Ring cyclization and eight-electron oxidation of 3a-(2-amino-2-carboxyethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydroquinoline-7,9-dicarboxylic-acid to PQQ. 6-(2-amino-2-carboxyethyl)-7,8-dioxo-1,2,3,4,7,8-hexahydroquinoline-2,4-dicarboxylate + 3 O2 = H(+) + 2 H2O + 2 H2O2 + pyrroloquinoline quinone Cofactor biosynthesis; pyrroloquinoline quinone biosynthesis. Belongs to the PqqC family.