Alpha-helical domains are sometimes large and complex

Alpha-helical domains are sometimes large and complex 

Fi re 3.9 Schematic diagram of the structure of one domain of a bacterial muramidase, comprising 450 amino acid residues. The structure is built up from 27 a helices arranged in a two-layered ring. The ring has a large central hole, like a doughnut, with a diameter of about 30 A. 

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