PROTEINS 



137 



Chromo proteins 



This group of proteins probably includes a larger number of important 

 proteins than any other subdivision. They arc functional rather than 

 structural, playing a role of the first order as carriers of gases, mediators 

 in the oxidation process, and intermediates in the phenomenon of vision. 



Hemoglobin. The chromoproteins contain colored prosthetic groups. 

 Hemoglobin, tlie red pigment of the blood cells, is the best known protein 

 of this type. Heme is the prosthetic group and globin, a histone, is the 

 protein j^art of the combination. Heme accounts for only 3.8 per cent 

 of the hemoglobin; globin comprises the other 96.2 per cent. Heme con- 

 tains four atoms of iron, which amounts to 0.33 per cent of the hemoglobin. 

 There are four heme units attached to one globin i)ai't to make up hemo- 

 globin. 



The exact nature of the attachment is not known, but it is generally 

 considered to be either an ionic or covalent bond between the iron of the 

 heme and the histidine of the globin. The union is weak and can be 

 easily broken by warming with acetic acid and sodium chloride. A salt, 

 hemin (C34H32N404FeCl), is formed and this crystalhzes readily. The 

 structural formula of hemin is 



CH=CH2 

 H 



CH3 



II 



V V 



0)/ I 



CH3--C 



(S)/C — N 

 CH3— C^x IV III 



-N, 



N= 



FcCl 

 \ 



N- 





,C— CH=CH2 



\ 



CH 



,C-CH3 



(5) 



H 



CH2 

 CH2 

 COOH 



CH3 



CH2 

 I 

 COOH 



Heme 



It consists of four pyrrole groups (I-IV) joined together through four 

 methene groups (a,/3,y,8). To the pyrrole groujis are also attached four 

 methyl groups (1,3,5,8), two vinyl groups (2 and 4), and two propionic 

 acid residues (6 and 7). In the center of the formula is a trivalent atom 



