Diverse and novel lanthanide-binding PQQ-dependent enzymes

First described pyrroloquinoline quinone-dependent (PQQ) eight-bladed {beta}-propeller proteins are calcium (Ca)-dependent, but many homologous bacterial enzymes are lanthanide (Ln)-dependent. Discovery of Ca-dependent six-bladed {beta}-propeller PQQ-dependent dehydrogenases motivated the search for Ln-dependent six-bladed {beta}-propeller PQQ-dependent enzymes in bacteria. Using in silico structural prediction of sequences from weathered rock, we identified [~]22,000 PQQ six-bladed {beta}-propeller proteins in bacteria from 77 phyla, of which 63% of sequences have the active site residues needed to bind Ln. PQQ and La binding was biochemically confirmed for enzymes from uncultivated Chloroflexi and Acidobacteria. In structural models, Ln-binding periplasmic proteins interact with TonB-dependent transporters that may enable Ln uptake. Most genomes also encode predicted Ln-and PQQ-dependent eight-bladed dehydrogenases that clade with diverse alcohol and sugar dehydrogenases. Thus, Ln-dependent six and eight-bladed PQQ-dependent {beta}-propeller proteins are implicated in diverse carbon substrate metabolisms in weathering rock and soil. We predict that many PQQ-dependent microbial enzymes are lanthanide dependent.