PQQ, or pyrroloquinoline-quinone, serves as a cofactor for a number of sugar and alcohol dehydrogenases in a limited number of bacterial species. Most characterized PQQ-dependent enzymes have multiple repeats of a sequence region described by pfam01011 (PQQ enzyme repeat), but this protein family in unusual in lacking that repeat. Below the noise cutoff are related proteins mostly from species that lack PQQ biosynthesis.
Jiyao W et al.(2023). "The conserved domain database in 2023.",