Structurally, proteins are polymers of amino acids. Amino acids are carbon compounds consisting of two side chains: basic amine (NH2) and acidic carboxylic acid (COOH) groups. The variable R-group is what gives amino acid its distinctive characteristic i.e. amino acids differ only due to the R-group. There are 20 different types of amino acids which makeup all the proteins in the body. An amino acid joins with another amino acid through a peptide bond i.e. a bond between carboxylic group of one and amine group of the next amino acid. Thus a polymer of amino acid is formed called polypeptide. Longer polypeptides eventually fold and a structural protein is formed.
The content and precise sequence of amino acids in a protein is encoded by the precise sequence of bases in the DNA. This precise sequence of amino acids is then responsible to determine the biological function of the protein. The sequence of amino acids also determines how the protein folds to make a 3D structure which is a prerequisite for its biological function. For e.g. many enzymes catalyze certain chemical reactions by binding with the substrate molecule. This binding is only possible when protein folds into a precise 3D structure which makes it possible for it to bind to the substrate in the precisely needed fashion. Just a little mistake and the precise binding does not possible thus the reaction fails to proceed. The field of protein folding, structure and stability has been one of the most important research fields for many years and still remain one of the unsolved mysteries.
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Structure of amino acids: Every amino acid consists of a basic amino and an acidic carboxyl group connected to a central Carbon molecule. The R-group attached to the central Carbon is what gives individual amino acid its identity.
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A polypeptide chain: Peptide bond between adjacent amino acids i.e. between carboxyl group from one and amine group from the other amino acid forms a polypeptide.
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Folding of a polynucleotide chain: A polypeptide chain of more than 100 amino acids can fold into a protein which has precise 3D structure and is biologically active
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