Protein and Amino Acids - Adequate Food for All - page 97

isolated the amino acid tryptophan by enzymic digestion of protein. T. Osborne and
L. B. Mendel reported in 1916 that rats fed the corn protein zein failed to grow unless
supplemented with tryptophan and lysine. This and further work established that
certain amino acids were dietary essentials (indispensable), and the last of these to
be discovered, threonine, was identified in 1935 by W. C. Rose and associates at the
University of Illinois.
PRoteIn functIons
s t r u C t u r a L r o L e s
Those proteins that serve structural roles in the body are commonly divided into
contractile proteins and fibrous proteins. The principal contractile proteins are actin
and myosin and are found in muscle, where they, when activated, power movement
of body parts, such as the limbs, head and neck, heart, lungs, and intestinal tract. The
fibrous proteins are characteristic of connective tissue and include collagen, elastin,
and keratin. They are found in mucus, cartilage, tendons, blood vessels, skin, matri-
ces of bones and teeth, hair, and nails.
e n z y m e s
Enzymes are proteins that act as catalysts to change the rate of metabolic reactions.
They may be found inside or outside body cells, depending on their function. They
combine selectively with substrate molecules to split them; to transfer atoms, elec-
trons, or functional groups; or to join molecules. Some enzymes require cofactors,
such as iron, copper, or zinc, or coenzyme forms of certain B vitamins to fulfill
their functions. Physiological processes that require enzymes include digestion of
food, generation of energy by tissues, generation and propagation of neurological
impulses, clotting of blood, and contraction of muscle.
h o r m o n e s
Hormones are chemical messengers that may regulate metabolic processes by pro-
moting enzyme synthesis or affecting enzyme activity. They are synthesized and
secreted by endocrine tissue, transported in the blood, and produce specific responses
in target organs. Some hormones are steroids derived from cholesterol. Others are
derived from amino acids that may be linked together or metabolically altered. These
include thyroid and parathyroid hormones, melatonin, insulin, growth hormone,
adrenocorticotropic hormone, antidiuretic hormone, glucagon, and calcitonin.
i m m u n i t y
Proteins called immunoglobulins , or antibodies, are produced by plasma cells,
derived from specialized white blood cells called B-lymphocytes. They function
by binding and inactivating foreign objects (antigens) such as bacteria or viruses
that enter the body with potential to cause disease. The immunoglobulin-antigen