Heme is broken down into iron and biliverdin, a process which is catalyzed by heme oxygenase.
Degradation begins inside macrophages of the spleen.
In the first step, heme is converted to biliverdin by the enzyme heme oxygenase (HO).[33]
The iron gets recycled, while biliverdin is reduced to create unconjugated bilirubin.
Heme is a ring of four pyrroles joined by carbon bridges and a central iron atom.
Bilirubin is generated by a two-stage sequential catalytic degradation reaction that primarily takes place in the cells of the reticuloendothelial system, notably the spleen.
Other cells include phagocytes and the Kupffer cells of the liver. Heme is taken up into These cells take up the heme, and enzyme heme oxygenase acts on them. The enzyme liberates the chelated iron by catalyzing the oxidation of the alpha carbon bridge. This reaction produces an equimolar amount of carbon monoxide which is excreted by the lungs and leads to the formation of the green pigment, biliverdin. This green pigment is acted upon further by the nicotinamide adenine dinucleotide phosphate (NADPH) dependent enzyme, biliverdin reductase. This process releases an orange-yellow pigment known as bilirubin. Heme oxygenase as mentioned above is present in high concentrations in the Kupffer cells of the liver and the cells of the reticuloendothelial system. Heme oxygenase is the rate-limiting factor in bilirubin production.
The final structure is highly compacted by hydrogen bonding rendering the molecule essentially insoluble in aqueous solutions at neutral pH. The fully bonded structure of bilirubin is designated as bilirubin IX-alpha-ZZ. Bilirubin, being insoluble in an aqueous solution, is carried in circulation bound to albumin which is a reversible and covalent type of bonding.
http://www2.csudh.edu/nsturm/CHE452/13_Heme%20Metabolism.htm#:~:text=Heme%20proteins%20are%20constantly%20synthesized,the%20hemoglobin%20molecule%20is%20degraded.
https://www.frontiersin.org/articles/10.3389/fphys.2021.655393/full#:~:text=Red%20cells%20have%20an%20average,from%20the%20circulation%20every%20second.