respiratory pigment

n.
Any of various colored conjugated proteins, such as hemoglobin, that occur in living organisms and function in oxygen transfer in cellular respiration.

Colored, metal-containing proteins which combine reversibly with oxygen, and which are found in the body fluids or tissues of invertebrate animals. 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).

Each of the pigments is composed of two parts, a large protein molecule to which is bound one or more small moieties called prosthetic groups, each of which is or contains a metal. The metal binds the oxygen, and this binding imparts the characteristic color to the pigment. In hemoglobins the prosthetic group is an iron porphyrin compound called heme. Chlorocruorin contains a similar iron porphyrin which differs from heme only in that a vinyl group in the molecule is replaced by formyl. The prosthetic group of hemerythrin consists of two adjacent iron atoms which bind an oxygen molecule between them. The prosthetic group of hemocyanin is analogous and consists of two adjacent copper atoms. Pigments containing vanadium have been found in tunicates, but these substances do not combine reversibly with oxygen and so cannot be considered respiratory pigments.

The protein part of the pigment confers reversibility upon the combination of the metal with oxygen. In the absence of protein, the prosthetic groups lose their capacity to combine with oxygen reversibly. Instead, the metals are irreversibly oxidized: Electrons are transferred from metal to oxygen. The bonds between metal and protein so alter the electronic energy levels of the metal that this transfer, if it occurs, is reversible. For this reason, the combination of hemoglobin with oxygen is described as oxygenation rather than oxidation. The protein is also responsible for certain physiological adaptations of the pigment to the environment. Thus the affinity of the pigment for oxygen is often highest in animals which inhabit environments with the lowest oxygen content. See also Hemoglobin.

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n.

Any of the oxygen-carrying substances in the blood and tissues, such as hemoglobin and myoglobin.

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