3₁₀ helices in other membrane proteins. (A) Graph comparing the frequency of 3₁₀ helices in soluble and in membrane proteins. Only 3₁₀ helices with lengths between 5 and 11 residues have been considered. The number of helices in each length category was normalized to the total number of detected helices with five or more residues. Inset shows in more detail the differences between membrane and soluble proteins for helices longer than six residues. The data for 3₁₀ helices from soluble proteins has been extracted from Enkhbayar et al. (2006). Examples of 3₁₀ helices in membrane proteins: 3₁₀ helices are shown in red, other protein regions are shown as a blue ribbon, and prosthetic groups are shown as CPK spheres, unless otherwise indicated. (B) 3₁₀ helix in cytochrome bc1 complex (Protein Data Bank accession no. 1FYU). Protoporphyrin IX containing Fe (heme) is shown in close proximity to the helix. (C) In bacterial nitrate reductase A (Protein Data Bank accession no. 1Q16), the iron atom from the heme is coordinated by histidine (yellow CPK spheres) from the 3₁₀ helix. Opposite the heme and on the top side of the figure, there is phosphatidyl glycerol molecule completing the packing of the 3₁₀ helix. (D) In the cyanobacterial photosystem II complex (Protein Data Bank accession no. 3BZ1), a 3₁₀ helix is found embedded in a layer formed by LMG (1,2-distearoyl-monogalactosyl-diglyceride), β carotene, and chlorophyll A.