Pyrroloquinoline quinone (PQQ) is an ortho-quinone cofactor of several prokaryotic oxidases. Widely available in the diet and necessary for the correct growth of mice, PQQ has been suspected to be a vitamin for eukaryotes. However, no PQQ-dependent eukaryotic enzyme had been identified to use the PQQ until 2014, when a basidiomycete enzyme catalyzing saccharide dehydrogenation using PQQ as a cofactor was characterized and served to define auxiliary activity family 12 (AA12). Here we report the biochemical characterization of the AA12 enzyme encoded by the genome of the ascomycete Trichoderma reesei (TrAA12). Surprisingly, only weak activity against uncommon carbohydrates likeL-fucose or D-arabinose was measured. The three-dimensional structure of TrAA12 reveals important similarities with bacterial soluble glucose dehydrogenases (sGDH). The enzymatic characterization and the structure solved in the presence of calcium confirm the importance of this ion in catalysis, as observed for sGDH. The structural characterization of TrAA12 was completed by modeling PQQ and L-fucose in the enzyme active site. Based on these results, the AA12 family of enzymes is likely to have a catalytic mechanism close to that of bacterial sGDH.