ATP sulfurylase, isolated from Escherichia coli K-12, is a GTPase - target complex that catalyzes and links the energetics of GTP hydrolysis to the synthesis of activated sulfate (APS). When the GTP concentration is saturating and held fixed with a regenerating system, the APS reaction reaches a steady state in which its mass ratio is shifted (5.4 x 106)-fold toward the product by the hydrolysis of GTP. If GTP is not regenerated, the shift toward the product is transient, producing a pulse-shaped progress curve. The mechanistic basis of this transience is the subject of this paper. The product transient is caused by the binding of GDP to the enzyme which establishes a catalytic pathway that allows the chemical potential that had been transferred to the APS reaction to 'leak' into the chemical milieu. The system leaks because the E·GDP complex catalyzes the uncoupled APS reaction. The addition of phosphate to the leaky GDP·E·APS·PP(i) complex converts it into the central P(i)·GDP·E·APS·PP(i) complex which catalyzes the energy- transfer reaction. Thus, P(i) binding directs the system through the coupled mechanism, 'plugging' the leak. GMPPNP, which also causes a leak, is used to demonstrate that the mass ratio of the APS reaction can be 'tuned' by adjusting flux through the coupled and uncoupled pathways. This energy- coupling mechanism provides a means for controlling the quantity of chemical potential transferred to the APS reaction. This versatile linkage might well be used to the cell's advantage to avoid the toxicity associated with an excess of activated sulfate.
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