Explain the various types of Protein Structure?

Protein Structure :  The structure of proteins can be examined at four levels of increasing complexity, with the primary structure being the simplest, and the quaternary structure being the most complex.

Primary Structure

The linear joining of amino acids forms the primary structure. The covalent bonds formed in the condensation reaction are called peptide bonds. The joined amino acids are peptides, or, as they form larger molecules, polypeptides.

Secondary Structure

Chains (primary structures) of amino acids can fold upon one another to form a right-handed coil (alpha helix), a folded, or pleated, sheet (beta sheet), or a twisted cable (triple helix). These secondary structures are held together by hydrogen bonds between amino acids in the chain.

The most common type of secondary structure is an alpha helix, or a-helix, which forms when hydrogen bonds arise between the amide group in one polypeptide and the carboxyl group of another. An example of the alpha helix is keratin, the structural basis of hair, skin, fingernails, and other protective tissues.

Secondary structures may also be formed as b-pleated sheets, in which the protein chains are extended and assembled into sheets by hydrogen bonds between adjacent chains. Beta sheets can form the basis for globular proteins. Globules can then become grouped to form various shapes joined by electrostatic interactions, hydrogen bonds, and occasionally by disulfide bonds.

In the third type of secondary structure, called a triple helix, three polypeptide chains are twisted together like strands of a cable. An example is collagen, a protein found in supporting structures such as cartilage, ligaments, and tendons, and other parts of the body. Cartilage is a translucent elastic substance that composes most of the skeleton of higher animals in early development but is eventually usually replaced by bone. Ligaments are the tough elastic fibers that hold bones in a joint together, such as those in your knees. Tendons are dense white fibers that connect muscle to bone.

Tertiary Structure

Secondary structures are further organized into more complex tertiary structures. Tertiary structures are formed when secondary protein structures such as alpha-helices or beta-pleated sheets become folded in very specific shapes.

Quarternary Structure

Some proteins are made up of two or more polypeptide chains that can be joined together in a certain way to form a functional protein called a quarternary structure. The red blood cell pigment hemoglobin, for example, is composed of four polypeptide strands that wrap themselves around a heme group, an iron-containing ring that binds oxygen.

Polypeptide chains can be made to unfold, or become denatured, by gentle heating or chemical treatments. If heat or chemical treatment is not too rigorous, under the proper conditions the protein can often reassemble to form the original quaternary structure.