# Production of Fungal Pigments: Molecular Processes and Their Applications
## 2 Pigmented Fungi and Biosynthesis of Pigments
### Melanins
These compounds are subsequently oxidized to DOPA-quinone by a phenol oxidase enzyme such as lactose or tyrosinase, depending on the substrates. The ensuing reactions towards melanin synthesis proceed very quickly, yielding the intermediate dihydroxyn indole, which in turn gives rise to melanin via polymerization [44]. Fungi may also synthesize melanin via the DHN pathway, as exemplified by the common mold and opportunistic human pathogen _A. fumigatus_. The DHN-melanin biosynthetic pathway is encoded in a gene cluster consisting of six genes--_abr1_, _abr2_, _agg1_, _arp1_, _arp2_, and _pksP/alb1_--located on its second chromosome [45]. The initial gene in the biosynthetic pathway is _pksP/alb1_, which encodes a polyketide synthase (PKS) catalyzing the conversion (\(\beta\)-ketoacyl condensation) from the acetyl-CoA and malonyl-CoA to the heptaketide  
Figure 2: Schematic representation of fungal melanin biosynthetic pathways: (**a**) DOPA-melanin production route as shown in _C. neoformans_; (**b**) biosynthetic pathway of DHN-melanin for deposit in conidial cell wall as manifested by _A. fumigatus_; (**c**) alternative pathway of pyomelanin biosynthesis linked to conidial germination of _A. fumigatus_. The dotted line in (**c**) means that the step is active only during conidial germination. YWA1: Yellow pigment intermediate of WA polyketide synthase. napthopyrox YWA1. The second step is hydrolysis, catalyzed by the _avg1_-coding enzyme, which converts YWA1 to 1,3,6,8-tetrahydrooxynaphthalene (1,3,6,8-THN). Subsequently, the pathway implements a series of reduction steps followed by aromatization/dehydration, which ends in an oxidative polymerization. Briefly speaking, the resultant intermediate 1,3,6,8-THN is reduced again by Arp2p (Arp2 protein), giving rise to vermelone, the latter of which is oxidized by the copper oxidase Abr1p to form the penultimate product 1,8-DHN. The last reaction generates DHN-melanin from 1,8-DHN, catalyzed by the lactose Abr2p [46]. The other type of melanin generated by _A. fumigatus_ is pyomelanin, a pigment with a dark brown hue. In contrast to DHN-melanin, the production of pyomelanin does not have its own biosynthetic pathway but rather proceeds via the polymerization of homogenisate, an intermediate derived from the degradation of L-tyrosine/L-phenylalanine, which is coupled with conidial germination [43; 47]. Recent studies with the commercial wood-ear mushroom _Auricularia auricula_ have shown its melanin in association with the mushroom cell wall, which is analogous to that of the model fungus _C. neoformans_, belonging to DOPA-melanin [48; 49]. See Figure 2a for the biosynthesis of melanin in _A. auricula_. Strikingly, _A. auricula_ represents the most feasible commercial melanin producer in terms of scalability and its non-pathogenic feature. It has been proposed that fungal melanins, regardless of their precursors and biosynthetic paths, likely share similar functional groups and analogous physicochemical traits [50]. This black mushroom is found to yield up to 10% melanin in fungal dry mass [48]. _A. auricula_ is a popular edible and medicinal mushroom in eastern Asia and is attracting increasing interest from consumers in Europe as well as North America. More recently, _A. auricula_ mushroom wastes have been reported as a rich source of melanin for extraction, with an output of up to 11% of fungal dry mass under optimal conditions [51]. Although the industrial process of fungal melanin production is still in its infancy, in-depth explorations with _A. auricula_ melanin--biosynthetic steps, fermentation, extraction, and isolation--are promising since this mushroom, in terms of a melanin provider, has several advantages over plant- and animal-derived sources, including independence of seasonal variations, low-cost maintenance, expedient operation, and affordable reaction conditions.