Role of Respiratory Pigments in Gaseous Exchange

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Colored, metal-containing proteins that combine reversibly with oxygen, found in the body fluids or tissues of multi-cellular invertebrate animals and microorganisms. The role of these pigments is primarily to aid in the transport of molecular oxygen. Thus they are distinguished from respiratory enzymes, which are concerned with the metabolic consumption of oxygen. Four distinctly colored groups of respiratory pigments exist among invertebrates: hemoglobins (purple, become orange-red with oxygen), chlorocruorins (green, become red with oxygen), hemocyanins (colorless, become blue with oxygen), and hemerythrins (colorless, become red with oxygen). Formerly, invertebrate hemoglobins were called erythrocruorins to distinguish them from functionally similar yet structurally distinct pigments of vertebrate bloods. 1. HEMOCYANINS : Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule (O2). Oxygenation causes a color change between the colorless deoxygenated form and the blue oxygenated form. Hemocyanins carry oxygen in the hemolymph of most molluscs, and some arthropods, including the horseshoe crab, Limulus polyphemus. They are second only to hemoglobin in frequency of use as an oxygen transport molecule. Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not bound to blood cells but are instead suspended directly in the hemolymph. Species using hemocyanin for oxygen transportation are commonly crustaceans living in cold environments with low oxygen pressure. Under these circumstances hemoglobin oxygen transportation is less efficient than hemocyanin oxygen transportation. Hemocyanins are one-fourth as efficient as hemoglobin at transporting oxygen per amount of blood. Hemocyanin is made of many individual subunit proteins, each of which contains two copper atoms and can bind one oxygen molecule (O2). Because of the large size of hemocyanin, it is usually found free-floating in the blood, unlike hemoglobin, which must be contained in cells because its small size would lead it to clog and damage blood-filtering organs such as the kidneys. This free-floating nature can allow for increased hemocyanin density over hemoglobin and increased oxygen carrying capacity. 2. HEMOGLOBIN : Hemoglobin (abbreviated Hb or Hgb) is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates. Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e., the tissues) where it releases the oxygen to burn nutrients to provide energy to power the functions of the organism, and collects the resultant carbon dioxide to bring it back to the respiratory organs to be dispensed from the organism. In mammals, haemoglobin makes up about 97% of the red blood cells' dry content, and around 35% of the total content (including water). Hemoglobin has an oxygen binding capacity of 1.34 ml O2 per gram of hemoglobin, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules. Hemoglobin is also found outside red blood cells and their progenitor lines. Other cells that contain hemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, and mesangial cells in the kidney. In these tissues, hemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism. Hemoglobin and hemoglobin-like molecules are also found in many invertebrates, fungi, and plants. In these organisms, hemoglobins may carry oxygen, or they may act to transport and regulate other things such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called...
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