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any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
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any of several complex proteins containing pigmented prosthetic (nonprotein) groups. The largest group of chromoproteins includes the enzymes catalase and peroxoidase and the respiratory pigments hemoglobin and myoglobin. This group of chromoproteins also includes the iron-containing hemoproteins, among which are the cytochromes, that is, the carriers of electrons in the processes of cell respiration and photosynthesis, and in hydroxylation systems. In many invertebrate animals the binding of oxygen is achieved by the hemoglobin-like proteins known as erythrocruorins. In the blood of certain poly-chaete worms, this function is performed by chlorocruorins.
The second group of chromoproteins consists of the respiratory pigments in the blood of invertebrates: hemerythrin, which contains non-heme iron, and hemocynanin, which contains copper. The third group of chromoproteins comprises enzymes whose prosthetic group consists of compounds derived from riboflavin—the flavoproteins, which are carriers of electrons and perform an important function in the oxidation-reduction reaction of all animal cells. Another chromoprotein is rhodopsin (visual purple), a pigment found in the retina that contains 11-cis retinal as its chromophore group.
The term “chromoprotein” is becoming obsolete in the above sense and is increasingly used to refer primarily to the respiratory pigments of the blood.