write the Peptide bond and protein
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Proteins are known to break down into peptides in stomach and duodenum under the influence of enzymes, pepsin being one of them which is secreted by stomach. Polypeptides are further broken down to ∝-amino acids. This implies that proteins are formed by connecting ∝-amino acids to each other. The bond that connects ∝-amino acids to each other is called peptide bond. Consider, for example, linking of a molecule of glycine with that of alanine. One way of doing this is to combine carboxyl group of glycine with ∝-amino group of alanine. This results in elimination of a water molecule and formation of a dipeptide called glycylalanine in which the two amino acid units are linked by a peptide bond It can be seen that a peptide bond or peptide linkage is same as what is described as secondary amide in organic chemistry. Combination of a third molecule of an ∝-amino acid with a dipeptide would result in formation of a tripeptide. Similarly linking of four, five or six ∝-amino acids results in formation of tetrapeptide, pentapeptide or hexapeptide respectively. When the number of ∝-amino acids linked by peptide bonds is more than ten, the products are called polypeptides. The -CHR- units linked by peptide bonds are referred to as ‘amino acid residues’. Proteins are polypeptides having more than hundred amino acid residues linked by peptide bonds. It may be, however, noted that distinction between proteins and polypeptides is not sharp. The two ends of a polypeptide chain of protein are not identical. The end having free carboxyl group is called C-terminal while the other end having free amino group is called N-terminal. In the dipeptide glycylalanine glycine residue is N-terminal and alanine residue is C-terminal.