help to lower cholesterol, and prevent breast and prostate cancer.^[7] Studies have indicated that isolated soy protein may have health benefits for people suffering from type 2 diabetes. These results have encouraged many Americans to consume more soy products, and many food companies also have been driven to add soy in their products.^[8] Nevertheless, it is important to realize that the overall recommendation for protein intake is to get a variety of proteins, and just two to four servings of soy protein a day are considered part of a balanced diet.

A Popular High-Protein Diet

One of the latest diet trends that revolve around proteins is the Atkins diet. It is based on a high protein, low carbohydrate intake. Part of the theory is that consuming high protein foods such as poultry, beef, fish, beans, cheese, etc., slows down the gastrointestinal tract, making the individual feel more satiated sooner, resulting in less calories consumed. One reason why people may have success with a high protein diet is because it is starving the body of carbohydrates. Carbohydrates are the bodies’ main source of energy, which is found in the form of glucose (C₆H₁₂O₆). By limiting the amount of carbohydrates, the body is forced to metabolize more protein, which is a more energy intensive process than glucose metabolism. In this carbohydrate-fasting state, the theory is that the body burns relatively more calories, which should increase the rate of fat-reduction for the individual (as long as the individual consumes fewer calories than he or she burns in a day).^[9]

Proteins and Disease

Proteins play a vital role in preventing disease. Without protein, our body would not be able to function, but too much protein can also be harmful. As mentioned earlier, it is important to keep balanced diet. Figure.2 is a picture of a food pyramid that is often used to illustrate a balanced diet.^[10] By eating a balanced diet of meat and vegetable protein, our body can properly sustain its everyday functions. In addition, some studies have shown that those who eat more protein are twenty five percent less likely to have heart disease. Nuts can not only provide protein but help lower LDL cholesterol and raise HDL cholesterol. Protein can have negative side effects when your body digests too much. While on high protein diets like the Atkins Diet, the protein neutralizes the body’s ability to absorb calcium and other buffering agents. People who continue on a high protein diet for too long may face excessive mineral loss, which can result in osteoporosis.^[11]

Protein Related Genetic Disease

The genetic disease, phenylketonuria, is an example of what can happen when the body cannot break down certain kinds of amino acids. Phenylketonuria results when an individual has a gene that encodes a non-functional or dysfunctional form of an enzyme important in the metabolism of the amino acid phenylalanine. The enzyme phenylalanine hydroxylase is defective because it does not contain the proper sequence of amino acids, which causes the enzyme’s shape to change. As mentioned above, the function of an enzyme depends on certain bonding patterns and intermolecular forces in its amino acid sequence, and a change in this sequence disables the enzyme’s ability to metabolize phenylalanine. See Figure.3 for a picture of the chemical structure of phenylalanine.^[12] Phenylalanine is toxic if it accumulates in the body, and can result in premature tooth decay, mental and physical retardation, seizures, and death.^[13]

Footnotes

1. ^ Harvard School of Public Health, “Proteins,” http://www.hsph.harvard.edu/nutritionsource/protein.html (accessed 9/25/2005).
2. ^ Freeman, Scott. Biological Science: The cell, Genetics, and Development. 2nd Edition. Pearson: Upper Saddle River, 2005; pp 53-65.
3. ^ “Tetrapeptide,” http://www.sp.uconn.edu/~terry/images/mols/ (accessed 11/6/2005).
4. ^ Silberg, Martin S., Chemistry. 3rd Edition. McGraw-Hill: New York, 2003; pp 700.
5. ^ “Medical Encyclopedia: protein in diet,” http://www.nlm.nih.gov/medlineplus/ency/article/002467.htm (accessed 11/3/2005).
6. ^ Greenberg, David M., “Protein Metabolism.” http://www.accessscience.com (accessed 9/25/2005)
7. ^ “Soybeans.” http://www.annecollins.com/diet_foods/soybeans.htm (accessed 11/3/05).
8. ^ Adams, Mike. “Isolated Soy Protein shown to Benefit Patients with Type 2 Diabetes.” http://www.newstarget.com/001620.html (accessed 9/24/2005).
9. ^ Gropper, Sareen S., Jack L. Smith, James L. Groff. Advanced Nutrition and Human Metabolism. 4th Edition. Thomdon Learning Inc, 2005.
10. ^ “Food and Digestion.” http://www.sciencepages.co.uk/. ../m13revision.php (accessed 11/2/2005).
11. ^ “Protein in Nutrition.” http://www.medical –library.net/sites/framer.html?/sites/_proteins_in_nutrition.html (accessed 10/14/2005).
12. ^ “Phenylalanine.” http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-P/Phenylalanine.html (accessed 11/4/2005).
13. ^ Brooker, Robert J. Genetics: Analysis and Principles. 2nd Ed. McGraw-Hill: New York; pp. 4.