Fig. 1.

Heme catabolic pathway. Heme contains a porphyrin ring and a central iron atom. In heme degradation, heme oxygenase breaks open the porphyrin ring with NADPH and oxygen (O₂) resulting in the release of iron (Fe), carbon monoxide (CO), and biliverdin. Biliverdin is reduced by biliverdin reductase to bilirubin. Reactive oxygen species (ROS) can recycle bilirubin back to biliverdin. In the liver, bilirubin is conjugated with glucuronic acid by UDP-glucuronosyltransferase 1A1 (UGT1A1) and excreted into the bile.