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They are composed of two or more straight chains (β strands) that are hydrogen bonded side by side.Amino acids with branches at the β-carbon atom (valine, threonine, and isoleucine).Close proximity of a pair of charged amino acids with similar charges.

It is very stable because of the linear hydrogen bondings.The pitch of the helix (the linear distance between corresponding points on successive turns) is 5.4 Å.The helix has an average of 3.6 amino acids per turn.Polypeptide chains can fold into regular structures such as.A hydrogen-bonded, local arrangement of the backbone of a polypeptide chain.the bond between the a-carbon and the carboxyl carbon of that residue (known as psi ).the bond between the -carbon and the amino nitrogen of that residue (know as phi ).The two bonds within each amino acid residue freely rotate.The mutation results in: 1) arrays of aggregates of hemoglobin molecules, 2) deformation of the red blood cell, and 3) clotting in blood vessels and tissues.It is caused by a change of amino acids in the 6th position of  globin (Glu to Val).A single amino acid substitution can give rise to a malfunctioning protein, as is the case with sickle-cell anemia.The primary structure of a protein determines the other levels of structure.The order in which the amino acids are covalently linked together.Quaternary structure describes the number and relative positions of the subunits in a multimeric protein Some proteins are made of multiple polypeptides crosslinked (connected) with each other.Tertiary structure: the three-dimensional structure and/or arrangement of all the amino acids residues of a polypeptide chain.Secondary structure: the localized organization of parts of a polypeptide chain.Primary structure: the sequence of amino acid residues.These active structures are known as native conformations (the 3-dimensioanl structure of a properly folded and functional protein).A protein may have gazillion possibilities of structures, but a few would be active.Proteins have different structures and some have repeating inner structures, other do not.