Figure 2

Secondary structure of proteins. A) Stick, ribbon, stick with line side chains and cartoon representations of a same α-helix. Dotted lines represent hydrogen bonds between the C' = O of residue n and the NH of residue n+4, each consecutive residue with approximate angles (-57º, -47º). The helical structure is formed by 3.6 residues per turn and a pitch of ~0.54 nm. B) Stick, ribbon, stick with line side chains and cartoon representations of a same antiparallel β-sheet, made of two β-strands. β-sheets are held together by hydrogen bonds between a C' = O residue of one β-strand and the NH residue of the apposing β-strand. Although very elongated, β-strands are also helical arrangements with two residues per turn and a pitch of ~0.7 nm. C) Cartoon representation of the protein myoglobin (PDB file 1MBN), mainly made of α-helices, showing the heme group as gray spheres and the oxygen molecule in red. It has a size of 2 × 3 × 4 nm³. D) Cartoon representation of membrane protein bacteriorhodopsin (PDB file 2at9), revealing the seven transmembrane α-helices, lipid molecules as brown sticks and the retinal in the protein core as black sticks. The yellow region represents the lipid bilayer with a typical thicknes of ~5 nm. E) Cartoon representation of muscle protein titin domain I91 (former I27) mainly formed by β-strands (PDB file 1TIU). F) Cartoon representation of outer membrane protein G, revealing its β-barrel conformation formed by 14 β-strands (PDB file 2F1C). The yellow region represents the lipid bilayer. The colour scale of images C to F goes from the N-terminus (blue) to the C-terminus (red), not to be confused with B-factors colour scale shown in Fig. 3B.