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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 organisms 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 functions 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 oxygen. 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 respiratory process needed to maintain life.
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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 molecules—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
Organic Chemistry and Biochemistry
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Chapter Eighteen
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 onboard diets. A daily ration of lemon or lime juice provided the necessary 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. Vitamin 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 vitamins 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 functions, and the possible consequences of deficient intake.
18-11 Fat-Soluble Vitamins
Vitamin
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
E
protects 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
Water-soluble vitamins can be excreted from the kidneys; therefore, 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 Messengers
Complex 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,
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Functions
3
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, polypeptide 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 researchers 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 compounds that will act against specific targets.
Organic Chemistry and Biochemistry
18-12 Water-Soluble Vitamins
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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