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responsible for signal transduction, e.g. G-protein-coupled receptors, ion channels, etc. In addition, a-helices are often found in fibrous (a-ceratin, collagen) and globular proteins (myoglobin), in the latter with an average length of 11 residues [5].

In a 0-sheet, the peptide chain adopts an extended conformation with alternating pairs of NH and C=O groups pointing to the top and bottom face. Thus, 0-sheets are sticky to the exterior. In proteins, a 5-15 amino acid strand is usually followed by a 0-turn-motif (often containing proline residues), which reverses the direction of the main chain. In this antiparallel arrangement, the NH and C=O pairs can form multiple linear hydrogen bonds, leading to a regular sheet structure with remarkable stiffness (Figure B.8.2) [6]. Here, the side chains are placed horizontally to the left or the right in alternating order.

Certain combinations of secondary superstructures are often found in proteins and control their structure and function. The most frequent is the bab-unit, where an a-helix bridges two 0-strands. This is the prevailing feature in most coenzyme-binding domains of dehydrogenases [7]. Other important superstructures include a,a-dimers, 0-meanders and 0-barrels.

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