Heme is perhaps the most ubiquitous cofactor found in nature and the most functionally diverse. Hemoproteins are involved in cell respiration (cytochromes), oxygen-binding and transport (hemoglobin and myoglobin), oxidative bio transformations (cytochrome P-450 and peroxidases), and most recently, as sensors in 2-component regulatory systems (guanylate cyclase, FixL, and CooA). The ability of hemoproteins to carry out extremely diverse reactions arises largely from the protein environment in which the heme molecule resides and specifically the nature of the heme-ligands. Other factors that contribute to the reactivity of the heme are intrinsic to the heme itself, including the substituents on the heme periphery and, in some cases, the covalent attachment of the heme to the protein. The structures of the most common heme-ligands and examples of the hemoproteins in which they occur are found in Table 1 .
Table 1. Heme Ligand Structure and Function
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