What porphyrins do

Changing the external protein environment of the heme (which partly changes the way the iron atom is bound) leads to completely different iron chemistry. Thus, in cytochrome c, the iron atom cycles through the +2 and +3 oxidation states, while performing the function of electron transfer in cell respiration. Thus, with a subtle blend of thermodynamics and kinetics, nature coaxes iron into doing its biochemical bidding. 

the accessibility of porphyrins and related macrocyclic rings is one of the factors that allows biological systems to capitalise on and to modify for their own purposes, the redox and coordination properties of metal ions. 

Plants use carbohydrates in their own metabolism, as do the animals that eat the plants. As the carbohydrates are metabolised back into carbon dioxide and water, the energy they contain is liberated and used by an organism to grow, move, reproduce, devise more efficient ways of killing other organisms—and write books about porphyrins. 

In chlorophyll, the bound metal is magnesium. Here, the function of the macrocycle is to capture photons of light in the near-ultraviolet (400 nm) and red (650-700 nm) regions of the visible spectrum. In the reduced por- 

In cytochrome c, on the other hand, the heme is closer to the surface of the protein. Also, the other ligands are bound so strongly that oxygen cannot coordinate. This microenvironment favours electron transfer, so that the iron cycles reversibly through its II and III oxidation states. 

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