PROTEINS 139 



nodules of leguminous plants and is believed to play a role in the fixa- 

 tion of atmospheric nitrogen by these nodules. 



Erythrocruorin and Chlorocruorin. Many proteins other than hemo- 

 globin contain iron porphyrins as prosthetic groups. The basic structure 

 of these iron porphyrins is the same as that of the heme in hemoglobin, 

 and in many forms of life they also serve as the oxygen-carrying agent. 

 An example of this type of compound is erythrocruorin, the respiratory 

 pigment of the common earthworm and other worms. Chlorocruorin, a 

 green pigment, serves the same purpose for marine worms, e.g., Spiro- 

 graphis. These pigments are dissolved in the blood, not contained in 

 blood cells as are the hemoglobins. They are usually of large molecular 

 weight, several million, and contain many heme groups, e.g., 190 in the 

 chlorocruorin of Spirographis. The side chains of the hemes differ from 

 those in hemoglobin, but the iron is in the ferrous state as in hemoglobin. 



Cytochromes. There are at least three cytochromes, a, b, and c, that 

 differ from one another in solubility, reaction to cyanide, and other 

 properties. The cytochrom.es occur in all oxygen-using cells and, hence, 

 are the most widely distributed heme proteins in nature. They form an 

 oxidation-reduction system and serve as carriers of hydrogen in the oxi- 

 dation scheme of cells. (See pp. 283 and 333.) 



Cytochrome c is the best known cytochrome and has been obtained 

 in a crystalline and homogeneous state. It is a small protein, molecular 

 weight 13,000, and contains only one heme per molecule. The iron con- 

 tent is 0.43 per cent and is present either in the ferrous or ferric state, 

 according to whether the cytochrome is in the reduced or oxidized state. 

 The heme part of the molecule is bound not only by an iron-to-histidine 

 bonding, as in hemoglobin, but, in addition, is joined by two covalent 

 linkages between vinyl groups of the heme and cysteine residues of the 

 globin. Thus, 



NH-CH'CO NH'CH-CO 



Globin part 



Heme part 



These bonds are very stable, and on hydrolysis the cysteine residues go 

 with the heme. In other words, the — S — C — linkage is more stable 

 than the peptide linkage — CO — N — . 



Heme- containing Enzymes. Catalases occur widely in plant and ani- 

 mal tissues. Beef liver catalase has been crystallized and contains 



