Although it is very difficult to discuss the functions of proteins in simple terms, and it is similarly challenging to explain exactly how they function in everyday life, it is not hard at all to name quite a few areas in which these highly important compounds are applied. As we noted earlier, much of our bodies' dry weight—that is, the weight other than water, which accounts for a large percentage of the total—is protein. Our bones, for instance, are about one-fourth protein, and protein makes up a very high percentage of the material in our organs (including the skin), glands, and bodily fluids.
Humans are certainly not the only organisms composed largely of protein: the entire animal world, including the animals we eat and the microbes that enter our bodies (see Digestion and Parasites and Parasitology) likewise is constituted largely of protein. In addition, a whole host of animal products, including leather and wool, are nearly pure protein. So, too, are other, less widely used animal products, such as hormones for the treatment of certain conditions—for example, insulin, which keeps people with diabetes alive and which usually is harvested from the bodies of mammals.
Proteins allow cells to detect and react to hormones and toxins in their surroundings, and as the chief ingredient in antibodies, which help us resist infection, they play a part in protecting our bodies against foreign invaders. The lack of specific proteins in the brain may be linked to such mysterious, terrifying conditions as Alzheimer and Creutzfeldt-Jakob diseases (discussed in Disease). Found in every cell and tissue and composing the bulk of our bodies' structure, proteins are everywhere, promoting growth and repairing bone, muscles, tissues, blood, and organs.
One particularly important type of protein is an enzyme, discussed in the essay on that topic. Enzymes make possible a host of bodily processes, in part by serving as catalysts, or substances that speed up a chemical reaction without actually participating in, or being consumed by, that reaction. Enzymes enable complex, life-sustaining reactions in the human body—reactions that would be too slow at ordinary body temperatures—and they manage to do so without forcing the body to undergo harmful increases in temperature. They also are involved in fermentation, a process with applications in areas ranging from baking bread to reducing the toxic content of wastewater. (For much more on these subjects, see Enzymes.)
Inside the body, enzymes and other proteins have roles in digesting foods and turning the nutrients in them—including proteins—into energy. They also move molecules around within our cells to serve an array of needs and allow healthful substances, such as oxygen, to pass through cell membranes while keeping harmful ones out. Proteins in the chemical known as chlorophyll facilitate an exceptionally important natural process, photosynthesis, discussed briefly in Carbohydrates.
The four blood types (A, B, AB, and O) are differentiated on the basis of the proteins present in each. This is only one of many key roles that proteins play where blood is concerned. If certain proteins are missing, or if the wrong proteins are present, blood will fail to clot properly, and cuts will refuse to heal. For sufferers of the condition known as hemophilia, caused by a lack of the proteins needed for clotting, a simple cut can be fatal.
Similarly, proteins play a critical role in forensic science, or the application of medical and biological knowledge to criminal investigations. Fingerprints are an expression of our DNA, which is linked closely with the operation of proteins in our bodies. The presence of DNA in bodily fluids, such as blood, semen, sweat, and saliva, makes it possible to determine the identity of the individual who perpetrated a crime or of others who were present at the scene. In addition, a chemical known as luminol assists police in the investigation of possible crime scenes. If blood has ever been shed in a particular area, such as on a carpet, no matter how carefully the perpetrators try to conceal or eradicate the stain, it can be detected. The key is luminol, which reacts to hemoglobin in the blood, making it visible to investigators. This chemical, developed during the 1980s, has been used to put many a killer behind bars.
These are just a very few of the many applications of proteins, including a very familiar one, discussed in more depth at the conclusion of this essay: nutrition. Given the importance and complexity of proteins, it might be hard to imagine that they can be produced artificially, but, in fact, such production is taking place at the cutting edge of biochemistry today, in the field of "designer proteins."
Many such designs involve making small changes in already existing proteins: for example, by changing three amino acids in an enzyme often used to improve detergents' cleaning power, commercial biochemists have doubled the enzyme's stability in wash water. Medical applications of designer proteins seem especially promising. For instance, we might one day cure cancer by combining portions of one protein that recognizes cancer with part of another protein that attacks it. One of the challenges facing such a development, however, is the problem of designing a protein that attacks only cancer cells and not healthy ones.
In the long term, scientists hope to design proteins from scratch. This is extremely difficult today and will remain so until researchers better understand the rules that govern tertiary structure. Nevertheless, scientists already have designed a few small proteins whose stability or instability has enhanced our understanding of those rules. Building on these successes, scientists hope that one day they may be able to design proteins to meet a host of medical and industrial needs.
Proteins are one of the basic nutrients, along with carbohydrates, lipids, vitamins, and minerals (see Nutrients and Nutrition). They can be broken down and used as a source of emergency energy if carbohydrates or fats cannot meet immediate needs. The body does not use protein from food directly: after ingestion, enzymes in the digestive system break protein into smaller peptide chains and eventually into separate amino acids. These smaller constituents then go into the bloodstream, from whence they are transported to the cells. The cells incorporate the amino acids and begin building proteins from them.
The protein content in plants is very small, since plants are made largely of cellulose, a type of carbohydrate (see Carbohydrates for more on this subject); this is one reason why herbivorous animals must eat enormous quantities of plants to meet their dietary requirements. Humans, on the other hand, are omnivores (unless they choose to be vegetarians) and are able to assimilate proteins in abundant quantities by eating the bodies of plant-eating animals, such as cows. In contrast to plants, animal bodies (as previously noted) are composed largely of proteins. When people think of protein in the diet, some of the foods that first come to mind are those derived from animals: either meat or such animal products as milk, cheese, butter, and eggs. A secondary group of foods that might appear on the average person's list of proteins include peas, beans, lentils, nuts, and cereal grains.
There is a reason why the "protein team" has a clearly defined "first string" and "second string." The human body is capable of manufacturing 12 of the 20 amino acids it needs, but it must obtain the other eight—known as essential amino acids —from the diet. Most forms of animal protein, except for gelatin (made from animal bones), contain the essential amino acids, but plant proteins do not. Thus, the nonmeat varieties of protein are incomplete, and a vegetarian who does not supplement his or her diet might be in danger of not obtaining all the necessary amino acids.
For a person who eats meat, it would be extremely difficult not to get enough protein. According to the U.S. Food and Drug Administration (FDA), protein should account for 10% of total calories in the diet, and since protein contains 4 calories per 0.035 oz. (1 g), that would be about 1.76 oz. (50 g) in a diet consisting of 2,000 calories a day. A pound (0.454 kg) of steak or pork supplies about twice this much, and though very few people sit down to a meal and eat a pound of meat, it is easy to see how a meat eater would consume enough protein in a day.
For a vegetarian, meeting the protein needs may be a bit more tricky, but it can be done. By combining legumes or beans and grains, it is possible to obtain a complete protein: hence, the longstanding popularity, with meat eaters as well as vegetarians, of such combinations as beans and rice or peas and cornbread. Other excellent vegetarian combos include black beans and corn, for a Latin American touch, or the eastern Asian combination of rice and tofu, protein derived from soybeans.
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