nesium (Mg+ + ) and manganese (Mn+ + ) are important as cofactors or 

 "activators" in a number of enzymatically controlled reactions (Figure 

 2-6). Metals are also important constituents of chromoproteins, many 

 of which are enzymes. Iron is an essential component of the enzymes 

 catalase, peroxidase, and cytochrome c. This same metal also forms an 

 integral part of the pyrrole ring of the hemoglobin protein molecule. In 

 plants, the green pigment, chlorophyll, contains magnesium and is com- 

 plexed with protein. 



^XTLr 



Substrate 



■h 



Enzyme - substrate 

 complex 



Figure 2-6. Diagram Showing One Possible Mechanism of an Enzyme- 

 catalized Reaction Involving Combination of the Enzyme (Apoenzyme) 

 with a Coenzyme or Metal. (From Harrow, B. and Mazur, A., 1958. "Text- 

 book of Biochemistry," 7th ed., W. B. Saunders Co., Philadelphia, Pa., Fig. 

 33, p. 125; after McElroy, W. D., 1947. "The Mechanism of Inhibition of 

 Cellular Activity by Narcotics," Quart. Rev. Biol., 22, Fig. 1, p. 26.) 



GENERAL MORPHOLOGY AND CHEMISTRY OF THE CELL / 23 



