ENZYMES 



279 



acteristic protein and a heme compound. There are at least three cyto- 

 chromes, designated as a, b, and c. The heme portion is more firmly 

 attached to the protein in these pigments than the corresponding func- 

 tional groups of the pyridino- or flavo-proteins. The cytochromes are 

 concerned with oxidation-reduction reactions, and their concentration in 

 aerobic organisms bears a direct relationship in many instances to the 

 respiratory activities of the cell. The best characterized of tliese respira- 

 tory pigments is cytochrome c. It contains 0.43 per cent of iron and 

 is believed to have a molecular weight of 13,000. The most probable 

 formula for the heme component of cytochrome c, according to the evi- 



PROTEIN 



S 

 I 



CH — CH3 CHj 

 I H I 



COOH 



Fig. 10-8. Cytochrome c. The heme component (prosthetic group) is 

 shown and also its attachment to the protein part of the molecule by two 

 sulfur linkages and the iron atom. 



dence available at present, is shown in Fig. 10-8. Cytochrome c (abbre- 

 viated Cyt. c) functions as an electron carrier in cellular oxidation- 

 reduction reactions by virtue of its iron atom which alternately changes 

 its valence from 2 to 3: 



Cyt. c (Fe+ + + ) + e ?=^ Cyt. c (Fe+ + ) 



6. Pyridoxal Phosphate. Enzymes which catalyze the decarboxylation 

 of histidine, tyrosine, lysine, and glutamic acid to form carbon dioxide 

 and the corresponding primary amine require pyridoxal phosphate as a 

 coenzyme. This coenzyme is also a necessary cofactor for transaminase 



