Phorphrins are a class of biologically significant pigments. They are composed of four pyrrole rings joined into flat macrocyclic compounds by one-carbon bridges in a conjugated system of 18 pi electrons. This lends such molecules exceptional stability. The parent molecule, porphine, has two hydrogens bonded to two of the central four nitrogen atoms, and is not found in nature.
Porphyrins are typically highly coloured, and form stable complexes with metallic ions which bind to each of the four central nitrogens and nestle in the exact middle of the molecule.
The most biologically important examples are haem, where an atom of Fe occupies the central position, and Chlorophyll, where the position is taken by an atom of magnesium. Haem is the oxygen-transport iron-porphyrin complex responsible for the red colour of arterial blood. It exists in red blood cells in a complex with the globin protein, as haemaglobin. The iron atom is complexed with the four porphyrin nitrogens but also has two additional coordination sites, one above and one below the plane of the porphyrin ring. One of these sites is occupied with an imadazole ring from a histidine unit in the globin proein, and the second site is available for binding with oxygen. Carbon monoxide can also bind to this site, with many times more affinity than oxygen, leading to the threat of suffocation in high CO environments.
The green colour of plants is due to chlorophyll, a magnesium complex of modified porphyrin. This pigment is found in large organelles called chloroplasts, one or more of which can be found in some types of cells, depending on the plant. Clorophyll-alpha is an ester of the long-chain alcohol phytol, which helps solubilize the pigment in the choloroplasts.