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Proteins  
 


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.

Some Functions of Proteins

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

Transport proteins

hemoglobin
serum albumin

Carries oxygen in blood
Carries fatty acids in blood

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.



 

Basic Structure of an Amino Acid
The letter R represents one of twenty different structures.

Click here to see the structural formulae including the
R groups of each of the 20 amino acids found in living cells.

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.
 


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

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

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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This web is lovingly dedicated to the memory of
Mr. James Dorsey
who so graciously and enthusiastically
donated his DNA to solve our family mystery. 


Jim Dorsey
2/12/1930 — 4-30-2002

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