Crystal structure of the bifunctional ATP sulfurylase-APS kinase from the chemolithotrophic thermophile Aquifex aeolicus

The thermophilic chemolithotroph, Aquifex aeolicus, expresses a gene product that exhibits both ATP sulfurylase and adenosine-5'-phosphosulfate (APS) kinase activities. These enzymes are usually segregated on two separate proteins in most bacteria, fungi, and plants. The domain arrangement in the Aquifex enzyme is reminiscent of the fungal ATP sulfurylase, which contains a C-terminal domain that is homologous to APS kinase yet displays no kinase activity. Rather, in the fungal enzyme, the motif serves as a sulfurylase regulatory domain that binds the allosteric effector 3'-phosphoadenosine-5'-phosphosulfate (PAPS), the product of true APS kinase. Therefore, the Aquifex enzyme may represent an ancestral homolog of a primitive bifunctional enzyme, from which the fungal ATP sulfurylase may have evolved. In heterotrophic sulfur-assimilating organisms such as fungi, ATP sulfurylase catalyzes the first committed step in sulfate assimilation to produce APS, which is subsequently metabolized to generate all sulfur-containing biomolecules. In contrast, ATP sulfurylase in sulfur chemolithotrophs catalyzes the reverse reaction to produce ATP and sulfate from APS and pyrophosphate. Here, the 2.3 A resolution X-ray crystal structure of Aquifex ATP sulfurylase-APS kinase bifunctional enzyme is presented. The protein dimerizes through its APS kinase domain and contains ADP bound in all four active sites. Comparison of the Aquifex ATP sulfurylase active site with those from sulfate assimilators reveals similar dispositions of the bound nucleotide and nearby residues. This suggests that minor perturbations are responsible for optimizing the kinetic properties for the physiologically relevant direction. The APS kinase active-site lid adopts two distinct conformations, where one conformation is distorted by crystal contacts. Additionally, a disulfide bond is observed in one ATP-binding P-loop of the APS kinase active site. This linkage accounts for the low kinase activity of the enzyme under oxidizing conditions. The thermal stability of the Aquifex enzyme can be explained by the 43% decreased cavity volume found within the protein core.     

Kinetic and stability properties of Penicillium chrysogenum ATP sulfurylase missing the C-terminal regulatory domain

ATP sulfurylase from Penicillium chrysogenum is a homohexameric enzyme that is subject to allosteric inhibition by 3'-phosphoadenosine 5'-phosphosulfate. In contrast to the wild type enzyme, recombinant ATP sulfurylase lacking the C-terminal allosteric domain was monomeric and noncooperative. All kcat values were decreased (the adenosine 5'-phosphosulfate (adenylylsulfate) (APS) synthesis reaction to 17% of the wild type value). Additionally, the Michaelis constants for MgATP and sulfate (or molybdate), the dissociation constant of E.APS, and the monovalent oxyanion dissociation constants of dead end E.MgATP.oxyanion complexes were all increased. APS release (the k6 step) was rate-limiting in the wild type enzyme. Without the C-terminal domain, the composite k5 step (isomerization of the central complex and MgPPi release) became rate-limiting. The cumulative results indicate that besides (a) serving as a receptor for the allosteric inhibitor, the C-terminal domain (b) stabilizes the hexameric structure and indirectly, individual subunits. Additionally, (c) the domain interacts with and perfects the catalytic site such that one or more steps following the formation of the binary E.MgATP and E.SO4(2-) complexes and preceding the release of MgPPi are optimized. The more negative entropy of activation of the truncated enzyme for APS synthesis is consistent with a role of the C-terminal domain in promoting the effective orientation of MgATP and sulfate at the active site.     

Structural and functional analysis of a truncated form of Saccharomyces cerevisiae ATP sulfurylase: C-terminal domain essential for oligomer formation but not for activity

ATP sulfurylase catalyzes the first step in the activation of sulfate by transferring the adenylyl-moiety (AMP approximately ) of ATP to sulfate to form adenosine 5'-phosphosulfate (APS) and pyrophosphate (PP(i)). Subsequently, APS kinase mediates transfer of the gamma-phosphoryl group of ATP to APS to form 3'-phosphoadenosine 5'-phosphosulfate (PAPS) and ADP. The recently determined crystal structure of yeast ATP sulfurylase suggests that its C-terminal domain is structurally quite independent from the other domains, and not essential for catalytic activity. It seems, however, to dictate the oligomerization state of the protein. Here we show that truncation of this domain results in a monomeric enzyme with slightly enhanced catalytic efficiency. Structural alignment of the C-terminal domain indicated that it is extremely similar in its fold to APS kinase although not catalytically competent. While carrying out these structural and functional studies a surface groove was noted. Careful inspection and modeling revealed that the groove is sufficiently deep and wide, as well as properly positioned, to act as a substrate channel between the ATP sulfurylase and APS kinase-like domains of the enzyme.     

Crystal structure of ATP sulfurylase from the bacterial symbiont of the hydrothermal vent tubeworm Riftia pachyptila.

In sulfur chemolithotrophic bacteria, the enzyme ATP sulfurylase functions to produce ATP and inorganic sulfate from APS and inorganic pyrophosphate, which is the final step in the biological oxidation of hydrogen sulfide to sulfate. The giant tubeworm, Riftia pachyptila, which lives near hydrothermal vents on the ocean floor, harbors a sulfur chemolithotroph as an endosymbiont in its trophosome tissue. This yet-to-be-named bacterium was found to contain high levels of ATP sulfurylase that may provide a substantial fraction of the organisms ATP. We present here, the crystal structure of ATP sulfurylase from this bacterium at 1.7 A resolution. As predicted from sequence homology, the enzyme folds into distinct N-terminal and catalytic domains, but lacks the APS kinase-like C-terminal domain that is present in fungal ATP sulfurylase. The enzyme crystallizes as a dimer with one subunit in the crystallographic asymmetric unit. Many buried solvent molecules mediate subunit contacts at the interface. Despite the high concentration of sulfate needed for crystallization, no ordered sulfate was observed in the sulfate-binding pocket. The structure reveals a mobile loop positioned over the active site. This loop is in a "closed" or "down" position in the reported crystal structures of fungal ATP sulfurylases, which contained bound substrates, but it is in an "open" or "up" position in the ligand-free Riftia symbiont enzyme. Thus, closure of the loop correlates with occupancy of the active site, although the loop itself does not interact directly with bound ligands. Rather, it appears to assist in the orientation of residues that do interact with active-site ligands. Amino acid differences between the mobile loops of the enzymes from sulfate assimilators and sulfur chemolithotrophs may account for the significant kinetic differences between the two classes of ATP sulfurylase.     

ATP sulfurylase from the hyperthermophilic chemolithotroph Aquifex aeolicus

ATP sulfurylase from the hyperthermophilic chemolithotroph Aquifex aeolicus is a bacterial ortholog of the enzyme from filamentous fungi. (The subunit contains an adenosine 5'-phosphosulfate (APS) kinase-like, C-terminal domain.) The enzyme is highly heat stable with a half-life >1h at 90 degrees C. Steady-state kinetics are consistent with a random A-B, ordered P-Q mechanism where A=MgATP, B=SO4(2-), P=PP(i), and Q=APS. The kinetic constants suggest that the enzyme is optimized to act in the direction of ATP+sulfate formation. Chlorate is competitive with sulfate and with APS. In sulfur chemolithotrophs, ATP sulfurylase provides an efficient route for recycling PP(i) produced by biosynthetic reactions. However, the protein possesses low APS kinase activity. Consequently, it may also function to produce PAPS for sulfate ester formation or sulfate assimilation when hydrogen serves as the energy source and a reduced inorganic sulfur source is unavailable.     

Temperature effects on the allosteric transition of ATP sulfurylase from Penicillium chrysogenum

The effects of temperature on the initial velocity kinetics of allosteric ATP sulfurylase from Penicillium chrysogenum were measured. The experiments were prompted by the structural similarity between the C-terminal regulatory domain of fungal ATP sulfurylase and fungal APS kinase, a homodimer that undergoes a temperature-dependent, reversible dissociation of subunits over a narrow temperature range. Wild-type ATP sulfurylase yielded hyperbolic velocity curves between 18 and 30 degrees C. Increasing the assay temperature above 30 degrees C at a constant pH of 8.0 increased the cooperativity of the velocity curves. Hill coefficients (n(H)) up to 1.8 were observed at 42 degrees C. The bireactant kinetics at 42 degrees C were the same as those observed at 30 degrees C in the presence of PAPS, the allosteric inhibitor. In contrast, yeast ATP sulfurylase yielded hyperbolic plots at 42 degrees C. The P. chrysogenum mutant enzyme, C509S, which is intrinsically cooperative (n(H) = 1.8) at 30 degrees C, became more cooperative as the temperature was increased yielding n(H) values up to 2.9 at 42 degrees C. As the temperature was decreased, the cooperativity of C509S decreased; n(H) was 1.0 at 18 degrees C. The cumulative results indicate that increasing the temperature increases the allosteric constant, L, i.e., promotes a shift in the base-level distribution of enzyme molecules from the high MgATP affinity R state toward the low MgATP affinity T state. As a result, the enzyme displays a true "temperature optimum" at subsaturating MgATP. The reversible temperature-dependent transitions of fungal ATP sulfurylase and APS kinase may play a role in energy conservation at high temperatures where the organism can survive but not grow optimally.     

Allosteric inhibition via R-state destabilization in ATP sulfurylase from Penicillium chrysogenum

The structure of the cooperative hexameric enzyme ATP sulfurylase from Penicillium chrysogenum bound to its allosteric inhibitor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS), was determined to 2.6 A resolution. This structure represents the low substrate-affinity T-state conformation of the enzyme. Comparison with the high substrate-affinity R-state structure reveals that a large rotational rearrangement of domains occurs as a result of the R-to-T transition. The rearrangement is accompanied by the 17 A movement of a 10-residue loop out of the active site region, resulting in an open, product release-like structure of the catalytic domain. Binding of PAPS is proposed to induce the allosteric transition by destabilizing an R-state-specific salt linkage between Asp 111 in an N-terminal domain of one subunit and Arg 515 in the allosteric domain of a trans-triad subunit. Disrupting this salt linkage by site-directed mutagenesis induces cooperative inhibition behavior in the absence of an allosteric effector, confirming the role of these two residues.     

Induction of positive cooperativity by amino acid replacements within the C-terminal domain of Penicillium chrysogenum ATP sulfurylase

ATP sulfurylase from Penicillium chrysogenum is an allosteric enzyme in which Cys-509 is critical for maintaining the R state. Cys-509 is located in a C-terminal domain that is 42% identical to the conserved core of adenosine 5'-phosphosulfate (adenylylsulfate) (APS) kinase. This domain is believed to provide the binding site for the allosteric effector, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Replacement of Cys-509 with either Tyr or Ser destabilizes the R state, resulting in an enzyme that is intrinsically cooperative at pH 8 in the absence of PAPS. The kinetics of C509Y resemble those of the wild type enzyme in which Cys-509 has been covalently modified. The kinetics of C509S resemble those of the wild type enzyme in the presence of PAPS. It is likely that the negative charge on the Cys-509 side chain helps to stabilize the R state. Treatment of the enzyme with a low level of trypsin results in cleavage at Lys-527, a residue that lies in a region analogous to a PAPS motif-containing mobile loop of true APS kinase. Both mutant enzymes were cleaved more rapidly than the wild type enzyme, suggesting that movement of the mobile loop occurs during the R to T transition.     

The complex structures of ATP sulfurylase with thiosulfate, ADP and chlorate reveal new insights in inhibitory effects and the catalytic cycle.

The ubiquitous enzyme ATP sulfurylase (ATPS) catalyzes the primary step of intracellular sulfate activation, the formation of adenosine 5'-phosphosulfate (APS). It has been shown that the enzyme catalyzes the generation of APS from ATP and inorganic sulfate in vitro and in vivo, and that this reaction can be inhibited by a number of simple molecules. Here, we present the crystal structures of ATPS from the yeast Saccharomyces cerevisiae complexed with compounds that have inhibitory effects on the catalytic reaction of ATPS. Thiosulfate and ADP mimic the substrates sulfate and ATP in the active site, but are non-reactive and thus competitive inhibitors of the sulfurylase reaction. Chlorate is bound in a crevice between the active site and the intermediate domain III of the complex structure. It forms hydrogen bonds to residues of both domains and stabilizes a "closed" conformation, inhibiting the release of the reaction products APS and PPi. These new observations are evidence for the crucial role of the displacement mechanism for the catalysis by ATPS.     

Human 3'-phosphoadenosine 5'-phosphosulfate synthetase (isoform 1, brain): kinetic properties of the adenosine triphosphate sulfurylase and adenosine 5'-phosphosulfate kinase domains

Recombinant human 3'-phosphoadenosine 5'-phosphosulfate (PAPS) synthetase, isoform 1 (brain), was purified to near-homogeneity from an Escherichia coli expression system and kinetically characterized. The native enzyme, a dimer with each 71 kDa subunit containing an adenosine triphosphate (ATP) sulfurylase and an adenosine 5'-phosphosulfate (APS) kinase domain, catalyzes the overall formation of PAPS from ATP and inorganic sulfate. The protein is active as isolated, but activity is enhanced by treatment with dithiothreitol. APS kinase activity displayed the characteristic substrate inhibition by APS (K(I) of 47.9 microM at saturating MgATP). The maximum attainable activity of 0.12 micromol min(-1) (mg of protein)(-1) was observed at an APS concentration ([APS](opt)) of 15 microM. The theoretical K(m) for APS (at saturating MgATP) and the K(m) for MgATP (at [APS](opt)) were 4.2 microM and 0.14 mM, respectively. At likely cellular levels of MgATP (2.5 mM) and sulfate (0.4 mM), the overall endogenous rate of PAPS formation under optimum assay conditions was 0.09 micromol min(-1) (mg of protein)(-1). Upon addition of pure Penicillium chrysogenum APS kinase in excess, the overall rate increased to 0.47 micromol min(-1) (mg of protein)(-1). The kinetic constants of the ATP sulfurylase domain were as follows: V(max,f) = 0.77 micromol min(-1) (mg of protein)(-1), K(mA(MgATP)) = 0.15 mM, K(ia(MgATP)) = 1 mM, K(mB(sulfate)) = 0.16 mM, V(max,r) = 18.7 micromol min(-1) (mg of protein)(-1), K(mQ(APS)) = 4.8 microM, K(iq(APS)) = 18 nM, and K(mP(PPi)) = 34.6 microM. The (a) imbalance between ATP sulfurylase and APS kinase activities, (b) accumulation of APS in solution during the overall reaction, (c) rate acceleration provided by exogenous APS kinase, and (d) availability of both active sites to exogenous APS all argue against APS channeling. Molybdate, selenate, chromate ("chromium VI"), arsenate, tungstate, chlorate, and perchlorate bind to the ATP sulfurylase domain, with the first five serving as alternative substrates that promote the decomposition of ATP to AMP and PP(i). Selenate, chromate, and arsenate produce transient APX intermediates that are sufficiently long-lived to be captured and 3'-phosphorylated by APS kinase. (The putative PAPX products decompose to adenosine 3',5'-diphosphate and the original oxyanion.) Chlorate and perchlorate form dead-end E.MgATP.oxyanion complexes. Phenylalanine, reported to be an inhibitor of brain ATP sulfurylase, was without effect on PAPS synthetase isoform 1.     

Isomerization couples chemistry in the ATP sulfurylase-GTPase system.

ATP sulfurylase catalyzes and couples the free energies of two reactions: GTP hydrolysis and the synthesis of activated sulfate, or APS. The GTPase active site undergoes changes during its catalytic cycle that are driven by events that occur at the APS-forming active site, which is located in a separate subunit. GTP responds to its changing environment by moving along its reaction path. The response, which may change the affinity or reactivity of GTP, can, in turn, produce alterations at the APS active site that drive APS synthesis. The resulting stepwise progression of the two reactions couples their free energies. The mechanism of ATP sulfurylase involves an enzyme isomerization that precedes and rate limits cleavage of the beta,gamma-bond of GTP. These fluorescence studies demonstrate that the isomerization is controlled by the binding of activators that drive ATP sulfurylase into forms that mimic different stages of the APS reaction. Only certain activators elicit the isomerization, suggesting that the APS reaction must proceed to a specific point in the catalytic cycle before the conformational "switch" that controls GTP hydrolysis is thrown. The isomerization is shown to require occupancy of the gamma-phosphate subsite of the GTP binding pocket. This requirement establishes that the isomerization results in a change in the interaction between the enzyme and the gamma-phosphate of GTP that emerges in the catalytic cycle during the transition from the nonisomerized to the isomerized E.GTP complex. The newly formed contact(s) appears to carry into the bond-breaking transition state, and to be essential for the enhanced affinity and reactivity of the nucleotide.     

Chemical modification and site-directed mutagenesis of conserved HXXH and PP-loop motif arginines and histidines in the murine bifunctional ATP sulfurylase/adenosine 5'-phosphosulfate kinase.

The sulfurylase domain of the mouse bifunctional enzyme ATP sulfurylase/adenosine 5'-phosphosulfate (APS) kinase contains HXXH and PP-loop motifs. To elucidate the functional importance of these motifs and of conserved arginines and histidines, chemical modification and site-directed mutagenesis studies were performed. Chemical modification of arginines and histidines with phenylglyoxal and diethyl pyrocarbonate, respectively, renders the enzyme inactive in sulfurylase, kinase, and overall assays. Data base searches and sequence comparison of bifunctional ATP sulfurylase/APS kinase and monofunctional ATP sulfurylases shows a limited number of highly conserved arginines and histidines within the sulfurylase domain. Of these conserved residues, His-425, His-428, and Arg-421 are present within or near the HXXH motif whereas His-506, Arg-510, and Arg-522 residues are present in and around the PP-loop. The functional role of these conserved residues was further studied by site-directed mutagenesis. In the HXXH motif, none of the alanine mutants (H425A, H428A, and R421A) had sulfurylase or overall activity, whereas they all exhibited normal kinase activity. A slight improvement in reverse sulfurylase activity (<10% residual activity) and complete restoration of forward sulfurylase was observed with R421K. Mutants designed to probe the PP-loop requirements included H506A, R510A, R522A, R522K, and D523A. Of these, R510A exhibited normal sulfurylase and kinase activity, R522A and R522K showed no sulfurylase activity, and H506A had normal sulfurylase activity but produced an effect on kinase activity (<10% residual activity). The single aspartate, D523A, which is part of the highly conserved GRD sequence of the PP-loop, affected both sulfurylase and kinase activity. This mutational analysis indicates that the HXXH motif plays a role only in the sulfurylase activity, whereas the PP-loop is involved in both sulfurylase and kinase activities. Residues specific for sulfurylase activity have also been distinguished from those involved in kinase activity.     

Affinity labeling of the catalytic and AMP allosteric sites of 3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase by 5'-p-fluorosulfonylbenzoyladenosine

The nucleotide analogue 5'-p-fluorosulfonylbenzoyladenosine (FSBA) reacts irreversibly with rat liver cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase kinase, causing a rapid loss of the AMP activation capacity and a slower inactivation of the catalytic activity. The rate constant for loss of AMP activation is about 10 times higher (kappa 1 = 0.112 min-1) than the rate constant of inactivation (kappa 2 = 0.0106 min-1). There is a good correspondence between the time-dependent inactivation of reductase kinase and the time-dependent incorporation of 5'-p-sulfonylbenzoyl[14C]adenosine ([14C]SBA). An average of 1.65 mol of reagent/mol of enzyme subunit is bound when reductase kinase is completely inactivated. The time-dependent incorporation is consistent with the postulate that covalent reaction of 1 mol of SBA/mol of subunit causes complete loss of AMP activation, whereas reaction of another mole of SBA/mol of subunit would lead to total inactivation. Protection against inactivation by the reagent is provided by the addition of Mg2+, AMP, Mg-ATP, or Mg-AMP to the incubation mixtures. In contrast, addition of ATP, 2'-AMP, or 3'-AMP has no effect on the rate constants. Mg-ATP protects preferentially the catalytic site against inactivation, whereas Mg-AMP at low concentration protects preferentially the allosteric site. Mg-ADP affords less protection than Mg-AMP to the allosteric site when both nucleotides are present at a concentration of 50 microM with 7.5 mM Mg2+. Experiments done with [14C]FSBA in the presence of some protectants have shown that a close correlation exists between the pattern of protection observed and the binding of [14C]SBA. The postulate is that there exists a catalytic site and an allosteric site in the reductase kinase subunit and that Mg-AMP is the main allosteric activator of the enzyme.     

ATP sulfurylase from Penicillium chrysogenum: measurements of the true specific activity of an enzyme subject to potent product inhibition and a reassessment of the kinetic mechanism

Homogeneous ATP sulfurylase from Penicillium chrysogenum has been reported to have an extremely low activity toward its physiological inorganic substrate, sulfate. This low activity is an artifact resulting from potent product inhibition by 5'-adenylylsulfate (APS) (Ki less than 0.25 microM). Assays based on 35S incorporation from 35SO4(2-) into charcoal-adsorbable [35S]APS are nonlinear with time, even in the presence of a large excess of inorganic pyrophosphatase. However, in the presence of excess APS kinase (along with excess pyrophosphatase), the ATP sulfurylase reaction is linear with time and the enzyme has a specific activity (Vmax) of 6 to 7 units mg protein-1 corresponding to an active site turnover number of at least 400 min-1. Monovalent oxyanions such as NO3-, ClO3-, ClO4-, and FSO3- are competitive with sulfate (or molybdate) and essentially uncompetitive with respect to MgATP. However, thiosulfate (SSO3(2-)), a true sulfate analog and dead-end inhibitor of the enzyme (competitive with sulfate or molybdate), exhibited clear noncompetitive inhibition against MgATP. Furthermore, APS was competitive with both MgATP and molybdate in the molybdolysis assay. These results suggest (a) that the mechanism of the normal forward reaction may be random rather than ordered and (b) that the monovalent oxyanions have a much greater affinity for the E X MgATP complex than for free E. In this respect, FSO3-, ClO4-, etc., are not true sulfate analogs although they might mimic an enzyme-bound species formed when MgATP is at the active site. The nonlinear ATP sulfurylase reaction progress curves (with APS accumulating in the presence of excess pyrophosphatase or PPi accumulating in the presence of excess APS kinase) were analyzed by means of "average velocity" plots based on an integrated rate equation. This new approach is useful for enzymes subject to potent product inhibition over a reaction time course in which the substrate concentrations do not change significantly. The analysis showed that ATP sulfurylase has an intrinsic specific activity of 6 to 7 units mg protein-1. Thus, the apparent stimulation of sulfurylase activity by APS kinase results from the continual removal of inhibitory APS rather than from an association of the two sulfate-activating enzymes to form a "3'-phospho-5'-adenylylsulfate synthetase" complex in which the sulfurylase has an increased catalytic activity. The progress curve analyses suggest that APS is competitive with both MgATP and sulfate, while MgPPi is a mixed-type inhibitor with respect to both substrates. The cumulative data point to a random sequence for the forward reaction with APS release being partially rate limiting.     

Adenine nucleotides as allosteric effectors of pea seed glutamine synthetase

The effects of adenine nucleotides on pea seed glutamine synthetase (EC 6.3.1.2) activity were examined as a part of our investigation of the regulation of this octameric plant enzyme. Saturation curves for glutamine synthetase activity versus ATP with ADP as the changing fixed inhibitor were not hyperbolic; greater apparent Vmax values were observed in the presence of added ADP than the Vmax observed in the absence of ADP. Hill plots of data with ADP present curved upward and crossed the plot with no added ADP. The stoichiometry of adenine nucleotide binding to glutamine synthetase was examined. Two molecules of [gamma-32P]ATP were bound per subunit in the presence of methionine sulfoximine. These ATP molecules were bound at an allosteric site and at the active site. One molecule of either [gamma-32P]ATP or [14C]ADP bound per subunit in the absence of methionine sulfoximine; this nucleotide was bound at an allosteric site. ADP and ATP compete for binding at the allosteric site, although ADP was preferred. ADP binding to the allosteric site proceeded in two kinetic phases. A Vmax value of 1.55 units/mg was measured for glutamine synthetase with one ADP tightly bound per enzyme subunit; a Vmax value of 0.8 unit/mg was measured for enzyme with no adenine nucleotide bound at the allosteric site. The enzyme activation caused by the binding of ADP to the allosteric sites was preceded by a lag phase, the length of which was dependent on the ADP concentration. Enzyme incubated in 10 mM ADP bound approximately 4 mol of ADP/mol of native enzyme before activation was observed; the activation was complete when 7-8 mol of ADP were bound per mol of the octameric, native enzyme. The Km for ATP (2 mM) was not changed by ADP binding to the allosteric sites. ADP was a simple competitive inhibitor (Ki = 0.05 mM) of ATP for glutamine synthetase with eight molecules of ADP tightly bound to the allosteric sites of the octamer. Binding of ATP to the allosteric sites led to marked inhibition.     

Mechanistic studies of Escherichia coli adenosine-5'-phosphosulfate kinase

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 ATPgammaS replaces ATP as substrate the V(max) 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 Mg(2+) or Mn(2+) for activity. APS kinase binds one Mn(2+) 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 Mn(2+) increases 23-fold when the enzyme is phosphorylated. Two Mn(2+) ions bind per subunit when both APS and the ATP analog AMPPNP are present, indicating a potential dual metal ion catalytic mechanism.     

Sulfate-activating enzymes of Penicillium chrysogenum. The ATP sulfurylase.adenosine 5'-phosphosulfate complex does not serve as a substrate for adenosine 5'-phosphosulfate kinase

At a noninhibitory steady state concentration of adenosine 5'-phosphosulfate (APS), increasing the concentration of Penicillium chrysogenum ATP sulfurylase drives the rate of the APS kinase-catalyzed reaction toward zero. The result indicates that the ATP sulfurylase.APS complex does not serve as a substrate for APS kinase, i.e. there is no "substrate channeling" of APS between the two sulfate-activating enzymes. APS kinase had no effect on the [S]0.5 values, nH values, or maximum isotope trapping in the single turnover of ATP sulfurylase-bound [35S]APS. Equimolar APS kinase (+/- MgATP or APS) also had no effect on the rate constants for the inactivation of ATP sulfurylase by phenylglyoxal, diethylpyrocarbonate, or N-ethylmaleimide. Similarly, ATP sulfurylase (+/- ligands) had no effect on the inactivation of equimolar APS kinase by trinitrobenzene sulfonate, diethylpyrocarbonate, or heat. (The last promotes the dissociation of dimeric APS kinase to inactive monomers.) ATP sulfurylase also had no effect on the reassociation of APS kinase subunits at low temperature. The cumulative results suggest that the two sulfate activating enzymes do not associate to form a "3'-phosphoadenosine 5'-phosphosulfate synthetase" complex.     

Localization of the high-affinity ATP site in adenosine-3':5'-monophosphate-dependent protein kinase type I. Photoaffinity labelling studies with 8-azidoadenosine 5'-triphosphate.

8-Azido-adenosine 5'-triphosphate (n8(3)ATP) appeared to be a suitable photoaffinity label for the protein kinase dependent on adenosine 3':5'-monophosphate (cAMP). It competes with ATP for the high-affinity ATP site in the undissociated form of the kinase and in the phosphotransferase reaction catalyzed by the catalytic subunit. Furthermore, it is accepted as a substrate in the phosphotransfer reaction. n8(3)ATP incorporated into the holoenzyme is covalently bound irradiation. Protection experiments with ATP indicated that this covalent attachment occurs in the high-affinity ATP site of the enzyme. Polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate shows that n8(3)ATP is bound to the catalytic subunit. After irradiation the enzyme was dissociated by cAMP. Proportional to the incorporated [gamma-32P]n8(3)ATP, a loss in phosphotransferase activity was found. These results support our model that both ATP sites coincide with respect to their adenine binding part. Thus binding of the regulatory subunit to the catalytic subunit would then transform the low-affinity catalytically active ATP site into a high-affinity inactive site.     

Crystal Structures of the Kinase Domain of the Sulfate-Activating Complex in Mycobacterium tuberculosis

In Mycobacterium tuberculosis the sulfate activating complex provides a key branching point in sulfate assimilation. The complex consists of two polypeptide chains, CysD and CysN. CysD is an ATP sulfurylase that, with the energy provided by the GTPase activity of CysN, forms adenosine-5’-phosphosulfate (APS) which can then enter the reductive branch of sulfate assimilation leading to the biosynthesis of cysteine. The CysN polypeptide chain also contains an APS kinase domain (CysC) that phosphorylates APS leading to 3’-phosphoadenosine-5’-phosphosulfate, the sulfate donor in the synthesis of sulfolipids. We have determined the crystal structures of CysC from M. tuberculosis as a binary complex with ADP, and as ternary complexes with ADP and APS and the ATP mimic AMP-PNP and APS, respectively, to resolutions of 1.5 Å, 2.1 Å and 1.7 Å, respectively. CysC shows the typical APS kinase fold, and the structures provide comprehensive views of the catalytic machinery, conserved in this enzyme family. Comparison to the structure of the human homolog show highly conserved APS and ATP binding sites, questioning the feasibility of the design of specific inhibitors of mycobacterial CysC. Residue Cys556 is part of the flexible lid region that closes off the active site upon substrate binding. Mutational analysis revealed this residue as one of the determinants controlling lid closure and hence binding of the nucleotide substrate.     

GTPase-mediated activation of ATP sulfurylase.

GTP stimulates the synthesis of APS (adenosine 5'-phosphosulfate) by the enzyme ATP sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4) via a GTPase mechanism. The activation of the enzyme, purified from Escherichia coli, is titratable with GTP. The initial rate of APS formation is increased 116-fold at a saturating concentration of GTP. The enzyme exhibits a GTPase activity that is stimulated by ATP and further enhanced by SO4; however, SO4 alone does not significantly stimulate GTP hydrolysis. The larger subunit of ATP sulfurylase, encoded by cysN, contains a GTP-binding consensus sequence common to other known GTP-binding proteins. This is the first evidence that the sulfate activation pathway is a metabolic target for regulation by a GTPase.