Heme in chlorocruorin, the source of its unique green color.
Erythrocruorin (from Greek eruthros "red" + Latin cruor "blood"), and the similar chlorocruorin (from Greek khlōros "green" + Latin cruor "blood"), are large oxygen-carrying hemeproteincomplexes, which have a molecular mass greater than 3.5 million daltons.[1] Both are sometimes called giant hemoglobinorhexagonal bilayer haemoglobin. They are found in many annelids and arthropods (including some insects).[2]
Two structures of erythrocruorin have been resolved. The protein is a highly symmetric assembly made from heme-binding globins and unique linker proteins.[1][6]
The only significant difference between chlorocruorin and erythrocruorin is that chlorocruorin carries an abnormal heme group structure. Both contain many 16–17 kDa myoglobin-like subunits arranged in a giant complex of over a hundred subunits with interlinking proteins as well with a total weight exceeding 3600 kDa. [6]
Giant hemoglobin is composed of multiple heme-containing globin chains and linker (InterPro: IPR031639) chains. Each species have different amounts of genes for these chains. For example, while a Lamellibrachia sp. has four kinds of globin chains and two kinds of linker chains, Sabella spallanzanii has three globin chains and three linker chains.[6] The exact stoichiometric ratios and arrangement is unknown, but is thought to resemble that of erythocrorins.
Erythrocruorin has a weaker affinity for oxygen than that of most hemoglobins. A dichromatic compound, chlorocruorin is noted for appearing green in dilute solutions, though it appears light red when found in concentrated solutions.[7][8][9]
This enormous macromolecule is typically found free floating in the plasma, and not contained within red blood cells.[6][10]
