Properties and Function of Pyomelanin

Melanin pigments are the most common pigments produced in nature and these complex biopolymers are found in species of all biological kingdoms. There are several categories of melanins which include eumelanins, pheomelanins and allomelanins. Eumelanins and pheomelanins are produced from oxidation of tyrosine or phenylalanine to odihydroxyphenylalanine (DOPA) and dopaquinone. Pheomelanin results from cysteinylation of DOPA. Allomelanins include a heterogeneous group of polymers that include pyomelanin. Melanin biochemistry and synthesis has been reviewed previously (Plonka and Grabacka 2006). This chapter will focus on the properties and function of pyomelanin and their potential utility in biotechnology. Pyomelanin originates from the catabolism of tyrosine or phenylalanine (Lehninger, 1975) (Fig. 1). Complete breakdown of tyrosine to acetoacetate and fumarate requires the enzymes 4-hydroxyphenylpyruvic acid dioxygenase (4-HPPD) and homogentisic acid oxidase (HGA-oxidase). In the absence of HGA-oxidase, or if homogentisic acid (HGA) production exceeds that of HGA-oxidase activity, HGA is over-produced and excreted from the cell (Yabuuchi and Ohyama 1972; Ruzafa et al. 1994; Katob et al. 1995). Autooxidation and selfpolymerization of HGA then results in pyomelanin. In addition, deletion of the gene that encodes for HGA-oxidase results in hyper production of pyomelanin while deletion of the gene that encodes for 4HPPD results in the inability to produce pyomelanin (Coon et al. 1994; Ruzafa et al. 1995). In humans with loss-of-function mutations in HGA-oxidase, pyomelanin (also known as alkapton or ochronotic pigment) forms in the urine due to the spontaneous auto oxidation of excess HGA (Beltrán-Valero de Bernabé, et al. 1999). This condition is known as alkaptonuria in humans and can result in arthritis in adults. Pyomelanin production in microorganisms often is associated with numerous survival advantages and was first characterized in bacteria among numerous species of the genus Pseudomonas (Yabuuchi & Ohyama 1972). Since then several fungi and a number of bacteria, especially in the ┛ Proteobacteria have been shown to produce pyomelanin.