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= Respiratory Pigments = A respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There are four major classifications of respiratory pigment: hemoglobin, hemocyanin, chlorocruorin, and hemerythrin. Hemoglobin is the most commonly-occurring respiratory pigment, occurring in at least 9 different phyla of animals.

Hemoglobin
Hemoglobin is thought to be a very ancient molecule, even acting as a molecular clock of sorts. It has even been used to date the separation of vertebrates and invertebrates more than 1 billion years ago. Hemoglobin enjoys a large biological distribution, not only occurring among more than 9 different phyla of animals but occurring in some fungi and bacteria as well, even being identified in nitrogen-fixing nodules on the roots of some leguminous plants. The isolation of the hemoglobin gene from plant root cells has suggested that the hemoglobin genes that were inherited from a common ancestor shared by plants and animals may be present in all plants.

There are multiple types of hemoglobin that have been found in the human body alone. Hemoglobin A is the “normal” hemoglobin, the variant of hemoglobin that is most common after birth. Hemoglobin A2 is a minor component of hemoglobin found in red blood cells. Hemoglobin A2 makes up less than 3% of total red blood cell hemoglobin. Hemoglobin F typically is only found in the fetal stage of development. While Hemoglobin F falls dramatically after birth, it is possible for some people to produce some levels of Hemoglobin F throughout their full life.

Leghemoglobin
Leghemoglobin is a molecule similar in structure to myoglobin that is currently being used in artificial meat products, such as the Impossible Burger, to simulate both the color of meat and taste. Similar in function to hemoglobin, leghemoglobin contains trace amounts of iron, but it is primarily found in plant roots.

Hemocyanin
Hemocyanin is a respiratory pigment that uses copper as its oxygen-binding molecule, as opposed to iron with hemoglobin. Hemocyanin is found in arthropods and Mollusca. While hemocyanin is present in both Arthropoda and Mollusca, it is thought that the molecule independently evolved in both phyla. There are several other molecules that exist in arthropods and Mollusca that are similar in structure to hemocyanin but serve entirely different purposes. For example, there are copper-containing tyrosinases that play significant roles in immune defense, wound healing, and the arthropod's cuticle. Molecules similar to hemocyanin in structure are grouped in under the hemocyanin superfamily.