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The functions of proteins are the essence of life itself. They make
up more than 50% of the dry mass of animals. Many of an organism’s
proteins are
enzymes, special proteins that speed up the
rate of chemical reactions in the cell. Enzymes are like
tiny molecular tools that temporarily combine with the ‘ingredients” for a
specific reaction and hold them at the correct angle for a reaction to
occur. The enzyme may also pull on bonds and loosen them. This
lowers the amount of energy needed for the reaction to proceed so it can
occur at a much lower temperature than would be necessary without the
enzyme (meaning that your cells don’t have to heat up every time a
reaction occurs:) Each of the approximately 2000 known enzymes is specific
to one particular reaction. In addition, proteins play a myriad of
other roles in cell.
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Some
Functions of Proteins |
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Type of
protein |
Example |
Function |
|
Enzymes |
amylase |
Promotes the break down of
starch to the simple sugar glucose. |
|
Structural proteins |
keratin, collagen |
Hair, wool, nails, horns,
hoofs, tendons, cartilage |
|
Hormones |
insulin, glucagon |
Regulates use of blood
sugar |
|
Contractile proteins |
actin, myosin
|
Contracting fibers in
muscle |
|
Storage proteins |
ferritin |
Stores iron in spleen |
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Transport proteins |
hemoglobin
serum albumin |
Carries oxygen in blood
Carries fatty acids in blood |
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Immunological proteins |
antibodies |
Rid the body of foreign
proteins |
|
Toxins |
neurotoxin |
Cobra venom blocker of
nerve functions |
Proteins are
organic compounds that contain nitrogen in addition to carbon, hydrogen
and oxygen.
The subunits of proteins are called
amino acids. There are twenty different kinds of
amino acids found in living cells. All amino acids have a central carbon
atom which makes four attachments. Three of the attachments are the same
for all kinds of amino acids. They are 1) a single hydrogen atom, 2) a COOH group on one side and 3) an NH2 group on the other.
The
fourth attachment differs for each of the 20 kinds of amino acids.
Proteins are
long, unbranched chains of amino acids that fold up into complex shapes
because of attractive and repulsive forces between the R groups of
different kinds of amino acids in the polypeptide chain. Some proteins are
composed of more than one polypeptide chain which fit together to form a
complex three-dimensional functional protein.
Scientists
describe the structure of a protein on four levels of complexity: primary,
secondary, tertiary, and quaternary levels.
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The
primary structure of a protein is the
sequence of amino acids.
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The
secondary protein structure occurs when the
sequence of
amino acids are linked by hydrogen bonds. This level of
structure takes the form of either
a pleated sheet
or a helix. |
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The
tertiary structure describes the
folding and other contortions of a
polypeptide chain that result from the
molecular
interactions among the R groups of the different amino
acids. |
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The arrangement of two or more polypeptide chains in a protein
make up its quaternary structure |
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The figure above is adapted
from the website of the
Office of Science Education and Outreach of the National Human Genome
Research Institute:
http://www.nhgri.nih.gov/DIR/VIP/Glossary/Illustration/protein.html
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Having certain
amino acids in certain positions is crucial to the protein’s overall shape
and consequently to its function. For example, the
change of just one amino acid alters
the shape of hemoglobin
enough to create the condition of sickle cell
anemia. Though proteins themselves do not mutate, a mutation in the
genetic material of an organism is expressed as a change in the order of
amino acids of a protein.
It is the order of the amino acids in a protein that
determines its shape and it is the shape that determines the function.
Clearly, it is critical for every cell to have
a process that guarantees
accurate ordering of amino acids
in every protein that it needs to carry out its life activities.
Find Out How That Happens. . .
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