7-Carboxy-7-deazaguanine synthase (QueE) is a radical S-adenosylmethionine (SAM) enzyme that catalyzes the conversion of 6-carboxy-5,6,7,8-tetrahydropterin (CPH 4 ) to 7-carboxy-7-deazaguanine (CDG). QueE also shows a clear dependence on Mg 2+ ion and is considered a new feature for a radical SAM enzyme. The catalytic mechanism of QueE from B. multivorans has been studied using a combined quantum mechanics and molecular mechanics (QM/MM) method. The results of our calculations reveal that the key ring-contraction step involves a bridged intermediate rather than a ring-opening one. For the QueE−Mg 2+ system, the elimination of ammonia is calculated to be rate limiting with a free energy barrier of 18.8 kcal/mol, which is basically in accordance with the estimated value (20.9 kcal/mol) from the experiment. For QueE−Na + complex, the rate-limiting step switches to the formation of the bridged intermediate with an energy barrier of 29.3 kcal/mol. Natural population analysis indicates that the metal ions do not act as Lewis acids; therefore, they mainly play a role in fixing the substrate in its reactive conformation. The different coordination of Mg 2+ and Na + with the substrate is suggested to be the main reason for leading to the different activities of QueE−Mg 2+ and QueE−Na + complexes.