Proteins

 Proteins are the most abundant organic compounds to be found in cells and comprise over 50% of their total dry weight. They are present in all types of cells and in all parts of the cell. Proteins perform many functions. They build many structures of the cell. All enzymes are proteins and in this way they control the whole metabolism of the cell. As hormones they regulate metabolic processes. some work as carrier and transport specific substances such as oxygen, lipids and metal ions etc. Some proteins called antibodies defend the body against pathogens. Blood clotting proteins prevent loss of blood from body after an injury. Movement of organs and organisms and movement of chromosomes during anaphase of cell division are caused by them. They are polymers of amino acids the compounds containing carbon, nitrogen, oxygen and hydrogen. The number of amino acids varies from a few to 3000 or even more in different proteins.

Amino acids: About 170 types of amino acids have been found to occur in cells and tissues. Of these about 25 are constituent of proteins. Most of the proteins are however made up of 20 types of amino acids. All amino acids have an amino group and a carboxyl group attached to the same carbon atom also known as alpha carbon. Amino acids are linked together to form polypeptides proteins. The amino group of one amino acid may react with carboxyl group of another releasing a molecule of water. for example glycine and alanine may combine. The linkage between the hydroxyl group of carboxyl group of one amino acid and hydrogen of amino group of another amino acid release water and C-N link to form a bond called peptide bond. The resultant compound glycylalanine has two amino acid subunits and is a dipeptide. A dipeptide has an amino group at one end and a carboxyl group at the other end of molecule. So both reactive parts are again available for further peptide bonds to produce tripeptides, tetrapeptides and pentapeptides etc. leading to polypeptide chains. 

Structure of proteins:

Primary structure: The primary structure comprises the number and sequence of amino acids in a protein molecule. F. Sanger was the first scientist who determined the sequence of amino acids in protein molecule. After ten year of careful work he concluded that insulin is composed of 51 amino acids in two chains. One of the chains had 21 amino acids and the other had 30 amino acids and they were held together by disulfide bridges. Hemoglobin is composed of four chains two alpha and two beta chains. Each alpha chain contains 141 amino while each beta chain contains 146 amino acids. The size of a protein molecule is determined by the type of amino acids and the number of amino acids comprising that particular protein molecule. Now we know that there are over 10000 proteins in human body which are composed of unique and specific arrangement of 20 types of amino acids. The arrangement of amino acids in a protein molecule is highly specific for its proper functioning. If any amino acid is not in its normal place the protein fails to carry on its normal function. The best example is the sickle cell hemoglobin of human beings. In this case only one amino acid in each beta chain out of the 574 amino acids do not occupy the normal place in protein( in fact this particular amino acid is replaced by some other amino acid ) and the hemoglobin fails to carry any or sufficient oxygen hence leading to death of patient. 

Secondary structure: The polypeptide chains in protein molecule usually do not lie flat. They usually coil into helix or into some other regular configuration. One of the common secondary structures is the alpha helix. It involves a spiral formation of the basic polypeptide chain. The alpha helix is a very uniform geometric structure with 3.6 amino acids in each turn of helix. The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in successive turns of the spiral. Beta pleated sheet is formed by folding back of the polypeptide.

Tertiary structure: Usually a polypeptide chain bends and folds upon itself forming a globular shape. This is the proteins tertiary conformation. It is maintained by three types of bonds namely ionic, hydrogen and disulfide. For example in aqueous environment the most stable tertiary conformation is that in which hydrophobic amino acids are buried inside while the hydrophilic amino acids are on the surface of molecules. 

Quaternary structure: In many highly complex proteins polypeptide tertiary chains are aggregated and held together by hydrophobic interactions, hydrogen and ionic bonds. This specific arrangement is the quaternary structure. hemoglobin the oxygen carrying protein of red blood cells exhibit such structure.

Classification of proteins:

Fibrous proteins: They consist of molecules having one or more polypeptide chains in the form of fibrils. Secondary structure is most important in them. They are insoluble in aqueous media. They are non crystalline and are elastic in nature. They perform structural roles in cells and organisms. Examples are silk fiber, myosin, fibrin and keratin.

Globular proteins: These are spherical or ellipsoidal due to multiple folding of polypeptide chains. Tertiary structure is most important in them. They are soluble in aqueous media such as salt solution, solution of acids or bases or aqueous alcohol. They can be crystallized. They disorganize with changes in the physical and physiological environment. Examples are enzymes, antibodies, hormones and hemoglobin.