Oxygen exchange between phosphate and water accompanies calcium-regulated ATPase activity of skinned fibers from rabbit skeletal muscle

The extent of oxygen exchange between phosphate and water has been measured for the calcium-regulated magnesium-dependent ATPase activity of chemically skinned fibers from rabbit skeletal muscle. The oxygen exchange was determined for isometrically held fibers by measuring with a mass spectrometer the distribution of 18O atoms in the product inorganic phosphate when ATP hydrolysis was carried out in H2(18)O. The extent of exchange was much greater in relaxed muscle (free Ca2+ less than 10(-8) M) than in calcium-activated muscle (free Ca2+ approximately equal to 3 X 10(-5) M). Activated fibers had an ATPase activity at least 30-fold greater than the relaxed fibers. These results correlate well with the extents of oxygen exchange accompanying magnesium-dependent myosin and unregulated actomyosin ATPase activities, respectively. In relaxed fibers, comparison of the amount of exchange with the ATPase activity suggests that the rate constant for the reformation of myosin-bound ATP from the myosin products complex is about 10 s-1 at 20 degrees C and pH 7.1. In each experiment the distribution of 18O in the Pi formed was incompatible with a single pathway for ATP hydrolysis. In the case of the calcium-activated fibers, the multiple pathways for ATP hydrolysis appeared to be an intrinsic property of the actomyosin ATPase in the fiber. These results indicate that in muscle fibers, as in isolated actomyosin, cleavage of protein-bound ATP is readily reversible and that association of the myosin products complex with actin promotes Pi release.     

Investigation of the regiospecificity and stereospecificity of proton transfer in the yeast inorganic pyrophosphatase catalyzed reaction

The regiospecificity and stereospecificity of proton transfer in the yeast inorganic pyrophosphatase (PPase) catalyzed hydrolysis of P1,P2-bidentate Mg(H2O)4(PPi)2- were probed with exchange-inert metal complexes of imidodiphosphate (PNP) and thiopyrophosphate (PPS). PPase was unable to catalyze the hydrolysis of Mg(H2O)4PNP and P1,P2-bidentate Co(NH3)4PNP under conditions that resulted in rapid hydrolysis of the corresponding metal-PPi complexes. PPase was found to catalyze the hydrolysis of Mg(H2O)4PPS at 17% the rate of Mg(H2O)4PPi hydrolysis. The Km of Mg(H2O)4PPS was determined to be 300 microM, which is a value 10-fold greater than that observed for Mg(H2O)4PPi. P1,P2-Bidentate Cr(H2O)4PPS and Co(NH3)4PPS (prepared from PPS) were both found to be substrates for PPase. The enzyme specifically catalyzed the hydrolysis of the Rp enantiomers of these complexes and not the Sp enantiomers. These results are accommodated by a reaction mechanism involving enzyme-mediated proton transfer to the pro-R oxygen atom of the incipient phosphoryl leaving group of the bound P1,P2-bidentate Mg(H2O)4PPi2- complex.     

Comparison of sucrase-free isomaltase with sucrase-isomaltase purified from the house musk shrew Suncus murinus

We purified sucrase-isomaltase and sucrase-free isomaltase from a normal and a sucrase-deficient line, respectively, of the house musk shrew Suncus murinus and examined the effects of mutation on enzyme structure and activities. Recent cDNA cloning studies have predicted that sucrase-free mutant isomaltase lacks the C-terminal 69 amino acids of normal isomaltase, as well as the entire sucrase. On SDS-polyacrylamide gel electrophoresis purified sucrase-free isomaltase gave a single protein band of 103 kDa, while sucrase-isomaltase gave two major protein bands of 106 and 115 kDa. The 115, but not 106, kDa band was quite similar to the 103 kDa band on Western blotting with Aleuria aurantia lectin and antibody against shrew sucrase-isomaltase, suggesting that the 115 and 103 kDa bands are due to normal and mutant isomaltases, respectively, in accordance with the above prediction. Purified isomaltase and sucrase-isomaltase were similar in Km and Vmax (based on isomaltase mass) values for isomaltose hydrolysis and in inhibition of isomaltase activity by antibody against rabbit sucrase-isomaltase, suggesting that the enzymatic properties of isomaltase are mostly unaffected by mutation.     

Modeling evolution of hydrogen bonding and stabilization of transition states in the process of cocaine hydrolysis catalyzed by human butyrylcholinesterase

Molecular dynamics (MD) simulations and quantum mechanical/molecular mechanical (QM/MM) calculations were performed on the prereactive enzyme-substrate complex, transition states, intermediates, and product involved in the process of human butyrylcholinesterase (BChE)-catalyzed hydrolysis of (-)-cocaine. The computational results consistently reveal a unique role of the oxyanion hole (consisting of G116, G117, and A199) in BChE-catalyzed hydrolysis of cocaine, compared to acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylcholine. During BChE-catalyzed hydrolysis of cocaine, only G117 has a hydrogen bond with the carbonyl oxygen (O31) of the cocaine benzoyl ester in the prereactive BChE-cocaine complex, and the NH groups of G117 and A199 are hydrogen-bonded with O31 of cocaine in all of the transition states and intermediates. Surprisingly, the NH hydrogen of G116 forms an unexpected hydrogen bond with the carboxyl group of E197 side chain and, therefore, is not available to form a hydrogen bond with O31 of cocaine in the acylation. The NH hydrogen of G116 is only partially available to form a weak hydrogen bond with O31 of cocaine in some structures involved in the deacylation. The change of the estimated hydrogen-bonding energy between the oxyanion hole and O31 of cocaine during the reaction process demonstrates how the protein environment can affect the energy barrier for each step of the BChE-catalyzed hydrolysis of cocaine. These insights concerning the effects of the oxyanion hole on the energy barriers provide valuable clues on how to rationally design BChE mutants with a higher catalytic activity for the hydrolysis of (-)-cocaine.     

A hydrophobic form of the small-intestinal sucrase-isomaltase complex

A large scale preparation of brush border membranes is described. Solubilized by either papain or Triton X-100, the sucrase-isomaltase complex is purified in a three-step procedure, including differential centrifugation, Sephadex G-200 and DEAE-cellulose chromatography. Detergent solubilized and protease solubilized sucrase-isomaltase differ in the tendency to aggregate but not in enzymatic characteristics. The chemical composition and the molecular weight of the two enzyme complexes are almost identical. Limited digestion of the Triton-solubilized sucrase-isomaltase complex by papain produces a protein electrophoretically indistinguishable from papain-solubilized sucrase-isomaltase together with low molecular proteolytic fragments.     

Biosynthesis of intestinal microvillar proteins. The intracellular transport of aminopeptidase N and sucrase-isomaltase occurs at different rates pre-Golgi but at the same rate post-Golgi

The kinetics of processing and microvillar expression of aminopeptidase N (EC 3.4.11.2) and sucrose alpha-D-glucohydrolase-oligo-1,6-glucosidase (sucrase-isomaltase, EC 3.2.1.48 and EC 3.2.1.10) were compared by labelling of pig small intestinal mucosal explants with [35S]methionine. The conversion from transient (high mannose glycosylated) to mature (complex glycosylated) form was 1.7-times slower for sucrase-isomaltase than for aminopeptidase N, indicating a slower rate of migration from the rough endoplasmic reticulum to the Golgi complex. Likewise, sucrase-isomaltase appeared in the microvillar fraction at a slower rate than aminopeptidase N. The relative pool sizes of mature and transient forms of both enzymes in intracellular membranes (Mg2+-precipitated fraction) were determined to obtain information on the relative time, spent pre- and post-Golgi, respectively, prior to microvillar expression. This ratio was 0.24 +/- 0.06 (mean +/- SD) for sucrase-isomaltase as compared to 0.40 +/- 0.04 (mean +/- SD) for aminopeptidase N. Considering the slower rate of pre-Golgi transport for sucrase-isomaltase, this indicates that the two microvillar enzymes have rather similar if not identical rates of post-Golgi transport.     

Catalytic properties of the F1-adenosine triphosphatase from Escherichia coli K-12 and its genetic variants as revealed by 18O exchanges

We have examined intermediate Pi-water oxygen exchange during [gamma-18O]ATP hydrolysis by the F1 adenosine triphosphatase from Escherichia coli K-12. Water oxygen incorporation into each Pi released was increased as ATP concentration was lowered as observed previously for the same reaction catalyzed by the enzyme from eukaryotic sources. Heterogeneous distributions of 18O in product Pi were produced by coexisting epsilon subunit-replete and epsilon subunit-depleted enzyme molecules. The epsilon-replete enzyme showed a much higher probability for oxygen exchange. These data imply that the epsilon subunit inhibits net ATP hydrolysis by imposing conformational constraints which reduce the cooperative conformational interactions that promote ADP and Pi release. Four enzyme variants altered in alpha or beta subunit structure with reduced net hydrolytic activity showed sharply increased oxygen exchange during ATP hydrolysis. Heterogeneity was apparent in the 18O distribution of the product Pi, however. That behavior could reflect hindered conformational interactions and/or increased affinity of the alpha 3 beta 3 gamma delta complex for the epsilon subunit. In contrast, enzyme from mutant uncA401 showed very little oxygen exchange accompanying hydrolysis of 20 microM ATP. This is the only enzyme so far reported with this unusual property. Its rate limitation appears to be in the hydrolytic rather than the product release step of the catalytic sequence.     

A hydrophobic form of the small-intestinal sucrase-isomaltase complex.

A large scale preparation of brush border membranes is described. Solubilized by either papain or Triton X-100, the sucrase-isomaltase complex is purified in a three-step procedure, including differential centrifugation, Sephadex G-200 and DEAE-cellulose chromatography. Detergent solubilized and protease solubilized sucrase-isomaltase differ in tendency to aggregate but not in enzymatic characteristics. The chemical composition and the molecular weight of the two enzyme complexes are almost identical. Limited digestion of the Triton-solubilized sucrase-isomaltase complex by papain produces a protein electrophoretically indistinguishable from papain-solubilized sucrase-isomaltase together with low molecular proteolytic fragments.     

Effects of alpha-D-glucosylglycerol on the in vitro digestion of disaccharides by rat intestinal enzymes

Alpha-D-glucosylglycerol (GG) is a mixture of 2-O-alpha-D-glucosylglycerol (GG-II), (2R)-1-O-alpha-D-glucosylglycerol (R-GG-I) and (2S)-1-O-alpha-D-glucosylglycerol (S-GG-I). GG has been found to be slightly hydrolyzed in vitro only by rat intestinal enzymes, but hardly at all by other digestive juices. GG suppressed the hydrolysis of maltose, sucrose and isomaltose by rat intestinal enzymes because the amount of glucose in the digestion of a mixture of GG and disaccharide was less than the sum of that in each individual digestion. The consumption of GG was suppressed by isomaltose, but promoted by maltose, with the hydrolysis of GG being suppressed. Sucrose appeared to suppress only the consumption of S-GG-I, suggesting that S-GG-I was hydrolyzed by the active site of sucrase in a sucrase-isomaltase complex. Transglucosylation seems to have occurred more frequently in the individual digestion of maltose and isomaltose than in that of GG and sucrose. GG seemed to promote transglucosylation in the presence of maltose, to suppress it with sucrose, and to delay it with isomaltose.     

The mode of anchoring and precursor forms of sucrase-isomaltase and maltase-glucoamylase in chicken intestinal brush-border membrane. Phylogenetic implications

Chicken intestinal sucrase-isomaltase and maltase-glucoamylase have been isolated in their intact form by detergent solubilization and characterized as to their subunit composition and mode of anchoring in the brush-border membrane. Both are heterodimeric enzyme complexes composed of two subunits each of approximately 140 and 130 kDa. Contrary to the mammalian sucrase-isomaltase, chicken isomaltase was identified as the smaller of the two subunits. As was shown by hydrophobic labeling, only one of the two subunits in each heterodimer is anchored in the bilayer, the smaller 130 kDa isomaltase subunit of the sucrase-isomaltase complex, and the larger 140 kDa subunit of the maltase-glucoamylase complex. Both preparations contain a high-molecular weight polypeptide of approximately 250 kDa which in the case of sucrase-isomaltase could be identified by peptide mapping as a single-chain precursor not (yet) proteolytically processed to the final heterodimer. These first data on the mode of membrane anchoring of non-mammalian glycosidases indicate that they are synthesized, inserted into the membrane, and processed in ways similar to the mammalian enzymes. The fundamental unity between avian and mammalian sucrase-isomaltases suggests that the partial gene duplication of an ancestral isomaltase gene and the subsequent mutation of one of the active sites resulting in pro-sucrase-isomaltase has occurred prior to the separation of mammals from reptiles, i.e. more than 300 million years ago.     

Immuno-electronmicroscopical localization of a microvillus membrane disaccharidase in the human small-intestinal epithelium with monoclonal antibodies

The cellular localization of the human intestinal disaccharidase, sucrase-isomaltase, was visualized in ultrathin cryosections by the use of specific monoclonal antibodies [25] followed by protein A-gold. The principle site of immunoreaction concerned the microvillus membrane, which supports current concepts of the localization of these hydrolases. One antibody against sucrase-isomaltase also showed labeling of the Golgi apparatus, apical vesicles, and lysosomes, but not of the basolateral membrane. The labeling of the Golgi complex was uniform, suggesting the absence of accumulation of sucrase-isomaltase in cisternae during its passage through this organelle. Absence of labeling of the basolateral membrane appears to support the view that newly synthesized sucrase-isomaltase is transferred directly from the Golgi complex to the microvillus membrane, bypassing the basolateral membrane. However, the results do not exclude the possibility of a very rapid passage through the basolateral membrane. A substantial fraction of the sucrase-isomaltase occurred in lysosomes, which indicates that this organelle plays a major role in the catabolism of microvillar hydrolases. Transport of sucrase-isomaltase to lysosomes might occur by endocytosis or via the crinophagic pathway. The latter was previously postulated to reflect a regulatory mechanism at the post-Golgi level for the surface expression of microvillar membrane proteins.     

Superoxide is produced by the reduced flavin in mitochondrial complex I: a single, unified mechanism that applies during both forward and reverse electron transfer

NADH:ubiquinone oxidoreductase (complex I) is a major source of reactive oxygen species in mitochondria and a contributor to cellular oxidative stress. In isolated complex I the reduced flavin is known to react with molecular oxygen to form predominantly superoxide, but studies using intact mitochondria contend that superoxide may result from a semiquinone species that responds to the proton-motive force (Δp) also. Here, we use bovine heart submitochondrial particles to show that a single mechanism describes superoxide production by complex I under all conditions (during both NADH oxidation and reverse electron transfer). NADH-induced superoxide production is inhibited by complex I flavin-site inhibitors but not by inhibitors of ubiquinone reduction, and it is independent of Δp. Reverse electron transfer (RET) through complex I in submitochondrial particles, driven by succinate oxidation and the Δp created by ATP hydrolysis, reduces the flavin, leading to NAD(+) and O(2) reduction. RET-induced superoxide production is inhibited by both flavin-site and ubiquinone-reduction inhibitors. The potential dependence of NADH-induced superoxide production (set by the NAD(+) potential) matches that of RET-induced superoxide production (set by the succinate potential and Δp), and they both match the potential dependence of the flavin. Therefore, both NADH- and RET-induced superoxide are produced by the flavin, according to the same molecular mechanism. The unified mechanism describes how reactive oxygen species production by complex I responds to changes in cellular conditions. It establishes a route to understanding causative connections between the enzyme and its pathological effects and to developing rational strategies for addressing them.     

Oxygen-exchange studies on the pathways for magnesium adenosine 5'-triphosphate hydrolysis by actomyosin

At an intermediate stage in the hydrolysis of magnesium adenosine 5'-phosphate (MgATP) by myosin or actomyosin, there is an exchange of oxygen between water and the P gamma group of enzyme-bound nucleotide. Starting with [P gamma-18O]ATP as substrate, the exchange is revealed in the [18O]Pi species that are ultimately released as product into the reaction medium. An analysis of the distribution of these labeled Pi species, which contain 3, 2, 1, or none of the 18O atoms originally on the P gamma of ATP, is used to probe intermediate stages of the hydrolytic mechanism. In recent years, studies of this kind by several groups have shown that more than one pathway of hydrolysis operates. The work reported here demonstrates that two of these pathways are spurious; one is a "nonexchanging MgATPase" that is present in fresh myosin preparations; the other is an induced slow exchange that develops in myosin during storage (-20 degrees C) and subsequent aging (4 degrees C). However, after correction for these artifacts, two normal pathways for actomyosin hydrolysis remain. These normal pathways differ in the mode of interaction between actin and myosin in the course of hydrolysis; one is the Lymn-Taylor pathway where oxygen exchange occurs at a stage when actin and myosin are dissociated; the other is a pathway in which actin and myosin are associated during oxygen exchange. Each of these two pathways contributes an equal amount of Pi to the product pool. Thus, on average, each myosin head uses each of these pathways half the time. The findings suggest, e.g., that during contraction, myosin can dissociate from the actin filament only during every other cycle of MgATP hydrolysis or that only half the heads, at any one time, can exchange oxygen while free of the actin filament.     

Mechanism of the Escherichia coli ADP-ribose pyrophosphatase,a Nudix hydrolase

Escherichia coli ADP-ribose (ADPR) pyrophosphatase (ADPRase), a Nudix enzyme, catalyzes the Mg(2+)-dependent hydrolysis of ADP-ribose to AMP and ribose 5-phosphate. ADPR hydrolysis experiments conducted in the presence of H(2)(18)O and analyzed by electrospray mass spectrometry showed that the ADPRase-catalyzed reaction takes place through nucleophilic attack at the adenosyl phosphate. The structure of ADPRase in complex with Mg(2+) and a nonhydrolyzable ADPR analogue, alpha,beta-methylene ADP-ribose, reveals an active site water molecule poised for nucleophilic attack on the adenosyl phosphate. This water molecule is activated by two magnesium ions, and its oxygen contacts the target phosphorus (P-O distance of 3.0 A) and forms an angle of 177 degrees with the scissile bond, suggesting an associative mechanism. A third Mg(2+) ion bridges the two phosphates and could stabilize the negative charge of the leaving group, ribose 5-phosphate. The structure of the ternary complex also shows that loop L9 moves fully 10 A from its position in the free enzyme, forming a tighter turn and bringing Glu 162 to its catalytic position. These observations indicate that as part of the catalytic mechanism, the ADPRase cycles between an open (free enzyme) and a closed (substrate-metal complex) conformation. This cycling may be important in preventing nonspecific hydrolysis of other nucleotides.     

Stereochemical analysis of peptide bond hydrolysis catalyzed by the aspartic proteinase penicillopepsin

The X-ray crystal structures of native penicillopepsin and of its complex with a synthetic analogue of the inhibitor pepstatin have been refined recently at 1.8-A resolution. These highly refined structures permit a detailed examination of peptide hydrolysis in the aspartic proteinases. Complexes of penicillopepsin with substrate and catalytic intermediates were modeled, by using computer graphics, with minimal perturbation of the observed inhibitor complex. A thallium ion binding experiment shows that the position of solvent molecule O39, between Asp-33(32) and Asp-213(215) in the native structure, is favorable for cations, a fact that places constraints on possible mechanisms. A mechanism for hydrolysis is proposed in which Asp-213(215) acts as an electrophile by protonating the carbonyl oxygen of the substrate, thereby polarizing the carbon-oxygen bond, a water molecule bound to Asp-33(32) (O284 in the native structure) attacks the carbonyl carbon as the nucleophile in a general-base mechanism, the newly pyramidal peptide nitrogen is protonated, either from the solvent after nitrogen inversion or by an internal proton transfer via Asp-213(215) from a hydroxyl of the tetrahedral carbon, and the tetrahedral intermediate breaks down in a manner consistent with the stereoelectronic hypothesis. The models permit the rationalization of observed subsite preferences for substrates and may be useful in predicting subsite preferences of other aspartic proteinases.     

Theoretical conformational analysis in the determination of productive conformations of substrates for acetylcholinesterase and butyrylcholinesterase

All the equilibrium conformations of 34 analogues of acetylcholine (ACh) with the general formula R-C(O)O-Alk-N+(CH3)3 are calculated by the method of molecular mechanics. In the series R-C(O)O-(CH2)2-N+(CH3)3, a reliable correlation is found between the molecular volume of the substrate and the rate of its hydrolysis by acetylcholinesterase (AChE); the absence of such a correlation is demonstrated for butyrylcholinesterase (BChE). Theoretical conformational analysis confirms that the completely extended tt conformation of ACh is productive for the hydrolysis by AChE, which agrees with the results of X-ray analysis of AChE. AChE is shown to hydrolyze only those substrates that form equilibrium conformers compatible in the mutual arrangement of trimethylammonium group, carbonyl carbon, and carbonyl oxygen with the tt conformation of ACh; in this case, the rate of substrate hydrolysis depends on the total population of these conformers. A reliable correlation was found between the population of the semifolded (tg-) conformation of the choline moiety of substrate molecules and the rate of their BChE hydrolysis. In a series of CH3-C(O)O-Alk-N+(CH3)3, the rate of BChE hydrolysis is demonstrated to depend on the total population of conformations compatible in the mutual arrangement of functionally important atoms with the tg- conformation of ACh. The tg- conformation of ACh is concluded to be productive for BChE hydrolysis. Similar orientations of the substrate molecules relative to the catalytic triads of both AChE and BChE are proven to coincide upon the substrate productive sorption in their active sites. It is hypothesized that the sorption stage is rate-limiting in cholinesterase hydrolysis and the enzyme hydrolyzes the ACh molecule in its energetically favorable conformation.     

Mechanism of oxygen exchange in actin-activated hydrolysis of adenosine triphosphate by myosin subfragment 1.

The gamma-phosphoryl groups of two intermediates (M-ATP and M-ADP-P1) in the pathway of MgATP hydrolysis by myosin undergo extensive oxygen exchange with water. Actin activates the overall rate of hydrolysis at a rate-limiting step which follows these exchange reactions. Thus, actin, by decreasing the turnover time of hydrolysis, would be expected to proportionately decrease the time available for oxygen exchange. Using subfragment 1 of myosin, the turnover time of hydrolysis can be varied over a wide range by changing the concentration of actin. An estimate for the rate constant of exchange can then be obtained by relating these turnover times to measured values for oxygen exchange (incorporation of 18O from H218O into the inorganic phosphate (Pi) released by hydrolysis). The results of such an experiment, with turnover times between 0.2 and 25 s, indicate that, for each gamma-phosphoryl group, one oxygen from the medium is added rapidly (to cleave the phosphoryl group or form a pentacoordinate phosphroyl complex); two more oxygens exchange with a rate constant, kc, of about 1 s-1; and a fourth oxygen exchanges slowly with ke about 0.2 s-1. The higher value is about 18 times smaller than the rate constant, 5-3, for the reverse cleavage step of the myosin pathway, which is postulated to be responsible for oxygen exchange. The data, then, indicate that the rate-limiting step for oxygen exchange is not k-3, but may be the rate of rotation of oxygens around the phosphorus atom, with one oxygen severely restricted by its binding to the active site. The finding that kc differs for the four oxygens in each phosphate group is related to past observations on myosin-catalyzed oxygen exchange.     

Stereochemistry and lifetime of the GTP hydrolysis intermediate at the active site of elongation factor Tu from Bacillus stearothermophilus as inferred from the 17O-55Mn superhyperfine interaction

Electron paramagnetic resonance spectroscopy has been used to obtain information on the structure and stability of the products of GTP cleavage at the active site of elongation factor Tu (EF-Tu) from Bacillus stearothermophilus. Using stereospecifically labelled (Sp)-(Rp)-[beta-17O]GTP (prepared by modification of a previously published procedure which is now also suitable for guanine nucleotides), it was found that only one of the two possible diastereomers (Sp) led to detectable line-broadening of the EPR spectrum of Mn2+ at the active site of EF-Tu (linewidth 1.5 mT), whereas the Rp isomer caused the same linewidth as unlabelled nucleotide (1.3 mT). From our earlier work and from a demonstration that the lifetime of the state giving the broadened spectrum is too long to be assigned to the EF-Tu.GDP.Mn complex [the rate constant for decay as measured by displacement of GDP by the fluorescent 2'(3')-O-(N-methylanthraniloyl)-GDP is 6.2 x 10(-3) s-1 at 25 degrees C and pH 6.8], we conclude that the broadened signal arises from the EF-Tu.Mn.GDP.Pi complex, the predominant steady-state species. During the hydrolysis of GTP the Mn2+ remains bound to the beta-phosphate oxygen of GDP which arises from the beta pro-S oxygen of GTP, possibly until GDP dissociates and certainly until Pi dissociates. Addition of elongation factor Ts (EF-Ts) to this intermediate leads to rapid reduction of the linewidth to that expected for random distribution of interactions of one 17O and two 16O atoms of GDP with Mn2+, and is not distinguishable from that exhibited by (Rp)-[beta-17O]GTP in the corresponding complex in the presence of EF-Ts.     

Substrate and solvent isotope effects on the fate of the active oxygen species in substrate-modulated reactions of putidamonooxin.

Using 4-methoxybenzoate monooxygenase from Pseudomonas putida, the substrate deuterium isotope effect on product formation and the solvent isotope effect on the stoichiometry of oxygen uptake, NADH oxidation, product and/or H2O2 (D2O2) formation for tight couplers, partial uncouplers, and uncouplers as substrates were measured. These studies revealed for the true, intrinsic substrate deuterium isotope effect on the oxygenation reaction a k1H/k2H ratio of < 2.0, derived from the inter- and intramolecular substrate isotope effects. This value favours a concerted oxygenation mechanism of the substrate. Deuterium substitution in a tightly coupling substrate initiated a partial uncoupling of oxygen reduction and substrate oxygenation, with release of H2O2 corresponding to 20% of the overall oxygen uptake. This H2O2 (D2O2) formation (oxidase reaction) almost completely disappeared when the oxygenase function was increased by deuterium substitution in the solvent. The electron transfer from NADH to oxygen, however, was not affected by deuterium substitution in the substrate and/or the solvent. With 4-trifluoromethylbenzoate as uncoupling substrate and D2O as solvent, a reduction (peroxidase reaction) of the active oxygen complex was initiated in consequence of its extended lifetime. These additional two electron-transfer reactions to the active oxygen complex were accompanied by a decrease of both NADH oxidation and oxygen uptake rates. These findings lead to the following conclusions: (a) under tightly coupling conditions the rate-limiting step must be the formation time and lifetime of an active transient intermediate within the ternary complex iron/peroxo/substrate, rather than an oxygenative attack on a suitable C-H bond or electron transfer from NADH to oxygen. Water is released after the monooxygenation reaction; (b) under uncoupling conditions there is competition in the detoxification of the active oxygen complex between its protonation (deuteronation), with formation of H2O2 (D2O2) and its further reduction to water. The additional two electron-transfer reactions onto the active oxygen complex then become rate limiting for the oxygen uptake rate.     

Two types of kinetic regulation of the activated ATPase in the chloroplast photophosphorylation system

The inhibition by light of chloroplast coupling factor ATPase is not due simply to competing photophosphorylation. This inhibition is only partially relieved by either an arsenate-pool trap for released phosphate, or a pyruvate kinase/phosphoenolpyruvate trap for ADP. Moreover, the amount of product return that does occur in the absence of trapping systems, ascertained by incorporation of 32Pi or [2-3H]ADP back into ATP during the hydrolysis reaction, is insufficient to account for the observed activity decrease. In intermediate pi:H2O oxygen exchange studies, the number of water oxygens incorporated into each molecule of Pi produced does not vary with light intensity during the ATPase assay. This indicates that the light-induced change in ATPase activity is not due to an alteration of rat constants involved in the forward and reverse partitioning of the E.ADP.Pi complex. In contrast, ammonium chloride, an uncoupler of photophosphorylation which stimulates membrane-bound coupling factor ATPase when added after light activation, causes a shift in the pattern of intermediate Pi:H2O oxygen exchange toward a lower number of water oxygens incorporated per Pi formed. The effect of NH4+ consistent with ATPase activity stimulation caused by enhanced partitioning forward of the E.products complex. These observations suggest the operation of two mechanisms of regulation of ATP ase activity during chloroplast de-energization. However, a direct effect of NH4+ on the coupling factor itself, independent of the membrane energization effect, cannot be ruled out by the present studies. Additional oxygen exchange experiments lead to the conclusion that the binding of ATP at a site catalyzing extensive ATP:H2O back exchange in the native chloroplast system ( Wimmer, M. J., and Rose, I. A. (1977) J. Biol. Chem. 252, 6769-6775) is different from the binding of ATP for net hydrolysis in the system activated for ATPase.