Within the accompanying paper in this issue (Reger et al. (2008) Biochemistry, 47, 8016-8025) we reported the X-ray structure of 4-chlorobenzoate:CoA ligase (CBL) bound with 4-chlorobenzoyl-adenylate (4-CB-AMP) and the X-ray structure of CBL bound with 4-chlorophenacyl-CoA (4-CP-CoA) (an inert analogue of the product 4-chlorobenzoyl-coenzyme A (4-CB-CoA)) and AMP. These structures defined two CBL conformational states. In conformation 1, CBL is poised to catalyze the adenylation of 4-chlorobenzoate (4-CB) with ATP (partial reaction 1), and in conformation 2, CBL is poised to catalyze the formation of 4-CB-CoA from 4-CB-AMP and CoA (partial reaction 2). These two structures showed that, by switching from conformation 1 to conformation 2, the cap domain rotates about the domain linker and thereby changes its interface with the N-terminal domain. The present work was carried out to determine the contributions made by each of the active site residues in substrate/cofactor binding and catalysis, and also to test the role of domain alternation in catalysis. In this paper, we report the results of steady-state kinetic and transient state kinetic analysis of wild-type CBL and of a series of site-directed CBL active site mutants. The major findings are as follows. First, wild-type CBL is activated by Mg (2+) (a 12-75-fold increase in activity is observed depending on assay conditions) and its kinetic mechanism (ping-pong) supports the structure-derived prediction that PP i dissociation must precede the switch from conformation 1 to conformation 2 and therefore CoA binding. Also, transient kinetic analysis of wild-type CBL identified the rate-limiting step of the catalyzed reaction as one that follows the formation of 4-CB-CoA (viz. CBL conformational change and/or product dissociation). The single turnover rate of 4-CB and ATP to form 4-CB-AMP and PP i ( k = 300 s (-1)) is not affected by the presence of CoA, and it is approximately 3-fold faster than the turnover rate of 4-CB-AMP and CoA to form 4-CB-CoA and AMP ( k = 120 s (-1)). Second, the active site mutants screened via steady-state kinetic analysis were ranked based on the degree of reduction observed in any one of the substrate k cat/ K m values, and those scoring higher than a 50-fold reduction in k cat/ K m value were selected for further evaluation via transient state kinetic analysis. The single-turnover time courses, measured for the first partial reaction, and then for the full reaction, were analyzed to define the microscopic rate constants for the adenylation reaction and the thioesterification reaction. On the basis of our findings we propose a catalytic mechanism that centers on a small group of key residues (some of which serve in more than one role) and that includes several residues that function in domain alternation.