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. Examples and functions are given below.
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 reactants 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.
Some Functions of Proteins
Type of protein
Enzymes
Structural
proteins
Hormones
Contractile
proteins
Storage proteins
Transport
proteins
Immunological
proteins
Toxins
Example
amylase
keratin,
collagen
insulin, glucagon
actin, myosin
Function
Promotes the break down of starch to the simple
sugar glucose.
Hair, wool, nails, horns, hoofs, tendons, cartilage
Regulates use of blood sugar
Contracting fibers in muscle
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
1
STRUCTURE OF PROTEINS
ELEMENTS found in proteins: Nitrogen in addition to C, H, O
Monomers of proteins : amino acids.
.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.
This complex 3-D structure is made up of
four “stages” or structures.
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.
2
Hemoglobin- A protein with quaternary structure
3
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.
4