Structure proteins – Each proteins has specific properties which are determined by the number and the specific sequence of amino acids in a molecule, and upon the shape which the molecule assumes as the chain folds into its final. Compact from. There are four levels of organization which are described below.
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 a protein molecule. After ten years of careful work, he concluded, that insulin in composed of 51 amino acids in two chains. One of the chains had 21 amino acids and the other had 30 amino acids in two alpha and two beta chains. Each alpha chain contains 141 amino acids, while each beta chain contains 146 amino acids and the number of amino acids comprising that particular protein molecule.
Fig. 2.11. polypeptide chains in keratin ( fibrous protein) and in hemoglobin (globular protein) are held together to form respective functional proteins.
Now we know that there are over 10,000 proteins in the human body which are composed of unique and specific arrangements of 20 types of amino acids. The sequence is determined by the order of nucleotides in the DNA. 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 the proteins (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 the patient.
Secondary Structure
The polypeptide chains in a protein molecule usually do not lie flat. They usually coil into a helix, or into some other regular configuration. One of the common secondary structures is the a-helix. It involves a spiral formation of the basic polypeptide chain. The a-helix is a very uniform geometric structure with 3.6 amino acids in each turn of the helix. The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in successive turns of the spiral. B-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 (-S-S-). 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 the molecule.
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, exhibits such a structure.
Fig. 2.12. three levels of protein structures compared with a telephone wire Classification of proteins
Because of the complexity of structure and diversity in their function. It is very difficult to classify proteins in a single well defined fashion. However, according to their structure, proteins are classified as follows:
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 (from silk worm, and spider’s web) myosin (in muscle cells), fibrin (of blood clot), and keratin (of nails and hair).
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.