Enzymes, Vitamins, and Hormones An organism that contains carbohydrates, proteins, and lipids may be complete structurally, but it still does not have all the compounds it needs to carry on its cellular processes. Specialized proteins called enzymes facilitate almost every reaction that takes place in living organisms. Vitamins are small but indispensable nutrients that or- ganisms cannot produce by themselves. Hormones serve as chemical messengers from one part of an organism to another. These too are the molecules of life. Enzymes: Organic Catalysts Enzymes are protein molecules that act as catalysts. For example, an equilibrium between carbon dioxide, water, and carbonic acid func- tions as the red blood cells pick up carbon dioxide from muscles, transport it to the lungs, and then release it during respiration. Once carbon dioxide is released, red blood cells immediately pick up oxy- gen. An enzyme named carbonic anhydrase catalyzes both of these reactions. Without this enzyme, one molecule of hemoglobin would react approximately every 100 seconds. With the enzyme, however, 100 000 molecules can react every second. In this reaction, the rate is multiplied by a factor of 10 7 . Without that increase, there would be no way for the lungs to exchange enough gases to support the respi- ratory process needed to maintain life. CO 2 + H 2 O carbonic anhydrase H 2 CO 3 Enzymes act with amazing precision. God, in His omnipotence, designed the enzymes so that they would select just the right types of molecules on which to act. An enzyme called trypsin catalyzes a reaction that breaks down protein chains. What is remarkable is that the enzyme splits protein chains only on a certain site of two amino acids; it ignores all other sites on the protein chain. How can enzymes work so fast, in so many applications, and with such precision? Researchers have proposed several models to account for what has been observed. One widely accepted model is the lock-and-key model. It holds that enzymes catalyze reactions by placing reactants in ideal positions for the reactions to occur. When an enzyme and the substance on which it works combine, collisions with other reactants are more effective than without the enzyme. A complete understanding of how enzymes work will involve a thorough knowledge of the shapes and sizes of the biological mole- cules—not an easy task, considering how complex large proteins can be. Like the study of steroids, enzyme research is currently one of the most exciting and active areas in biochemistry. Vitamins: Micronutrients As far as biochemistry is concerned, sailors had it rough in the old days. For British seamen, long trips almost certainly meant bleeding gums, loose teeth, cuts that did not heal, and weight loss. Japanese navigators faced stiff lower limbs and possible paralysis. The causes of these plights had nothing to do with the sea, but rather with the © BJU Press. Unauthorized reproduction prohibited. 1 Organic Chemistry and Biochemistry
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Enzymes, Vitamins, and Hormones
An organism that contains carbohydrates, proteins, and lipids may be complete structurally, but it still does not have all the compounds it needs to carry on its cellular processes. Specialized proteins called enzymes facilitate almost every reaction that takes place in living organisms. Vitamins are small but indispensable nutrients that or-ganisms cannot produce by themselves. Hormones serve as chemical messengers from one part of an organism to another. These too are the molecules of life.
Enzymes: Organic CatalystsEnzymes are protein molecules that act as catalysts. For example, an equilibrium between carbon dioxide, water, and carbonic acid func-tions as the red blood cells pick up carbon dioxide from muscles, transport it to the lungs, and then release it during respiration. Once carbon dioxide is released, red blood cells immediately pick up oxy-gen. An enzyme named carbonic anhydrase catalyzes both of these reactions. Without this enzyme, one molecule of hemoglobin would react approximately every 100 seconds. With the enzyme, however, 100 000 molecules can react every second. In this reaction, the rate is multiplied by a factor of 107. Without that increase, there would be no way for the lungs to exchange enough gases to support the respi-ratory process needed to maintain life.
CO2 + H2O carbonic anhydrase H2CO3
Enzymes act with amazing precision. God, in His omnipotence, designed the enzymes so that they would select just the right types of molecules on which to act. An enzyme called trypsin catalyzes a reaction that breaks down protein chains. What is remarkable is that the enzyme splits protein chains only on a certain site of two amino acids; it ignores all other sites on the protein chain.
How can enzymes work so fast, in so many applications, and with such precision? Researchers have proposed several models to account for what has been observed. One widely accepted model is the lock-and-key model. It holds that enzymes catalyze reactions by placing reactants in ideal positions for the reactions to occur. When an enzyme and the substance on which it works combine, collisions with other reactants are more effective than without the enzyme.
A complete understanding of how enzymes work will involve a thorough knowledge of the shapes and sizes of the biological mole-cules—not an easy task, considering how complex large proteins can be. Like the study of steroids, enzyme research is currently one of the most exciting and active areas in biochemistry.
Vitamins: MicronutrientsAs far as biochemistry is concerned, sailors had it rough in the old days. For British seamen, long trips almost certainly meant bleeding gums, loose teeth, cuts that did not heal, and weight loss. Japanese navigators faced stiff lower limbs and possible paralysis. The causes of these plights had nothing to do with the sea, but rather with the
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unbalanced diets of the seamen. They had the vitamin-deficiency diseases now known as scurvy and beriberi.
Vitamins are organic substances that are essential for normal nutrition, but they are not carbohydrates, lipids, proteins, or fats. The British sailors lacked ascorbic acid, or vitamin C, in their on-board diets. A daily ration of lemon or lime juice provided the neces-sary nutrient to help control the symptoms of scurvy and earned the sailors the nickname “limeys.” The Japanese navy fought the disease of beriberi in the late 1800s by introducing portions of wheat and barley and unpolished rice into their diets. The hulls of these grains contain vitamin B1, which relieved the sailors’ suffering. The doctor who first isolated the organic substance that cured beriberi found that it contained an amine (NH2) group; he called it a “vital amine,” or vitamin.
Vitamins are vital to the proper functioning of enzymes. Vita-min C is connected with the formation of the intercellular “glue” in bones, connective tissue, and cartilage. The B vitamins help enzymes break down carbohydrates. Without them, pyruvic acid, a product of incomplete carbohydrate breakdown, accumulates in the muscles and causes the pain of beriberi. Today, approximately twenty-one vi-tamins are known, but researchers suspect that several others exist. The known vitamins can be classified as either water soluble or fat soluble. Tables 18-11 and 18-12 list the common vitamins, their func-tions, and the possible consequences of deficient intake.
Water-soluble vitamins can be excreted from the kidneys; there-fore, they need to be replenished constantly. Unlike water-soluble vitamins, excessive amounts of fat-soluble vitamins can accumulate in the fatty tissue. Although vitamins are essential to good health, megadoses of some vitamins can be harmful. Excessive amounts of vitamin A can result in brain and kidney damage. Too much vitamin C has been linked to kidney stones. (See Table 18-12.)
Hormones: Chemical MessengersComplex organisms have many specialized body parts that must constantly communicate with each other. The brain must know when the eyes see something as significant as a charging rhinoceros, the adrenal gland must know when it is time to secrete adrenaline,
18-11 Fat-Soluble Vitamins
Vitamin Functions Deficiency symptoms Dietary sources
A contributes to visual pigments in the eye
inflammation of eyes, night blindness, and drying of mucous membranes
eggs, butter, cheese, liver, dark green and deep orange vegetables
D aids in absorption and deposition of calcium rickets (defective bone growth) fish-liver oil, fortified milk
Eprotects blood cells, unsaturated fatty acids, and vitamin A from oxidation
anemia, bursting of red blood cells
wheat germ, whole grain cereals, liver, margarine, vegetable oil, leafy green vegetables, egg yolk
K aids clotting of blood slow-clotting blood liver, cabbage, potatoes, peas, leafy green vegetables
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and the muscles must know when to respond to the danger with increased activity. Nerves, which carry electrical messages, handle many of the body’s communications, but they are not alone in this task. Hormones are chemical messengers that are produced by the endocrine glands and are transported by the bloodstream to various areas of the body.
Hormones are a chemically diverse lot. They can be steroids, poly-peptide chains, or proteins. These compounds, when released into the bloodstream or other body fluids, travel throughout the body. Despite the fact that they come into contact with many cells, they act only on their target cells. Research findings suggest that the “target” cells contain receptor molecules that recognize specific hormones by their shapes.
Hormones are also found in insects and plants. Biochemical re-searchers use insect sex attractant hormones called pheromones to trap and disorient insect pests. The pheromone for the gypsy moth, for example, is effective at a level of 1 × 10 -13 gram. Some herbicides are modeled after plant hormones in order to be very selective in the plants that they kill. Continued biochemical research will enable us to replace broad-spectrum pesticides and herbicides with com-pounds that will act against specific targets.
18-12 Water-Soluble Vitamins
Vitamin Functions Deficiency symptoms Dietary sources
B1 (thiamine) aids in carbohydrate metabolism fatigue, beriberi, accumulation of body fluids
pasta, bread, lima beans, wheat germ, nuts, milk, liver, peas, pork
niacin aids in energy utilization inflammation of nerves and mucous membranes, dermatitis
meat, whole grains, poultry, fish, peanuts
B2 (riboflavin) aids in protein metabolism dermatitis, inflammation of the tongue, anemia
milk, meat, eggs, mushrooms, dark green vegetables, pasta, bread, beans, peas
B6 (pyridoxine) aids in amino acid metabolism
convulsions in infants, inflammation of the tongue, increased susceptibility to infections
muscle meats, liver, whole grains, poultry, fish
B12
aids in the formation of nucleic acids
retarded growth, spinal cord degeneration
meat, fish, eggs, milk, kidneys, liver
C (ascorbic acid) builds strong connective tissues in bones, cartilage, and blood vessels
slow wound healing, scurvy, anemia
citrus fruits, melons, tomatoes, green peppers, berries, leafy green vegetables
pantothenic acid aids in respiration gastrointestinal disturbances, depression, mental confusion whole grain cereals, bread
folic acid aids in formation of heme groups and nucleotides various types of anemia
kidneys, liver, leafy green vegetables, wheat germ, peas, beans
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