| Popis: |
ATP sulfurylase from Penicillium chrysogenum is a homohexamer that contains three free sulfhydryl groups/subunit, only one of which (designated SH-1) can be modified by disulfide, maleimide, and halide reagents under nondenaturing conditions. Modification of SH-1 has only a small effect on kcat but causes the [S]0.5 values for MgATP and SO4(2-) (or MoO4(2-) to increase by an order of magnitude. Additionally, the velocity curves become sigmoidal with a Hill coefficient (nH) of about 2 (Renosto, F., Martin, R. L., and Segel, I. H. (1987) J. Biol. Chem. 262, 16279-16288). Direct equilibrium binding measurements confirmed that [32P]MgATP binds to the SH-modified enzyme in a positively cooperative fashion (nH = 2.0) if a sulfate subsite ligand (e.g. FSO3-) is also present. [35S]Adenosine 5'-phosphosulfate (APS) binding to the SH-modified enzyme displayed positive cooperativity (nH = 1.9) in the absence of a PPi subsite ligand. The results indicate that positive cooperativity requires occupancy of the adenylyl and sulfate (but not the pyrophosphate) subsites. [35S]APS binding to the native enzyme displayed negative cooperativity (or binding to at least two classes of sites). Isotope trapping profiles for the single turnover of [35S]APS: (a) confirmed the equilibrium binding curves, (b) indicated that all six sites/hexamer are catalytically active, and (c) showed that APS does not dissociate at a significant rate from E.APS.PPi. The MgPPi concentration dependence of [35S]APS trapping was indicative of MgPPi binding to two classes of sites on both the native and SH-modified enzyme. Inactivation of the native or SH-modified enzyme by phenylglyoxal in the presence of saturating APS was biphasic. The semilog plots suggested that only half of the sites were highly protected. The cumulative data suggest a model in which pairs of sites or subunits can exist in three different states designated HH (both sites have a high APS affinity, as in the native free enzyme), LL (both sites have a low APS affinity as in the SH-modified enzyme), and LH (as in the APS-occupied native or SH-modified enzyme). Thus, the HH----LH transition displays negative cooperativity for APS binding while the LL----LH transition displays positive cooperativity. The relative reactivities of like-paired SH-reactive reagents were in the order: N-phenylmaleimide greater than N-ethylmaleimide; dithionitropyridine greater than dithionitrobenzoate; thiolyte-MQ greater than thiolyte-MB. The log kmod versus pH curve indicates that the pKa of SH-1 is greater than 9.(ABSTRACT TRUNCATED AT 400 WORDS) |
