Mechanism of heme catabolism

In adult humans, the catabolism of heme is the only known source of bilirubin IXa, 300-500 mg of the pigment being manufactured daily. It is a puzzle just why mammals should go to the trouble of reducing the water-soluble green biliverdin to insoluble yellow bilirubin. The body then has to esterify bilirubin with various sugars in order to solubilise it ready for excretion, via the bile duct, into the intestinal tract. Here, bilirubin undergoes further transformation to urobilinoids prior to its final excretion in feces. 

Perhaps the most convincing piece of evidence that the two enzymes are different is that purified heme oxygenase (from the pig spleen) contains no detectable cytochrome P450. So, although the hydroxylating activity of the two enzymes is similar, heme itself is the substrate for heme oxygenase. 

The enzyme-bound heme has a ligand attached to it that provides a strong ligand field. This means that the Fe(III) cation (which has a 3d outer electron configuration) is in a low-spin state, with the majority of its 3d electrons spin-paired. In this configuration, the Fe(III) cation is easily reduced to low-spin Fe(II) by a suitable electron donor (such as NADPH). Ready 

The next step in the catabolism of heme is the reduction of biliverdin to bilirubin. Birds, reptiles, and amphibians are quite happy to excrete water-soluble biliverdin directly—hence the green colour of bird droppings—but not mammals or fish. 

The reduction of biliverdin to bilirubin creates problems. Bilirubin is both 

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