Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes

Figure 9

The pentaketide pathway (DHN-melanin formation): In this pathway, acetyl-CoA coming from intermediate metabolism is carboxylated to malonyl-CoA and five molecules give place to a pentaketide coenzyme A thioester. Subsequent cyclization by the pentaketide synthase complex yields 1,3,6,8-tetrahydroxynaphthalene. This tetrahydroxy derivative is transformed by hydration and dehydration reactions in 1,8-dighydroxynaphathalene (1,8-DHN), the main melanin precursor, passing through the formation of scytalone and vermelone as key intermediates, although other routes are possible in different fungal species. Fungal laccases and other phenol oxidases are able to catalyze their oxidation to naphthalenequinone that dimerize and polymerize to naphthalene melanin (DHN-melanin). The final polymer has undefined structure, as other types of melanin.