PROTEINS AND PROTEIN STRUCTURE
OVERVIEW
The functions of proteins are the essence of life itself. They make up more than 50% of
the dry mass of animals. There are thousands of different proteins within the cells of
living things.
STRUCTURE OF PROTEINS
ELEMENTS found in proteins: Nitrogen in addition to C, H, O, some R groups have S
(see next page).
AMINO ACIDS: MONOMERS OF PROTEINS
.Proteins are formed from chains of amino acids. Amino
acids are SIMILAR in that each has an amino group NH2, a
central carbon, and a carboxyl group (COOH). What makes
amino acids different from each other are their side chains
– usually labeled as R (see next page)
The R group of each amino acid is circled. The R group
contributes the unique properties of each of the different amino acids.
Proteins are long, unbranched chains of amino acids that fold up into complex shapes . It is
the order of the amino acids in a protein that determines its shape and it is the shape that
determines the function. Proteins have distinctive shapes because of interactions between
the R groups of amino acids in different parts of the chain. This folding forms a 3
dimensional structure which actually allows the protein to be functional.
Proteins are long, unbranched chains of amino acids that fold up into complex shapes . It is
the order of the amino acids in a protein that determines its shape and it is the shape that
determines the function. Proteins have distinctive shapes because of interactions between
the R groups of amino acids in different parts of the chain. This folding forms a 3
dimensional structure which actually allows the protein to be functional.
This complex 3-D structure is made up of four “stages” or structures. (see next page)
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The primary structure of a protein is the
sequence of amino acids.
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.
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.
The arrangement of two or more polypeptide
chains in a protein make up its quaternary
structure. An example of this type of protein is
hemoglobin.
What is an R in the Amino Acid structure?
An R is NOT an element. Believe it or not, the letter R stands for “ it might be anything”.
What makes amino acids different from each other are their R groups. On the next page you
will see that all R groups are circled. R groups make each amino acid unique. Although there
are only 20 amino acids, it is the SEQUENCE (order) of the amino acids that creates the
specific FUNCTION of the protein. So even though we have only 20 amino acids, we have
thousands of different types of proteins in living organisms.
1. What is the name of the amino acid that has the “simplest” R group?
2. Two amino acids have a sulfur in the R group. Find the two amino acids that contain
sulfur and list them in the space below:
3. Look through all the R groups. Find the five elements that are found in the R groups.
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Examples of Proteins and their Functions:
Many of an organism’s proteins are enzymes, special proteins that speed up the rate of
chemical reactions in the cell. We will talk more about enzymes.
Enzymes are like tiny molecular tools that temporarily combine with the reactants and hold
them at the correct angle for a reaction to occur. Each enzyme has only ONE specific
function.
Some enzymes put molecules together by putting them in close contact with each other so
that bonding can occur
Other enzymes break apart molecules. These enzymes exert a force on the bonds in the
molecule they breakdown to loosen or break the bonds.
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.
There are many other types of proteins that are not enzymes, examples of these proteins
and the function of the “example” are given below.
Some Functions of Proteins
Type of protein
Enzymes
Example
amylase
Function
Promotes the break down of starch to the simple
sugar glucose.
Hair, wool, nails, horns, hoofs, tendons, cartilage
Structural
proteins
Hormones
Contractile
proteins
keratin,
collagen
insulin, glucagon
actin, myosin
Storage proteins
Transport
proteins
Immunological
proteins
Toxins
ferritin
Stores iron in spleen
hemoglobin
serum albumin
antibodies
Carries oxygen in blood
Carries fatty acids in blood
Rid the body of foreign proteins
neurotoxin
Cobra venom blocker of nerve functions
Regulates use of blood sugar
Contracting fibers in muscle
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CLOSE READING: Please read this article and answer the questions provided.
Hemoglobin- A protein with quaternary structure
PROBLEMS IN PROTEINS
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. 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.
Hemoglobin is comprised of four polypeptide subunits (each has tertiary structure). All four
components carry a heme group that can bind to oxygen, and all four components must be
present to form hemoglobin. The shape of the hemoglobin affects its ability to carry oxygen,
and travel freely throughout the circulatory system.
A condition that is a result of a malformed hemoglobin unit is sickle-cell anemia. In this
condition, a specific glutamic acid is replaced by a valine, and an ionic cross-link is not formed.
The result is a severe change of shape of the tertiary structure of the hemoglobin. The new
shape of the red blood cell is a crescent or sickle which reduces the oxygen carrying capacity
of the red blood cell.
The body recognizes sickled cells as “defective” and so they are removed from circulation
faster than normal cells. The result is that most sickle cell patients suffer from anemia. The
sickled cells can also clump together causing blockages, pain and organ damage3.
References
Davies, J., Shaffer Littlewood, B., Elementary Biochemistry – An Introduction to the Chemistry
of Living Cells, Prentice-Hall Inc, New Jersey, 1979.
Koolman, J., Rohm, K-H, Colour Atlas of Biochemistry, Thieme, Stuttgart, 1996.
Timberlake, K.C., Chemistry – 5th Edition, Harper-Collins Publishers Inc, NY, 1992.
Devlin, T.M., The Textbook of Biochemistry – 3rd Edition, Wiley-Liss Inc, NY, 1992.
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