140 



PROTEINS 



approximately 0.1 per cent of iron. On the basis of 225,000 for the 

 molecular weight, the iron content corresponds to four atoms per mole- 

 cule. The iron appears to be in the ferric state, and the heme unit is 

 the same as that in hemoglobin. 



Two peroxidases, horse radish peroxidase II and cytochrome c peroxi- 

 dase from yeast, have been obtained in crystalline, or highly purified, 

 form and found to contain the same heme as hemoglobin. Two other 

 peroxidases, myeloperoxidase from leucocytes and lactoperoxidase from 

 milk, contain green-colored hemes and, hence, are also called verdo- 

 peroxidases. Both have hemes containing iron, but the structures of 

 these hemes have not yet been determined. 



Other M etal- containing Proteins. Ferritin is a brown-colored protein 

 containing up to 23 per cent of iron. The iron is present as colloidal 

 iron oxide or phosphate and is very loosely bound to the protein. Ferritin 

 occurs in the liver, spleen, and bone marrow and is believed to serve as 

 a storage form of iron. 



Hemocyanins serve as respiratory proteins for the lobster, octopus, 

 and other marine animals. The blood of the lobster becomes blue when 

 aerated, hence the term hemocyanin, literally blue blood (a term that man 

 has applied to himself as a mark of distinction, without considering its 

 connotations). Hemocyanin contains copper (about 0.35 per cent), but 

 the nature of the prosthetic group, if any, carrying the copper is still 

 an unsettled question. The molecular weights ascribed to the hemo- 

 cyanins are enormous, 2 to 5 million. 



Additional copper-containing proteins are : hemocuprein from red blood 

 corpuscles, hepatocuprein from the liver and several oxidizing enzymes, 

 e.g., tyrosinase, ascorbic acid oxidase, etc. Other metals forming com- 

 plexes with proteins, but not having color, are magnesium in carboxylase, 

 zinc in insulin and carbonic anhydrase, and manganese in arginase. 



Other Colored Proteins. Flavoproteins or "yellow enzymes" have ribo- 

 flavin phosphate, or a dinucleotide of riboflavin and adenine, as the pros- 

 thetic group and are yellow in color, hence the name "yellow enzyme" 

 given to them. About a dozen such enzyme proteins have been reported. 

 They play an important role as hydrogen transport agents and can exist 

 in either a reduced or oxidized state (see Fig. 10-4). 



Rhodopsin is a red, light-sensitive pigment found in the retina of land 

 and marine animals, e.g., man, cattle, squid, and plays an important 

 role in vision. It is a chromoprotein that consists of the prosthetic group, 

 cis-retinene 1, and a protein called opsin. Retinene 1 is the aldehyde 

 (C19H07CHO) corresponding to vitamin Aj (C19H27CH2OH). In fresh 

 water vertebrates, such as fish, a somewhat different chromoprotein called 

 porphyropsin takes the place of rhodopsin. Porphyropsin contains 

 retinene 2, which corresponds to vitamin Ao. Opsins from different sources 



