Question

What is pleated sheet organization? Can there be quaternary structures?

Answer 1

Hint: Here, protein structure is being talked about, pleated sheets, also known as beta sheets are a type of structural organization of proteins.

Complete answer:
Proteins tend to fold in a 3-D structure. They automatically fold in their native structure following the sequence of amino acids present in it. The structure of protein can be primary, secondary, tertiary and quaternary. Beta pleated sheets fall under secondary proteins.
The β-sheet or β-pleated sheet is a motif of regular secondary protein structure. Beta sheets consist of beta strands connected laterally by hydrogen bonds, forming a twisted, pleated sheet. A β-strand is a stretch of polypeptide chain, it is 3 to 10 amino acids long with backbone where the conformation is extended. The β-strands are arranged adjacent to one another and form an extensive hydrogen bond network among them in which the N−H groups in the backbone of one strand establish hydrogen bonds with the C=O groups in the backbone from the adjacent strands. In a fully extended β-strand, the side chains point straight up, then straight down in continuous fashion. Adjacent β-strands in a β-sheet are aligned so that their alpha carbon atoms are adjacent and their side chains pointing in the same direction. The tetrahedral chemical bonding at the alpha carbon atom gives the pleated appearance. Beta sheets contain beta loops, also known as β-hairpin in which two antiparallel strands are linked by a short loop of amino acid residues. Either glycine or a proline is present which forms the dihedral-angle conformations required for a turn or a β- loop. The secondary structures are formed of primary structures. They together form tertiary structure which may contain both beta sheets as well as alpha helix. Several tertiary structures together form the quaternary structure.

Note:Several secondary and tertiary protein molecules together form the quaternary structure of protein called oligo protein. Beta sheets can be present in the quaternary structure but quaternary structure cannot be present in a pleated sheet.