is called heme. Hemes are prosthetic groups in some important proteins. These heme-containing proteins include hemoglobin, myoglobin, and cytochrome c. Complexes can also be formed with other metal ions, such as zinc. Biosynthesis The compound is synthesized from acyclic precursors via a mono-pyrrole (porphobilinogen) then a tetrapyrrole (a porphyrinogen, specifically uroporphyrinogen III). This precursor is converted to protoporphyrinogen IX, which is oxidized to protoporphyrin IX. The last step is mediated by the enzyme protoporphyrinogen oxidase. Protoporphyrin IX is an important precursor to biologically essential prosthetic groups such as heme, cytochrome c, and chlorophylls. As a result, a number of organisms are able to synthesize this tetrapyrrole from basic precursors such as glycine and succinyl-CoA, or glutamic acid. Despite the wide range of organisms that synthesize protoporphyrin IX, the process is largely conserved from bacteria to mammals with a few distinct exceptions in higher plants. In the biosynthesis of those molecules, the metal cation is inserted into protoporphyrin IX by enzymes called chelatases. For example, ferrochelatase converts the compound into heme B (i.e. Fe-protoporphyrin IX or protoheme IX). In chlorophyll biosynthesis, the enzyme magnesium chelatase converts it into Mg-protoporphyrin IX. Described metalloprotoporphyrin IX derivatives Protoporphyrin IX reacts with iron salts in air to give the complex FeCl(PPIX). Heme coordinated with chlorine is known as hemin. Many metals other than Fe form Heme-like complexes when coordinated to PPIX. Of particular interest are cobalt derivatives because they also function as oxygen carriers. Other metals - nickel, tin, chromium - have been investigated for their therapeutic value. Palepron is the disodium salt of protoporphyrin IX. History Laidlaw may have first isolated PPIX in 1904. See also Carbon monoxide-releasing molecules Heme oxygenase Biosynthesis of chlorophylls Biosynthesis of hemes References Porphyrins