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The anaerobic acetogenic bacteriumAcetobacterium woodii couples reduction of caffeate with electrons derived from molecular hydrogen to the synthesis of ATP by a chemiosmotic mechanism with sodium ions as coupling ions. Caffeate is activated to caffeyl coenzyme A (caffeyl-CoA) prior to its reduction, and the caffeate reduction operon encodes an ATP-dependent caffeyl-CoA synthetase that is thought to catalyze the initial caffeate activation. The operon also encodes a potential CoA transferase, the product ofcarA , which was thought to be involved in subsequent ATP-independent caffeate activation. To prove the proposed function ofcarA , we overproduced its protein inEscherichia coli and then purified it. Purified CarA drives the formation of caffeyl-CoA from caffeate with hydrocaffeyl-CoA as the CoA donor. The dependence of the reaction on caffeate and hydrocaffeyl-CoA followed Michaelis-Menten kinetics, with apparentKm values of 75 ï¿½ 5 μM for caffeate and 8 ï¿½ 2 μM for hydrocaffeyl-CoA. The enzyme activity had broad ranges of pH and temperature optima. In addition to being able to use caffeate, CarA could usep -coumarate and ferulate but not cinnamate, sinapate, orp -hydroxybenzoate as a CoA acceptor. Neither acetyl-CoA nor butyryl-CoA served as the CoA donor for CarA. The enzyme uses a ping-pong mechanism for CoA transfer and is the first classified member of a new subclass of family I CoA transferases that has two catalytic domains on one polypeptide chain. Apparently, CarA catalyzes an energy-saving CoA loop for caffeate activation in the steady state of caffeate respiration.

Caffeate Respiration in the Acetogenic Bacterium Acetobacterium woodii: a Coenzyme A Loop Saves Energy for Caffeate Activation