A Broad-Spectrum Copper Oxidase Synthesizes Fungal Peptidomelanin

Peptidomelanin is a fungal biopolymer secreted during
the germination
of Aspergillus niger melanoliber spores. Peptidomelanin
is composed of an insoluble core polymer derived from L-DOPA, surrounded
and solubilized by short, heterogeneous peptide chains. In this work,
we report the identification and biochemical characterization of BroSCO
(gene ID: ABHI18_005222), a broad spectrum copper oxidase responsible
for synthesizing peptidomelanin. Our model of BroSCO possesses a type
3 dicopper center housed in a large, solvent accessible catalytic
center that can accommodate substrates possessing a range of sizes.
BroSCO polymerizes L-DOPA into melanin along the Raper-Mason pathway.
BroSCO oxidizes thiol groups from cysteine and cysteinylated peptides,
allowing them to copolymerize with L-DOPA, forming peptidomelanin
in the latter case. Our identification of BroSCO as the enzyme responsible
for synthesizing peptidomelanin lead to the characterization of a
previously unknown biochemical pathway, expanding our understanding
of fungal biochemistry and melanogenesis.