Figure 1. Representative structures of haem-containing proteins. Cartoon representations of several haem proteins showing the location of the haem in the protein. Shown from left to right. Top row: cytochrome b₅₆₂ (four-helix bundle), myoglobin (globin fold), nitrophorin (beta barrel). Middle row: cytochrome c, cytochrome f, bovine catalase. Bottom row, nitrite reductase (nine-haem form). (Figure prepared using Pymol.)

Figure 2. Close-up views of the haem environment. (a) Space-filling representation of the haem environment of cytochrome c. Only the edge of the haem ring (in grey) is visible, the rest of the haem is enveloped by the protein, including the propionic acid groups that point into the centre of the protein). The orange atom at the front of the pocket is the sulfur of Cys14 which is covalently bound to the haem ring. (b) Space-filling representation of the haem environment in cytochrome P450 showing that most of the distal surface of the haem (grey) is solvent-exposed. Water molecules occupying the distal pocket have been removed. The iron (orange) in the centre of the haem is solvent-exposed. (c) The surroundings of the propionic acid substituents of yeast isozyme-1-cytochrome c. The carboxylate substituent of pyrrole ring D is completely buried in the protein and forms an ion pair with Arg38. The other carboxylate is positioned closer to the surface, but is still completely surrounded by protein residues and is not solvated. (Figure prepared using Molscript.)

Figure 3. Structure of an electron-transfer complex. (Left) Yeast cytochrome bc₁ complex and its electron-transfer partner cytochrome c. The large assembly is the normally membrane bound yeast cytochrome bc₁ complex, the small red protein on the bottom in the box is cytochrome c. The green and red domains at the extreme bottom left and bottom right are antibody fragments needed for crystallization. (Right) An enlargement of the boxed area showing the end-on close approach (4.5 Å) between the vinyl substituents of the haems in cytochrome c and the cytochrome c₁ of the cytochrome bc₁ complex. (Figure prepared using Molscript.)

Figure 4. Substrate entrance and exit paths in haem proteins. (Top left) The conformation of P450_BM-3 (open) in the absence of substrate. (Bottom left) The same protein closed around the substrate, palmityloleic acid. The green helicies on the left side of the figure undergo the largest movements, moving towards the blue -sheet region on the right. (Right) The myoglobin structure showing cavities (grey spheres) thought to be occupied by substrates as they diffuse through the protein. After subjecting myoglobin crystals to xenon gas at liquid nitrogen temperatures, subsequent structure determinations have found xenon trapped in these sites. (Figure prepared using Pymol and Molscript.)