Protein
I Introduction
Protein,
any of a large number of organic compounds that make up living organisms and are
essential to their functioning. First discovered in 1838, proteins are now
recognized as the predominant ingredients of cells, making up more than 50
percent of the dry weight of animals. The word protein is coined from the
Greek proteios, or "primary."
Protein molecules range from
the long, insoluble fibers that make up connective tissue and hair to the
compact, soluble globules that can pass through cell membranes and set off
metabolic reactions. They are all large molecules, ranging in molecular weight
from a few thousand to more than a million, and they are specific for each
species and for each organ of each species. Humans have an estimated 30,000
different proteins, of which only about 2 percent have been adequately
described. Proteins in the diet serve primarily to build and maintain cells, but
their chemical breakdown also provides energy, yielding close to the same 4
calories per gram as do carbohydrates (see Metabolism).
Besides their function in
growth and cell maintenance, proteins are also responsible for muscle
contraction. The digestive enzymes are proteins, as are insulin and most other
hormones. The antibodies of the immune system are proteins, and proteins such as
hemoglobin carry vital substances throughout the body.
II NUTRITION
Whether found in humans or in single-celled bacteria,
proteins are composed of units of about 20 different amino acids, which, in
turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
In a protein molecule these acids form peptide bonds—bonds between amino and
carboxyl (COOH) groups—in long strands (polypeptide chains). The almost
numberless combinations in which the acids line up, and the helical and globular
shapes into which the strands coil, help to explain the great diversity of tasks
that proteins perform in living matter.
To synthesize its life-essential proteins, each species
needs given proportions of the 20 main amino acids. Although plants can
manufacture all their amino acids from nitrogen, carbon dioxide, and other
chemicals through photosynthesis, most other organisms can manufacture only some
of them. The remaining ones, called essential amino acids, must be derived from
food. Eight essential amino acids are needed to maintain health in humans:
leucine, isoleucine, lysine, methionine, phenylalanine, theonine, tryptophan,
and valine. All of these are available in proteins produced in the seeds of
plants, but because plant sources are often weak in lysine and tryptophan,
nutrition experts advise supplementing the diet with animal protein from meat,
eggs, and milk, which contain all the essential acids.
Most diets—especially in the United States, where
animal protein is eaten to excess—contain all the essential amino acids.
(Kwashiorkor, a wasting disease among children in tropical Africa, is due to an
amino acid deficiency.) For adults, the Recommended Dietary Allowance (RDA) for
protein is 0.79 g per kg (0.36 g per lb) of body weight each day. For children
and infants this RDA is doubled and tripled, respectively, because of their
rapid growth (see Nutrition, Human).
III STRUCTURE
OF PROTEINS
The most basic level of protein structure, called the
primary structure, is the linear sequence of amino acids. Different sequences of
the acids along a chain, however, affect the structure of a protein molecule in
different ways. Forces such as hydrogen bonds, disulfide bridges, attractions
between positive and negative charges, and hydrophobic
("water-fearing") and hydrophilic ("water-loving") linkages
cause a protein molecule to coil or fold into a secondary structure, examples of
which are the so-called alpha helix and the beta pleated sheet. When forces
cause the molecule to become even more compact, as in globular proteins, a
tertiary protein structure is formed. When a protein is made up of more than one
polypeptide chain, as in hemoglobin and some enzymes, it is said to have a
quaternary structure.
IV INTERACTION
WITH OTHER PROTEINS
Polypeptide chains are sequenced and coiled in such a
way that the hydrophobic amino acids usually face inward, giving the molecule
stability, and the hydrophilic amino acids face outward, where they are free to
interact with other compounds and especially other proteins. Globular proteins,
in particular, can join with a specific compound such as a vitamin derivative
and form a coenzyme (see Enzyme), or join with a specific protein and
form an assembly of proteins needed for cell chemistry or structure.
V FIBROUS
PROTEINS
The major fibrous proteins, described below, are
collagen, keratin, fibrinogen, and muscle proteins.
A Collagen
Collagen, which makes up bone, skin,
tendons, and cartilage, is the most abundant protein found in vertebrates. The
molecule usually contains three very long polypeptide chains, each with about
1000 amino acids, that twist into a regularly repeating triple helix and give
tendons and skin their great tensile strength. When long collagen fibrils are
denatured by boiling, their chains are shortened to form gelatin.
B Keratin
Keratin, which makes up the outermost
layer of skin and the hair, scales, hooves, nails, and feathers of animals,
twists into a regularly repeating coil called an alpha helix. Serving to protect
the body against the environment, keratin is completely insoluble in water. Its
many disulfide bonds make it an extremely stable protein, able to resist the
action of proteolytic (protein-hydrolyzing) enzymes. In beauty treatments, human
hair is set under a reducing agent, such as thioglycol, to reduce the number of
disulfide bonds, which are then restored when the hair is exposed to oxygen.
C Fibrinogen
Fibrinogen is a blood plasma protein
responsible for blood clotting. With the catalytic action of thrombin,
fibrinogen is converted into molecules of the insoluble protein fibrin, which
link together to form clots.
D Muscle
Proteins Myosin, the protein chiefly
responsible for muscle contraction, combines with actin, another muscle protein,
forming actomyosin, the different filaments of which shorten, causing the
contracting action.
VI GLOBULAR
PROTEINS
Unlike fibrous proteins, globular proteins are
spherical and highly soluble. They play a dynamic role in body metabolism.
Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and
protein hormones. The albumins and globulins are classes of soluble proteins
abundant in animal cells, blood serum, milk, and eggs. Hemoglobin is a
respiratory protein that carries oxygen throughout the body and is responsible
for the bright red color of red blood cells. More than 100 different human
hemoglobins have been discovered, among which is hemoglobin S, the cause of
sickle-cell anemia, a hereditary disease suffered mainly by blacks.
A Enzymes
All of the enzymes are globular proteins
that combine rapidly with other substances, called substrate, to catalyze the
numerous chemical reactions in the body. Chiefly responsible for metabolism and
its regulation, these molecules have catalytic sites on which substrate fits in
a lock-and-key manner to trigger and control metabolism throughout the body.
B Protein
Hormones These proteins, which come from
the endocrine glands, do not act as enzymes. Instead they stimulate target
organs that in turn initiate and control important activities—for example, the
rate of metabolism and the production of digestive enzymes and milk. Insulin,
secreted by the islands of Langerhans, regulates carbohydrate metabolism by
controlling blood glucose levels. Thyroglobulin, from the thyroid gland,
regulates overall metabolism; calcitonin, also from the thyroid, lowers blood
calcium levels. Angiogenin, a protein structurally determined in the mid-1980s,
directly induces the growth of blood vessels in tissues.
C Antibodies
Also called immunoglobulins, antibodies (see
Antibody) make up the thousands of different proteins that are generated in the
blood serum in reaction to antigens (body-invading substances or organisms). A
single antigen may elicit the production of many antibodies, which combine with
different sites on the antigen molecule, neutralize it, and cause it to
precipitate from the blood.
D Microtubules
Globular proteins can also assemble into minute, hollow
tubes that serve both to structure cells and to conduct substances from one part
of a cell to another. Each of these microtubules, as they are called, is made up
of two types of nearly spherical protein molecules that pair and join onto the
growing end of the microtubule, adding on length as required. Microtubules also
make up the inner structure of cilia, the hairlike appendages by which some
microorganisms propel themselves.
Contributed By:
Mary Lynn Hendrix
