Isomerization of pyrogallol to phloroglucin. 1,2,3,5-tetrahydroxybenzene + 1,2,3-trihydroxybenzene = 1,2,3,5-tetrahydroxybenzene + 1,3,5-trihydroxybenzene Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Heterodimer of a large and a small subunit. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.