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COURSE BIOMOLECULES Lesson Amino acids and Peptide bonds Dr. Praveen Kumar Agrawal
AMINO ACIDS . Amino acids are the building blocks of the proteins. On Polymerisation, amino acids give rise to proteins, which are essential for growth and development of the body. (You will learn more about proteins in the next chapter) Although over 300 types of amino acids occur in nature but only 20 types are biologically important. These amino acids are held together by means of peptide bonds, to form proteins. This bond formation in living organism takes place on ribosomes. 20 types of amino acids may be arranged in a number of ways to form innumerable types of proteins. This arrangement or sequence of amino acids in a protein is determined by the DNA (gene). The DNA contains the message for this sequencing in the form of genetic codes.
Chemistry Amino acids are small organic molecules, which contain at least a carboxylic (-COOH) and an amine (-NHH) roup. (Hence they are called amino acids). Proline is the only amino acid which has no NHH group. It has only NH . g group. So in fact it is an imino acid Both carboxylic and amine groups (in all biologically important 20 amino acid) are attached to the same carbon HOOC CH-NHH The shaded part is common in all 20 amino acids. The only difference is of side chain -R atom. Such amino acids are called a amino acids. This carbon atom may also contain a side chain (R) The general formula for amino acids is given in adjacent box Therefore all biologically important amino acids are a- amino acids and differ in the nature of their R chain.
S.No Nature of R chain Examples 0. 1. R chain is simple hydrocarbon chain 2. R chain contains hydroxilic groups 3. R chain containing sulphur atom Glycine, Alanine, Valine, Leucine, Isoleucine Serine, Threonine, Tyrosine Cysteine, Methionine Aspartic acid, Asparagine, Glutamic acid, Glutamine Arginine, Lysine, Histidine Histidine, Phenylalanine, Tyrosine, Tryptophan Proline 4. R chain contains acidic group or their amides 5. R chain contains basic groups 6. R chain contains aromatic rings 7. R chain is an imino acid
Peptide Bond During formation of a protein, various amino acids get linked together by peptide bonds. The structure so obtained is called polypeptide chain or simple protein. Peptide bond formation takes place be tween amino group ( NH2) of one amino acid and carboxylic group (COOH) of an other amino acid as follows. It is also a de hydration synthesis, as a water molecule is removed per peptide bond formed. A structure of two amino acids linked by peptide bond is called dipeptide. A chain of many amino acids is called polypeptide chain. A polypeptide chain represents a simple protein.
HOOC-CH NHH +HOOC CH NHH R' H,O HOOC-CH NHOC-CH NHH Peptide bond
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