AHelix and bSheet

Functional proteins are characterized not only by their amino acid sequence (called the primary structure) but also by their exact geometrical arrangement in space (called the secondary, tertiary, and quaternary structure) [1]. Only correctly folded proteins can perform their biological function correctly. This complicated process is currently under active investigation; a whole class of folding helper proteins, the chaperones [2], are used by Nature to prevent misfolding events with their often dramatic consequences (TSE, Alzheimer's disease, etc., vide infra).

Most proteins contain one or both of the fundamental regular peptide backbone arrays, i.e. the a-helix or the ¿-sheet. These not only confer mechanical stability on the protein but also protect active centers, create the correct microenvironment for the catalyzed reaction, or even participate in the catalytic process. In a-helices all NH groups point in one direction and hydrogen-bond to the respective C=O groups of the i + 4 residues, all of which point into the opposite direction. By means of linear, intramolecular hydrogen bonds a right-handed helix is formed with a helical pitch of 0.54 nm (Figure B.8.1). Non-polar, aliphatic side-chains are compatible with the torsion angles necessary for an a-helix (only several areas of torsion angles y and f are allowed; these can be visualized in Ramachandran plots) [3]. Thus, helices are often found in transmembrane regions of proteins

Fig. B.8.1. Right-handed a-helix with intramolecular hydrogen bonds (dotted lines) [4].

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