Mechanistic Studies of Escherichia coli Adenosine-5′-phosphosulfate Kinase

Abstract

Adenosine-5′-phosphosulfate kinase (APS kinase) catalyzes the formation of 3′-phosphoadenosine 5′-phosphosulfate (PAPS), the major form of activated sulfate in biological systems. The enzyme from Escherichia coli has complex kinetic behavior, including substrate inhibition by APS and formation of a phosphorylated enzyme (E-P) as a reaction intermediate. The presence of a phosphorylated enzyme potentially enables the steady-state kinetic mechanism to change from sequential to ping-pong as the APS concentration decreases. Kinetic and equilibrium binding measurements have been used to evaluate the proposed mechanism. Equilibrium binding studies show that APS, PAPS, ADP, and the ATP analog AMPPNP each bind at a single site per subunit; thus, substrates can bind in either order. When ATPγS replaces ATP as substrate the Vmax is reduced 535-fold, the kinetic mechanism is sequential at each APS concentration, and substrate inhibition is not observed. The results indicate that substrate inhibition arises from a kinetic phenomenon in which product formation from ATP binding to the E · APS complex is much slower than paths in which product formation results from APS binding either to the E · ATP complex or to E-P. APS kinase requires divalent cations such as Mg2+ or Mn2+ for activity. APS kinase binds one Mn2+ ion per subunit in the absence of substrates, consistent with the requirement for a divalent cation in the phosphorylation of APS by E-P. The affinity for Mn2+ increases 23-fold when the enzyme is phosphorylated. Two Mn2+ ions bind per subunit when both APS and the ATP analog AMPPNP are present, indicating a potential dual metal ion catalytic mechanism.