Biochemestry protein : Most interesting topic of the day

Most proteins consist of linear polymersbuilt from series of up to 20 differentL-α- amino acids. All proteinogenic amino acidspossess common structural features, including an α-carbonto which an aminogroup, a carboxylgroup, and a variable side chainare bonded. Only prolinediffers from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. [ 1 ]The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; it is the combined effect of all of the amino acid side chains in a protein that ultimately determines its three-dimensional structure and its chemical reactivity. [ 2 ]The amino acidsin a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called aresidue,and the linked series of carbon, nitrogen, and oxygen atoms are known as themain chainorprotein backbone. [ 3 ] The peptide bond has two resonanceforms that contribute some double-bondcharacter and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar. The other two dihedral anglesin the peptide bond determine the local shape assumed by the protein backbone. [ 4 ]The end of the protein with a free carboxyl group is known as the C-terminusor carboxy terminus, whereas the end with a free amino group is known as the N-terminusor amino terminus. The wordsprotein,polypeptide,and peptideare a little ambiguous and can overlap in meaning.Proteinis generally used to refer to the complete biological molecule in a stable conformation, whereaspeptideis generally reserved for a short amino acid oligomers often lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and usually lies near 20–30 residues. [ 5 ]Polypeptidecan refer to any single linear chain of amino acids, usually regardless of length, but often implies an absence of a defined conformation.