Crystal structure of a calcium(II)-pyrroloquinoline quinone (PQQ) complex outside a protein environment.

Pyrroloquinoline quinone (PQQ) is an important cofactor of calcium- and lanthanide-dependent alcohol dehydrogenases, and has been known for over 30 years. Crystal structures of Ca-MDH enzymes (MDH is methanol dehydrogenase) have been known for some time; however, crystal structures of PQQ with biorelevant metal ions have been lacking in the literature for decades. We report here the first crystal structure analysis of a Ca-PQQ complex outside the protein environment, namely, poly[[undecaaquabis(μ-4,5-dioxo-4,5-dihydro-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylato)tricalcium(II)] dihydrate], {[Ca(CHNO)(HO)]·2HO}. The complex crystallized as CaPQQ·13HO with Ca in three different positions and PQQ, including an extensive hydrogen-bond network. Similarities and differences to the recently reported structure with biorelevant europium (EuPQQ) are discussed.