Novel Dual-Coenzyme Specificity and Thermostability of Malate Dehydrogenase Identified in the Cyanobacterium Microcystis aeruginosa PCC7806

Malate dehydrogenase (MDH) is a key energy metabolic enzyme with distinct coenzyme specificity for either NAD⁺ or NADP⁺ in all domains of life. Here, we characterize a novel MDH from the bloom-forming cyanobacterium <i>Microcystis aeruginosa</i> PCC7806 (MaMDH), which displays dual-coenzyme specificity with comparable efficiency for both NAD⁺ and NADP⁺, albeit with a slight preference for NAD⁺. MaMDH exists as a 72.1 kDa homodimer with a subunit mass of 36.2 kDa in solution. Kinetic measurements yielded <i>K</i>_m values of 33.140 μM for NAD⁺ and 113.200 μM for NADP⁺, with a <i>k</i>_cat ratio (NAD⁺/NADP⁺) of 3.64. The enzyme exhibited optimal activity at pH 8.0 and 40 °C, along with notable thermostability, retaining over 90% activity after incubation at 70 °C for 20 min. Through structure-guided mutagenesis of the predicted coenzyme-binding motif, we shifted MaMDH cofactor preference from NAD⁺ toward NADP⁺, supporting the hypothesis that dual-specificity MDHs may represent evolutionary intermediates in the emergence of NADP⁺-dependent chloroplast MDHs. This study provides new insights into the molecular evolution mechanisms of coenzyme specificity within the MDH family.