Enzyme/enzymatic heme containing

Frequently, metal ions are associated with the prosthetic group or cofactor. Heme rings usually contain a chelated iron atom. Occasionally, however, these metals are merely bound within folded polypeptide regions with no additional organic constituents required. Many metal ions are known to participate in enzymatic activity. One or more of the ions of Na, K, Ca, Zn, Cu, Mg, Mn, as well as Co and Mo are often required by enzymes to maintain activity. 

Several factors may limit the overall rate of enzymatic reductive reactions. First, the electron transfer to the reactive metal (e.g., Co, Fe, or Ni) may be limiting. It is also possible that access of the organic substrates to the reduced metals contained within enzyme microenvironments may be limited. Mass transfer limitation is even more important in intact bacterial cells. For example, Castro et al. (1985) found that rates of heme-catalyzed reductive dehalogenations were independent of the heme content of the cells. 

Attempts to resolve the enzyme systems, which are tightly bound to mitochondrial or endoplasmic membranes, usually yield enzymatically inactive preparations. Only for the steroid ll)6-hydroxylase system of adrenocortical mitochondria has resolution into the following three components been accomplished (IS, 21, 22, 31) (1) a particulate fraction containing cytochrome P-450 (2) a fiavine-adeninedinucleotide fiavoprotein (MW 60,000) and (3) a non-heme iron protein (MW 20,000). The latter two constitute the TPNH-cytochrome P-450 reducing system. Full 11 -hy-... 

---