The trifunctional sulfate-activating complex (SAC) of Mycobacterium tuberculosis

The sulfate activation pathway is essential for the assimilation of sulfate and, in many bacteria, is comprised of three reactions: the synthesis of adenosine 5′-phosphosulfate (APS), the hydrolysis of GTP, and the 3′-phosphorylation of APS to produce 3′-phosphoadenosine 5′-phosphosulfate (PAPS), whose sulfuryl group is reduced or transferred to other metabolites. The entire sulfate activation pathway is organized into a single complex in Mycobacterium tuberculosis. Although present in many bacteria, these tripartite complexes have not been studied in detail. Initial rate characterization of the mycobacterial system reveals that it is poised for extremely efficient throughput: at saturating ATP, PAPS synthesis is 5800 times more efficient than APS synthesis. The APS kinase domain of the complex does not appear to form the covalent E-P intermediate observed in the closely related APS kinase from Escherichia coli. The stoichiometry of GTP hydrolysis and APS synthesis is 1:1, and the APS synthesis reaction is driven 1.1 × 10 ⁶-fold further during GTP hydrolysis; the system harnesses the full chemical potential of the hydrolysis reaction to the synthesis of APS. A key energy-coupling step in the mechanism is a ligand-induced isomerization that enhances the affinity of GTP and commits APS synthesis and GTP hydrolysis to the completion of the catalytic cycle. Ligand-induced increases in guanine nucleotide affinity observed in the mycobacterial system suggest that it too undergoes the energy-coupling isomerization.