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C Grubmeyer

Publications and source records attributed to C Grubmeyer.

At least 37 records · Page 2Linked to original sources
PubMed1996

Limited proteolysis of Salmonella typhimurium nicotinic acid phosphoribosyltransferase reveals ATP-linked conformational change.

Nicotinic acid phosphoribosyltransferase (NAPRTase;EC 2.4.2.11) couples stoichiometric ATP hydrolysis with formation of nicotinate mononucleotide (NAMN) from nicotinic acid and alpha-D-5-phosphoribosyl 1-pyrophosphate (PRPP). Trypsin rapidly inactivated the ATPase and NAMN synthesis activities of NAPRTase in parallel, with cleavages at Arg-384 and Lys-374 of the 399-residue protein. ATP and PRPP each provided protection against tryptic cleavage. Limited chymotryptic proteolysis of NAPRTase exhibited very similar behavior, with specific cleavage at Phe-382 and protection by substrates. Results suggest that a solvent-exposed loop encompassing Lys-374, Phe-382, and Arg-384 is protected by ATP- or PRPP-induced conformational changes. The ability of ATP to protect even under conditions in which enzyme phosphorylation was prevented by EDTA provides evidence for a distinct ATP-induced protein conformation that acts as an intermediate in energy coupling.

Adenosine Triphosphate↗
PubMed1996

Transition state structure of Salmonella typhimurium orotate phosphoribosyltransferase.

Orotate phosphoribosyltransferase (OPRTase) catalyzes the magnesium-dependent conversion of alpha-D-phosphoribosylpyrophosphate (PRPP) and orotate to orotidine 5'-monophosphate (OMP) and pyrophosphate. We have determined kinetic isotope effects on the reaction of OMP with pyrophosphate and with the pyrophosphate analog phosphonoacetic acid. In the latter case, full expression of the kinetic isotope effects allowed us to calculate the structure of the transition state for the pyrophosphorylytic reaction. The transition state resembles a classical oxocarbonium ion. Using the recently reported three-dimensional structures of the OPRTase-OMP (Scapin et al., 1994) and the OPRTase-PRPP complexes (Scapin et al., 1995a), we have modeled the calculated transition state structure into the active site of OPRTase. We propose a detailed chemical mechanism which is consistent with these results.

Carbon Radioisotopes↗
PubMed1995

The crystal structure of the orotate phosphoribosyltransferase complexed with orotate and alpha-D-5-phosphoribosyl-1-pyrophosphate.

The three-dimensional structure of Salmonella typhimurium orotate phosphoribosyltransferase (OPRTase) in complex with the ribose 5-phosphate donor alpha-D-5--phosphoribosyl-1-pyrophosphate (PRPP) and the nitrogenous base orotic acid has been solved and refined with X-ray diffraction data extending to 2.3 A resolution to a crystallographic R-factor of 18.7%. The complex was generated by carrying out catalysis in the crystal. Comparison of this structure with the previously reported structure of the orotidine 5'-monophosphate (OMP) complex [Scapin, G., Grubmeyer, C., and Sacchettini, J. C. (1994) Biochemistry 33, 1287-1294] revealed that the enzyme backbone undergoes only small movements. The most significant differences occur near the active site, at Ala71-Gly74, with the largest difference involving the side chains of Lys73, Val127-Ala133, the 5'-phosphate binding loop, and a long, solvent-exposed loop at the dimer interface. The position of the ribose moiety is, on the other hand, very different in the OMP and PRPP.orotate complexes, with its anomeric carbon moving approximately 7 A across the binding cavity. In the PRPP.orotate complex the highly conserved acidic side chain of Asp124 interacts with the ribose of PRPP, whereas there are no interactions of this aspartate with the substrate in the OMP complex.

Binding Sites↗
PubMed1995

Locations and functional roles of conserved lysine residues in Salmonella typhimurium orotate phosphoribosyltransferase.

Salmonella typhimurium orotate phosphoribosyltransferase (OPRTase) catalyzes the formation of orotidine 5'-monophosphate (OMP) from orotate and alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP). There are five highly conserved lysine residues (Lys-19, -26, -73, -100, and -103) in S. typhimurium OPRTase. Here, we report the results of mutagenesis and substrate analog studies to investigate the functional roles of these lysines. Together with information from X-ray crystallography [Scapin, G., Grubmeyer, C., & Sacchettini, J. C. (1994) Biochemistry 33, 1287-1294; Scapin, G., Ozturk, D. H., Grubmeyer, C., & Sacchettini, J. C. (1995) Biochemistry 34, 10744-10754], sequence comparisons, and chemical modification [Grubmeyer, C., Segura, E., & Dorfman, R. (1993) J. Biol. Chem. 268, 20299-20304], this work permits the assignment of functions of the five conserved lysines. Lys-19 is external to the active site, and its mutation to glutamine had little effect on enzyme activity. Lys-26 forms a hydrogen bond to OMP at the 3'-hydroxyl group, and its mutation produced 3-10-fold decreases in kcat. Lys-73 extends into the active site, and a conformational change allows it to interact with either the 5'-phosphate of OMP or the 2-hydroxyl and alpha-phosphoryl oxygen of PRPP in their respective substrate complexes. Mutation of Lys-73 produced a 50-100-fold decrease in kcat and an 8-12-fold increase in the KM value for PRPP. Mutation of Lys-100 produced a 5-fold decrease in kcat and a 3-fold increase in the KM for PRPP, consistent with its location within the active site, near the pyrophosphate moiety of PRPP.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1995

Structure and function of Salmonella typhimurium orotate phosphoribosyltransferase: protein complementation reveals shared active sites.

A solvent-exposed loop, comprising residues 98-119 of S. typhimurium orotate phosphoribosyltransferase (OPRTase), is at the subunit interface of the dimeric enzyme, and its amino acid side chains potentially contact active sites on either subunit. A portion of the loop (103-107) appears to be mobile on the basis of the X-ray structures of enzyme.OMP [Scapin, G., Grubmeyer, C., & Sacchettini, J. C. (1994) Biochemistry 33, 1287-1294] and enzyme.PRPP.orotate complexes [Scapin, G., Ozturk, D. H., Grubmeyer, C., & Sacchettini, J. C. (1995) Biochemistry 34, 10744-10754]. Lys-103, which is essential for activity [Ozturk, D. H., Dorfman, R. H. Scapin, G., Sacchettini, J. C., & Grubmeyer, C. (1995) Biochemistry 34, 10755-10763], may thus be functional in the active site formed by the adjacent subunit. Asp-125 is an essential residue that is in the middle of the active site. Equimolar mixtures of the nearly inactive K103A and D125N mutant ORPTase subunits produced approximately 21-23% of the enzymatic activity of the wild-type OPRTase. Heterodimer formation in the complemented mixtures was evidenced by various physical methods. Thus, the active site of OPRTase requires Asp-125 from one subunit and Lys-103 from the adjacent subunit. As predicted from the three-dimensional structure, increased activity resulting from complementation was also observed with mixtures of the K103A mutant and the poorly active K73A and K73Q mutants but not with mixtures of D125N and either K73A or K73Q mutants. Neither K103A nor D125N mutants exhibited negative complementation with the wild-type enzyme. A K103A/D125N double mutant enzyme was also constructed and was able to inactivate wild-type enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1994

The crystal structure of human hypoxanthine-guanine phosphoribosyltransferase with bound GMP.

The crystal structure of HGPRTase with bound GMP has been determined and refined to 2.5 A resolution. The enzyme has a core alpha/beta structure resembling the nucleotide-binding fold of dehydrogenases, and a second lobe composed of residues from the amino and carboxy termini. The GMP molecule binds in an anti conformation in a solvent-exposed cleft of the enzyme. Lys-165, which forms a hydrogen bond to O6 of GMP, appears to be critical for determining the specificity for guanine and hypoxanthine over adenine. The location of active site residues also provides evidence for a possible mechanism for general base-assisted HGPRTase catalysis. A rationalization of the effects on stability and activity of naturally occurring single amino acid mutations of HGPRTase is presented, including a discussion of several mutations at the active site that lead to Lesch-Nyhan syndrome.

Amino Acid Sequence↗
PubMed1994

Crystal structure of orotate phosphoribosyltransferase.

Phosphoribosyltransferases (PRTases) are enzymes involved in the synthesis of purine, pyrimidine, and pyridine nucleotides. They utilize alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP) and a nitrogenous base to form a beta-N-riboside monophosphate and pyrophosphate (PPi), and their functional significance in nucleotide homeostasis is evidenced by the devastating effects of inherited diseases associated with the decreased activity and/or stability of these enzymes. The 2.6-A structure of the Salmonella typhimurium orotate phosphoribosyltransferase (OPRTase) complexed with its product orotidine monophosphate (OMP) provides the first detailed image of a member of this group of enzymes. The OPRTase three-dimensional structure was solved using multiple isomorphous replacement methods and reveals two major features: a core five-stranded alpha/beta twisted sheet and an N-terminal region that partially covers the C-terminal portion of the core. PRTases show a very high degree of base specificity. In OPRTase, this is determined by steric constraints and the position of hydrogen bond donors/acceptors of a solvent-inaccessible crevice where the orotate ring of bound OMP resides. Crystalline OPRTase is a dimer, with catalytically important residues from each subunit available to the neighboring subunit, suggesting that oligomerization is necessary for its activity. On the basis of the presence of a common PRPP binding motif among PRTases and the similar chemistry these enzymes perform, we propose that the alpha/beta core found in OPRTase will represent a common feature for PRTases. This generality is demonstrated by construction of a model of the human hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) from secondary structure predictions for HGPRTase and the three-dimensional structure of OPRTase.

Amino Acid Sequence↗
PubMed1993

A new paradigm for biochemical energy coupling. Salmonella typhimurium nicotinate phosphoribosyltransferase.

The pncB gene of Salmonella typhimurium was used to develop an overexpression system for nicotinate phosphoribosyltransferase (NAPRTase, EC 2.4.2.11), which forms nicotinate mononucleotide (NAMN) and PPi from nicotinate and alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP). NAPRTase hydrolyzes ATP in 1:1 molar stoichiometry to NAMN synthesis. Hydrolysis of ATP alters the ratio of products/substrates for the reaction nicotinate + PRPP <--> NAMN + PPi from its equilibrium value of 0.67 to a steady-state value of 1100. The energy for the maintenance of this ratio must come from ATP hydrolysis. However, in contrast to other ATP-utilizing enzymes, when all ATP is hydrolyzed the unfavorable product/substrate ratio collapses. ATP/ADP exchange results suggest that the overall reaction involves a phosphoenzyme (E-P) arising from E.ATP. Km values for nicotinate and PRPP each decreased by 200-fold when ATP was present to phosphorylate the enzyme. PPi stimulated the ATPase activity of the enzyme to Vmax values, suggesting that PPi formation during catalysis provides a trigger for cleavage of the putative E-P in the overall reaction and regenerates the low affinity form of the enzyme. A model is presented in which alternation of high and low affinity forms of NAPRTase provides a "steady-state" coupling between ATP hydrolysis and NAMN formation.

Cloning, Molecular↗
PubMed1993

Active site lysines in orotate phosphoribosyltransferase.

Orotate phosphoribosyltransferase (OPRTase; EC 2.4.2.10) catalyzes the formation of the nucleotide orotidine-5'-monophosphate from orotate and 5-phosphoribosyl-1-pyrophosphate (PRPP). The bacterial enzyme, unlike its mammalian homolog, is monofunctional and is a dimer of M(r) 23,000 subunits. The availability of large amounts of highly purified crystalline Salmonella typhimurium OPRTase have enabled us to being structure/function studies on the enzyme. Like other phosphoribosyltransferases, OPRTase binds the highly charged MgPRPP complex, as well as anionic orotate, suggesting an active site containing basic residues. The S. typhimurium sequence (Scapin, G., Sacchettini, J. C., Dessen, A., Bhatia, M., and Grubmeyer, C. (1993) J. Mol. Biol. 230, 1304-1308) contains 12 lysine and 13 arginine residues, of which Lys-26, Arg-99, Lys-100, Lys-103, and Arg-156 are conserved as identities among the sequences of OPRTases from other organisms, with Lys-19 and Arg-161 replaced by the alternative basic residue in one or more sequences. The lysine modifier 2,4,6-trinitrobenzene sulfonate inactivated S. typhimurium OPRTase in a pseudo first-order process, and OMP and PRPP provided good protection against inactivation. Spectral quantitation of trinitrophenyl (TNP) group incorporation showed that inactivation was correlated with incorporation of one TNP group per subunit. Surprisingly, tryptic proteolysis followed by high performance liquid chromatography and amino acid sequence analysis revealed that four peptides, containing three distinct lysines, had been modified. Peptides modified at Lys-26, Lys-100 and Lys-103, as well as a doubly modified peptide containing TNP groups at Lys-100 and Lys-103, were identified. Inactivation kinetics showed that the 3 lysine residues were modified at equal rates. Protection studies demonstrated that Lys-26, and to a lesser extent Lys-100 and Lys-103, were protected against modification by OMP, whereas PRPP protected Lys-26, Lys-100 and Lys-103. Pyrophosphate protected Lys-100 and Lys-103. The results suggest active site locations for the sequence-conserved and TNP-modified lysine residues, with Lys-26 interacting with the ribose-5-phosphate moiety of OMP and PRPP, and Lys-100 and Lys-103 interacting with the pyrophosphate moiety of PRPP.

Amino Acid Sequence↗
PubMed1993

Conserved cysteine residues of histidinol dehydrogenase are not involved in catalysis. Novel chemistry required for enzymatic aldehyde oxidation.

The 4-electron oxidoreductase L-histidinol dehydrogenase (HDH, EC 1.1.1.23) oxidizes the amino alcohol histidinol to histidine via an aldehyde-level intermediate at a single active site. The enzyme contains two Zn2+ per dimer, and treatment with metal chelators causes a metal-reversible inactivation. NAD-linked aldehyde oxidations, for which glyceraldehyde-3-phosphate dehydrogenase has served as the major paradigm, are thought to proceed via cysteine-based thiohemiacetals. Sequenced forms of HDH contain two conserved cysteine residues, Cys-116 and Cys-153 in the Salmonella typhimurium enzyme, and in previous work we have shown that HDH is inactivated by active site modification of Cys-116 by the reagent 4-nitro-7-chlorobenzadioxazole. Thus, Cys-116 is an excellent candidate for the active site nucleophile in HDH. In the current studies we show that treatment of HDH with the Zn2+ chelator 1,10-phenanthroline exposes Cys-116 to specific modification by iodoacetate, resulting in irreversible loss of activity. Site-specific mutagenesis was used to explore the roles of the conserved cysteine residues. The mutant enzymes C116S, C153S, C116A, and C153A and the double mutant C116,153A were each overproduced and purified to homogeneity. All mutant enzymes showed normal kcat and Km values for catalysis. The double mutant protein was unstable, and the single mutants also lose significant activities over a 3-h period during which wild-type enzyme retains full activity. The C116S mutant, and to a lesser extent the C116A mutant, were sensitive to the presence of EDTA in the assay medium, but the other mutants or wild-type enzyme were not, suggesting that Cys-116 may be near, but probably not liganded to, the bound metal ion. The results clearly indicate that HDH does not use a cysteine-based thiohemiacetal as a catalytic intermediate, requiring a new paradigm for NAD-linked aldehyde oxidation. Some models for the reaction are presented and discussed.

Alcohol Oxidoreductases↗
PubMed1993

Primary structure and crystallization of orotate phosphoribosyltransferase from Salmonella typhimurium.

Orotate phosphoribosyltransferase (OPRTase; EC 2.4.2.10) catalyzes phosphoribosyl group transfer between alpha-D-5-phosphoribosyl-1-pyrophosphate and orotate to form orotidine-5'-monophosphate and pyrophosphate, the nucleotide-forming step in pyrimidine biosynthesis. It is one of ten PRTases that perform vital roles in de novo and salvage pathways for purine, pyrimidine and pyridine nucleotides. Although the PRTases are important drug targets, they are poorly understood mechanistically, and no three-dimensional structures exist. Here, we report the complete sequence of the Salmonella typhimurium pyrE gene and the deduced sequence of the OPRTase gene product. OPRTase forms tetragonal crystals from polyethylene glycol solutions; these crystals diffract to better than 2 A resolution, and are stable to radiation damage. The space group is P4(1)2(1)2 (or P4(3)2(1)2) with unit cell dimensions of a = b = 48.5 A, c = 210.5 A, and alpha = beta = gamma = 90 degrees. A crystalline form of the selenomethionine derivative of the protein is also reported.

Amino Acid Sequence↗
PubMed1993

The role of divalent magnesium in activating the reaction catalyzed by orotate phosphoribosyltransferase.

Orotate phosphoribosyltranferase (OPRTase) catalyzes the formation of orotidine 5'-monophosphate from the nitrogenous base orotate and alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP). While it is known that Mg2+ is necessary for catalysis, the mechanism of activation of the phosphoribosyl transfer by Mg2+ remains unclear. The divalent cation may activate the phosphoribosyl transfer by binding to either or both substrates PRPP and orotate or/and the enzyme. In this work we chose to explore the role of divalent magnesium in activating the phosphoribosyl transfer in bacterial OPRTase. Studies on the effect of Mg2+ on the OPRTase-catalyzed reaction indicated that the divalent metal was necessary for catalysis. A maximal rate of 70 units/mg was achieved at 2 mM MgCl2. Mn2+ could replace Mg2+ as the divalent metal. Orotate methyl ester (OAME) and uracil, neither of which form chelates with divalent metal, were found to be substrates for OPRTase. The KM for OAME and uracil were 190 microM and 2.63 mM and kcat/KM were 0.91 x 10(5) and 6 M-1 s-1, respectively. These values compare with a KM of 27 microM for orotate, 44 microM for PRPP, and a kcat/KM of 1.3 x 10(6) M-1 s-1 for orotate. Spectroscopic studies failed to reveal the existence of Mg(2+)-orotate complexes. Thus we have concluded that an orotate-metal complex is not necessary for OPRTase catalysis. Metal-enzyme binding studies indicate that only weak metal-enzyme complexes may form in bacterial OPRTase. Thus the role of divalent metal in bacterial OPRTase must be to bind PRPP.

Cations, Divalent↗
PubMed1993

The Salmonella typhimurium nadC gene: sequence determination by use of Mud-P22 and purification of quinolinate phosphoribosyltransferase.

The Salmonella typhimurium nadC gene and its product, quinolinic acid phosphoribosyltransferase (QAPRTase), were characterized at the molecular and biochemical levels. Fusions of Mud-lac elements isolated in the nadC gene were converted to Mud-P22 insertions. Starting with six original Mud-lac fusions, the entire sequence of the nadC gene was readily obtained. The sequence shows a long open reading frame with two potential initiator methionines, one of which is preceded by the Shine-Dalgarno sequence GGAG-7-nucleotide-ATG. The protein predicted from this second open reading frame is 297 residues in length. The nadC gene was subcloned into a T7-based expression system, allowing for facile purification of the QAPRTase (EC 2.4.2.19) protein to homogeneity. Upon gel filtration, the protein gave an M(r) of 72,000, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a subunit M(r) of 35,000. Automated Edman degradation of several tryptic peptides confirmed the amino acid sequence predicted from the DNA sequence. Chromatography of the apparently homogeneous enzyme on reverse-phase high-performance liquid chromatography resolved two protein species. One of these species failed to give an amino-terminal sequence, while the other yielded the amino-terminal sequence predicted by the second open reading frame and lacked the initiator methionine. The mass of the mature protein, predicted from its DNA sequence, was 32,428 Da. Electrospray mass spectrometry gave masses of 32,501 and 32,581 Da for the two peptides. Steady-state kinetics on the purified QAPRTase indicated Km values of 32 microM for 5-phosphoribosyl-1-pyrophosphate and 20 microM for quinolinate. Vmax was 0.9 U/mg, similar to values reported for this enzyme by other sources.

Amino Acid Sequence↗
PubMed1990

Kinetic mechanism of orotate phosphoribosyltransferase from Salmonella typhimurium.

The chemical mechanism of the phosphoribosyltransferases (PRTases), although largely unknown, may proceed either via a concerted direct-transfer mechanism or with a two-step mechanism involving a carboxonium-like intermediate. To study this question, we have cloned the Salmonella typhimurium pyrE gene, coding for the enzyme orotate phosphoribosyltransferase (EC 2.2.4.10, OPRTase), and developed a bacterial strain that overproduces the enzyme, which we have purified to homogeneity. Initial velocity and product inhibition studies indicated that S. typhimurium OPRTase follows a random sequential kinetic mechanism. This result was further confirmed by equilibrium isotope exchange studies on two substrate-product pairs, PRPP-PPi and OMP-orotate. In addition, the rates of the individual equilibrium isotope exchanges allowed us to conclude that PPi release and PRPP release were the rate-determining steps in the forward and reverse reactions, respectively. Although partial reactions between the two substrate-product pairs, PRPP-PPi and OMP-orotate, were observed, further studies revealed that these exchanges were a result of contaminations. Our results are significant in that S. typhimurium OPRTase, like most PRTases but in contrast to its yeast homologue, follows sequential kinetics. The artifactual partial isotope exchanges found in this work may have implications for similar prior work on the yeast enzyme. In view of the careful isotope effect studies of Parsons and co-workers [Goitein, R.K., Chelsky, D., & Parsons, S.M. (1978) J. Biol. Chem. 253, 2963-2971] and the results obtained by us, we propose that PRTases may involve a direct-transfer mechanism but with low bond order to the leaving pyrophosphate moiety and attacking base.

Carbon Radioisotopes↗
PubMed1989

L-histidinol dehydrogenase, a Zn2+-metalloenzyme.

The enzymatic activity of L-histidinol dehydrogenase from Salmonella typhimurium was stimulated by the inclusion of 0.5 mM MnCl2 in the assay medium. At pH 9.2 the stimulation was correlated with binding of 1 g-atom of 54Mn2+/mol dimer, KD = 37 microM. ZnCl2, which prevented the MnCl2 stimulation, also bound to the enzyme, 1.2 g-atom/mol dimer, KD = 51 microM, and prevented Mn2+ binding. Enzyme activity was lost when histidinol dehydrogenase was incubated in 8 M urea. Reactivation was observed when urea-denatured enzyme was diluted into buffer containing 2-mercaptoethanol but required either MnCl2 or ZnCl2. Histidinol dehydrogenase was inactivated by the transition metal chelator 1,10-phenanthroline or by high levels of 2-mercaptoethanol. The nonchelating 1,7-phenanthroline was not an inactivator, and inactivation by either 1,10-phenanthroline or 2-mercaptoethanol was prevented by MnCl2. Enzyme inactivated by 1,10-phenanthroline could be reactivated by addition of MnCl2 or ZnCl2 in the presence of 2-mercaptoethanol. Reactivation was correlated with the binding of 1.5 g-atom 54Mn2+/mol dimer. Atomic absorption analysis of the native enzyme indicated the presence of 1.65 g-atom Zn/mol dimer, and no Mn was detected. The results demonstrate, therefore, that histidinol dehydrogenase contains two metal binding sites per enzyme dimer, which normally bind Zn2+, but which may bind Mn2+ while retaining enzyme function. Histidinol dehydrogenase is thus the third NAD-linked oxidoreductase in which Zn2+ fulfills an essential structural and/or catalytic role.

Alcohol Oxidoreductases↗
PubMed1987

Evidence for catalytic cooperativity during ATP hydrolysis by beef heart F1-ATPase. Kinetics and binding studies with the photoaffinity label BzATP.

The photoaffinity analog of ATP, 3'-O-(4-benzoyl) benzoyl ATP (BzATP), was used to covalently modify the catalytic sites on the beef heart mitochondrial F1-ATPase. In the absence of actinic illumination, BzATP was a slow substrate for the enzyme (Vmax = 0.19 mumol min-1 mg-1; kcat/Km = 2.2 X 10(6) M-1s-1) and behaved as a classical competitive inhibitor versus ATP (Ki = 0.85 microM). Under photolytic conditions, BzATP inactivated F1 with pseudo first-order kinetics, and the photoinactivation reaction showed rate saturation suggesting specific, reversible binding of BzATP to F1 prior to covalent bond formation. ATP protected against F1 photoinactivation (Kprotect = 0.3 microM) and partially covalently modified F1 yielded the same Km for ATP as unmodified enzyme. These results strongly suggested that BzATP was bound to catalytic sites on the enzyme. In the absence of photolysis, BzATP saturated two binding sites on the F1 (KD = 1.6 microM), and under photolytic conditions, 1 mol of BzATP was shown to be covalently liganded to the beta subunit of the enzyme coincident with 100% loss in ATPase activity. Previous studies with the mitochondrial F1-ATPase have suggested a mechanism involving catalytic cooperativity during ATP hydrolysis. Our demonstration of a molar stoichiometry of 1 for photoinactivation is in accord with this mechanism. It is suggested that either F1 is unable to hydrolyze covalently bound BzATP, or that subsequent to hydrolysis, the BzADP product can not be released from the catalytic site. It is therefore inferred that F1 hydrolytic activity requires cooperativity between multiple, viable catalytic sites and that covalent modification of a single catalytic site is sufficient for complete enzyme inactivation.

Adenosine Diphosphate↗