Adenosine 5'-phosphosulfate kinase from Penicillium chrysogenum. Determining ligand dissociation constants of binary and ternary complexes from the kinetics of enzyme inactivation.

Adenosine 5-phosphosulfate (APS) kinase from Penicillium chrysogenum is irreversibly inactivated by trinitrobenzene sulfonate in a pseudo-first order process. Under standard assay conditions kapp was 1.9 X 10(-3) s-1. Saturating MgATP or MgADP decreased Kapp to a limit of 4.1 X 10(-4) s-1. There are several explanations for the partial protection, including the presence of two essential lysyl side chains, only one of which is at the active site. Analysis of the inactivation kinetics by means of linear plots derived for partial protection yielded dissociation constants for E X MgATP (Kia) and E X MgADP (Kiq) of 2.9 mM and 1.8 mM, respectively. Low concentrations of APS alone provided no protection against trinitrobenzene sulfonate inactivation, but in the presence of 1 mM MgADP, as little as 2 microM APS provided additional protection while 100 microM APS reduced kapp to the limit of 4.1 X 10(-4) s-1. The results confirm the formation of a dead end E X MgADP X APS proposed earlier as the cause of the potent substrate inhibition by APS. Linear plots of 1/delta k versus 1/[MgADP] at different fixed [APS] and of 1/delta k versus 1/[APS] at different fixed [MgADP] were characteristic of the ordered binding of MgADP before APS (or the highly synergistic random binding of the two ligands). The true APS dissociation constant of the dead end E X MgADP X APS complex (K'ib) was determined to be 1.9 microM. From the value of K'ib and the previously reported value of KIB (apparent inhibition constant of APS as a substrate inhibitor of the catalytic reaction at saturating MgATP), the ratio of the MgADP and PAPS release rate constants (k4/k3) was calculated to be 11. Inactivation kinetics was used to study the effects of Mg2+ and high salt on ADP and APS binding. The results indicated that free ADP binds to the enzyme more tightly than does MgADP at low ionic strength. High salt decreased free ADP binding, but had little effect on MgADP binding. APS binds more tightly to E X MgADP in the absence or presence of salt than to E X ADP.