![]() For example, the dipeptide alan ylglycine can be written as Ala-Gly, and the tripeptide glyc ylhistid yllysine as Gly-His-Lys. For polypeptides, one-letter abbreviations of the amino acid residues are usually written in a sequence from N-terminus to C-terminus. For example, dipeptide Ala-Gly is written AG, and tripeptide Gly-His-Lys is written as GHL. The primary structure of peptides or proteins is the sequence of amino acids linked together by peptide bonds. The primary structure of a protein is shown as a sequence of amino acids written from N-terminus-to-C-terminus.įor example, the primary structures of the dipeptide and tripeptides shown above are Ala-Gly and Gly-His-Lys. When the sequence of amino acids is known, three-letter abbreviations are shown separated by hyphens, e.g., Gly-His-Lys. When the sequence of amino acids in a peptide is not known, the three-letter abbreviations of the constituent amino acid are listed, separated by commas. For example, Ala, Gly could mean Ala-Gly or Gly-Ala which are different compounds with different properties, related to each other as constitutional isomers. Similarly, Gly, His, Lys could mean any one of the following six constitutional isomers: Gly-His-Lys, Gly-Lys-His, His-Lys-Gly, His-Gly-Lys, Lys-Gly-His, or Lys-His-Gly. The number of constitutional isomers increases exponentially as the number of amino acids in the peptide increases. ![]() Constitutional isomers of a polypeptide of n amino acids chosen from 20 amino acids commonly found in proteins are given by 20 n. ![]()
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