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BIOMOLECULES
PROTEINS
Biomolecules include both micromolecules, e.g. amino acids, nitrogenous bases, fatty acids, sugar, etc. and macromolecules, such as carbohydrates, proteins, lipids and nucleic acids.
PROTEIN
Protein structures are made by condensation of amino acids forming peptide bonds. The sequence of amino acids in a protein is called its primary structure. The secondary structure is determined by the dihedral angles of the peptide bonds, the tertiary structure by the folding of protein chains in space. Association of folded polypeptide molecules to complex functional proteins results in quaternary structure.
Define Protein Structure
Protein structure is defined as a polymer of amino acids joined by peptide bonds. We can thus see that the peptide bond (-CO-NH) is formed between the amine group of one molecule and the carboxyl group of the adjacent molecule followed by the elimination of a water molecule. This bond is otherwise an amide linkage. When peptide bonds are established among more than ten amino acids, they together form a polypeptide chain. Very often, when a polypeptide chain has a mass exceeding 10000u and the number of amino acids in the chain exceeding 100, we get a protein.
Classification of Proteins
Based on the molecular shape, proteins can be classified into two types.
1. Fibrous Proteins:
When the polypeptide chains run parallel and are held together by hydrogen and disulfide bonds, then the fiber-like structure is formed. Such proteins are generally insoluble in water. These are water-insoluble proteins. Example – keratin (present in hair, wool, and silk) and myosin (present in muscles), etc.
2. Globular Proteins:
This structure results when the chains of polypeptides coil around to give a spherical shape. These are usually soluble in water. Example – Insulin and albumins are common examples of globular proteins. Levels of Protein Structure
1. Primary Structure of Protein The Primary structure of proteins is the exact ordering of amino acids forming their chains. The exact sequence of the proteins is very important as it determines the final fold and therefore the function of the protein. The number of polypeptide chains together form proteins. These chains have amino acids arranged in a particular sequence which is characteristic of the specific protein. Any change in the sequence changes the entire protein. The following picture represents the primary protein structure (an amino acid chain). As you might expect, the amino acid sequence within the polypeptide chain is crucial for the protein’s proper functioning. This sequence is encrypted in the DNA genetic code. If mutation is present in the DNA and the amino acid sequence is changed, the protein function may be affected.
(a) α – Helix:
α – Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right-handed screw with the - NH group of each amino acid residue hydrogen-bonded to the - CO of the adjacent turn of the helix. The polypeptide chains twisted into a right-handed screw.
(b) β – pleated sheet:
In this arrangement, the polypeptide chains are stretched out beside one another and then bonded by intermolecular H-bonds. In this structure, all peptide chains are stretched out to nearly maximum extension and then laid side by side which is held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery and therefore is known as β – pleated sheet
3. Tertiary Structure of Protein This structure arises from further folding of the secondary structure of the protein. H-bonds, electrostatic forces, disulphide linkages, and Vander Waals forces stabilize this structure.
The tertiary structure of proteins represents overall folding of the polypeptide chains, further folding of the secondary structure. It gives rise to two major molecular shapes called fibrous and globular. The main forces which stabilize the secondary and tertiary structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.
4. Quaternary Structure of Protein The spatial arrangement of various tertiary structures gives rise to the quaternary structure. Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure. The exact amino acid sequence of each protein drives it to fold into its own unique and biologically active three-dimensional fold also known as the tertiary structure. Proteins consist of different combinations of secondary elements some of which are simple whereas others are more complex. Parts of the protein chain, which have their own three-dimensional fold and can be
