447 Pyrroles, Porphyrins and Related Compounds 



CH3 



I 

 HOOC— CHo— CH.— CO— CoA ^ HOOC— CH— CO— CoA 



Succinyl Coenzyme A Methylmalonyl Coenzyme A 



The final step in the conversion of succinate to propionate 

 is the biotin-dependent decarboxylation : -'^ 



CH3 

 Methylmalonyl Coenzyme A Propionyl Coenzyme A 



The total process can be written: 



(1) Acetyl CoA + Succinate ^ Succinyl CoA + Acetate 



B12 coenzyme 



(2) Succinyl CoA , Methylmalonyl CoA 



(3) Methylmalonyl CoA + Biotinenzyme :^ CO2 Biotinenzyme + Propionyl CoA 



(4) Propionyl CoA + Acetate ^ Acetyl CoA + Propionate 



Perhaps it is significant that propionibacteria are rela- 

 tively rich sources of vitamin Bjo and of bio tin. This 

 scheme also shows how propionic acid can be oxidized by 

 entry into the carboxylic acid cycle. 



The precise mechanism by which these interesting rear- 

 rangements are promoted by the Bj^ coenzymes remains 

 to be determined. It has been pointed out^^ that, in effect, 

 what is accomplished is a transpropionation. 



A monograph on vitamin Byj has been published. ^^ 



The cytochromes are heme proteins important in elec- 

 tron transport. The most studied is cytochrome c. The 

 commonest source is muscle, but yeast cytochrome c has 

 been crystallized.'"' Classification is made by spectrum, 

 and the proteins are species specific. 



The prosthetic group of cytochrome c is protoporphyrin 

 IX bound firmly to the apoenzyme by covalent bonds be- 

 tween the thiol groups of cysteine and the vinyl groups of 

 the porphyrin.^' Four of the iron coordination bonds are 



^^ E. Lester Smith, "Vitamin Bi-," John Wiley & Sons, Inc., New 

 York, 1960. 



°^ Bunji Hagihara, Takekazu Horlo, Kazuo Okunuki, Jinpei 

 Yamashita and Mitsuhiro Nozaki, Nature 178 629 (1956). 



^" K. Zeile and H. Meyer, Hoppe-Seylers Z. physiol. Chem. 262 178 

 (1939); H. Theorell, Enzymologia 6 88 (1939); Karl-Gustav Paul, 

 Acta Chem. Scand. 5 389 (1951). 



