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Substitution of a conserved catalytic dyad into 2‐KPCC causes loss of carboxylation activity

The characteristic His–Glu catalytic dyad of the disulfide oxidoreductase (DSOR) family of enzymes is replaced in 2‐ketopropyl coenzyme M oxidoreductase/carboxylase (2‐KPCC) by the residues Phe–His. 2‐KPCC is the only known carboxylating member of the DSOR family and has replaced this dyad potentially to eliminate proton‐donating groups at a key position in the active site. Substitution of the Phe–His by the canonical residues results in production of higher relative concentrations of acetone versus the natural product acetoacetate. The results indicate that these differences in 2‐KPCC are key in discriminating between carbon dioxide and protons as attacking electrophiles.