Magnesium ion dependent rabbit skeletal muscle myosin guanosine and thioguanosine triphosphatase mechanism and a novel guanosine diphosphatase reaction.

The mechanism of the Mg2+-dependent myosin subfragment 1 catalyzed hydrolysis of GTP and 2-amino-6-mercapto-9-beta-ribofuranosylpurine 5'-triphosphate (thioGTP) has been investigated by rapid-reaction techniques. The myosin was isolated from rabbit skeletal muscle. The steady-state intermediate of these reactions consists pre-dominantly of a protein-substrate complex unlike the myosin subfragment 1 ATPase reaction which has a protein-products complex as the principal steady-state component. The mechanism of GTP hydrolysis catalyzed by subfragment 1 has other marked differences from the ATPase mechanism. The second-order rate constant of binding of GTP to subfragment 1 is tenfold greater than that for GDP binding. The dissociation rate constant of GDP from subfragment 1 is 0.06 s-1 compared with the subfragment 1 catalytic center activity for GTP hydrolysis of 0.5 s-1 at pH 8.0 and 20 degrees C. This shows that GDP bound to subfragment 1 forms a complex which is not kinetically competent to be an intermediate of the GTPase mechanism. GDP is hydrolyzed in the presence of subfragment 1 to GMP and Pi. The subfragment 1 GTPase mechanism has a nuber if features in common with that of the elongation factor Tu GTPase of the protein biosynthetic system of Escherichia coli.     

The covalent modification of myosin's proteolytic fragments by a purine disulfide analog of adenosine triphosphate. Reaction at a binding site other than the active site.

A purine disulfide analog of ATP, 6,6'-dithiobis(inosinyl imidodiphosphate), forms mixed disulfide bonds between the 6 thiol group on the purine ring and certain key cysteines on myosin, heavy meromyosin, and subfragment one. The EDTA ATPase activities of myosin and heavy meromyosin were completely inactivated when 4 mol of thiopurine nucleotide was bound. When similarly inactivated, subfragment one, depending on its method of preparation, incorporated either 1 or 2 mol of thiopurine nucleotide. Modification of a single cysteine on subfragment one resulted in an inhibition of both the Ca2+ and the EDTA ATPase activities, but the latter always to a greater extent. Modification of two cysteines per head of heavy meromyosin had the same effect suggesting that the active sites were not blocked by the thiopurine nucleotides. Direct evidence for this suggestion was provided by equilibrium dialysis experiments. Heavy meromyosin and subfragment one bound 1.9 and 0.8 mol of [8-3H]adenylyl imidodiphosphate per mol of enzyme, respectively, with an average dissociation constant of 5 X 10(-7) M. Heavy meromyosin with four thiopurine nucleotides bound or subfragment one with two thiopurine nucleotides bound retained 65-80% of these tight adenylyl imidodiphosphate binding sites confirming the above suggestion. Thus previous work assuming reaction of thiopurine nucleotide analogs at the active site of myosin must be reevaluated. Ultracentrifugation studies showed that heavy meromyosin which had incorporated four thiopurine nucleotides did not bind to F-actin while subfragment one with one thiopurine nucleotide bound interacted only very weakly with F-actin. Thus reaction of 6,6'-dithiobis(inosinyl imidodiphosphate) at nucleotide binding sites other than the active sites reduces the rate of ATP hydrolysis and inhibits actin binding. It is suggested that these second sites may function as regulatory sites on myosin.     

Cross-linked long-pitch actin dimer forms stoichiometric complexes with gelsolin segment 1 and/or deoxyribonuclease I that nonproductively interact with myosin subfragment 1

Actin dimer cross-linked along the long pitch of the F-actin helix by N-(4-azido)-2-nitrophenyl (ANP) was purified by gel filtration. Purified dimers were found to polymerize on increasing the ionic strength, although at reduced rate and extent in comparison with native actin. Purified actin dimer interacts with the actin-binding proteins (ABPs) deoxyribonuclease I (DNase I) and gelsolin segment-1 (G1) as analyzed by gel filtration and native gel electrophoresis. Complex formation of the actin dimer with these ABPs inhibits its ability to polymerize. The interaction with rabbit skeletal muscle myosin subfragment 1 (S1) was analyzed for polymerized actin dimer and dimer complexed with gelsolin segment 1 or DNase I by measurement of the actin-stimulated myosin S1-ATPase and gel filtration. The data obtained indicate binding of subfragment 1 to actin dimer, albeit with considerably lower affinity than to F-actin. Polymerized actin dimer was able to stimulate the S1-ATPase activity to about 50% of the level of native F-actin. In contrast, the actin dimer complexed to DNase I or gelsolin segment 1 or to both proteins was unable to significantly stimulate the S1-ATPase. Similarly, G1:dimer complex at 20 microM stimulated the rate of release of subfragment 1 bound nucleotide (mant-ADP) only 1.6-fold in comparison to about 9-fold by native F-actin at a concentration of 0.5 microM. Using rapid kinetic techniques, a dissociation constant of 2.4 x 10 (-6) M for subfragment 1 binding to G1:dimer was determined in comparison to 3 x 10 (-8) M for native F-actin under identical conditions. Since the rate of association of subfragment 1 to G1:dimer was considerably lower than to native F-actin, we suspect that the ATP-hydrolysis by S1 was catalyzed before its association to the dimer. These data suggest an altered, nonproductive mode for the interaction of subfragment 1 with the isolated long-pitch actin dimer.     

Distance measurement between the active site and cysteine-177 of the alkali one light chain of subfragment 1 from rabbit skeletal muscle

F?rster energy-transfer techniques have been applied to labeled myosin subfragment 1 from rabbit skeletal muscle to determine an intramolecular distance and whether this distance changes during magnesium-dependent ATPase activity. The alkali one light chain was labeled at Cys-177 with N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine (1,5-IAEDANS) and then exchanged into subfragment 1. High specificity of labeling was indicated by high-performance liquid chromatography analysis of a tryptic digest of the labeled light chain. 2'(3')-O-(2,4,6-Trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP) was bound to the labeled protein at the ATPase active site. The efficiency of energy transfer between the probes was 0.09 when measured by both steady-state and time-resolved fluorescence. Anisotropy measurements of the bound AEDANS indicated considerable freedom of motion of the probe. The probable distance between the probes was 57 A. This distance was unchanged during triphosphatase activity. Two further sites of TNP-ADP interaction with subfragment 1 were found. The effect of these interactions on the energy-transfer measurements was reduced to a minimum by careful choice of reaction conditions.     

Cooperativity between the two heads of rabbit skeletal muscle heavy meromyosin in binding to actin.

An extensive series of experiments in this laboratory has shown that the binding of actin to rabbit skeletal muscle myosin subfragment-1 (a single-headed subfragment) can be described by a two-step model, with formation of a weakly bound complex, the A-state, followed by an isomerization to a more tightly bound complex, the R-state. In this paper, we report on additional experiments comparing the subfragment-1 with heavy meromyosin (a two-headed subfragment). Using a modeling approach, we have quantitated the two-step binding for each of the two heads. This indicates that the binding is cooperative and leads to a more complex view of the acto-myosin interaction than has previously been acknowledged. Implications for the dynamic behavior of the two heads during muscle contraction are discussed.     

Analysis of cloned mRNA sequences encoding subfragment 2 and part of subfragment 1 of alpha- and beta-myosin heavy chains of rabbit heart

Two cardiac myosin heavy chain cDNA clones, pMHC alpha 252 and pMHC beta 174, were constructed using rabbit ventricular mRNA isolated from adult thyrotoxic and normal hearts, respectively. The complete DNA sequences of the 2.2- and 1.4-kilobase inserts of pMHC beta 174 and pMHC alpha 252, respectively, were obtained. The 736 amino acids specified by pMHC beta 174 begin 439 (1.3 kilobases) residues from the heavy chain NH2 terminus and include a 400-amino acid segment of subfragment 1 and the entire subfragment 2 region. Clone pMHC alpha 252 encodes 465 amino acids encompassing all of subfragment 2 and a portion of light meromyosin. Comparison of these two clones revealed extensive sequence overlap which included 1107 nucleotides specifying a 369-amino acid segment corresponding to subfragment 2. Within this region 78 (7%) base and 32 (8.7%) amino acid mismatches were noted. These differences were clustered within discrete regions, with the subfragment 1/subfragment 2 junctional region being particularly divergent. Structural differences between pMHC alpha 252 and pMHC beta 174 indicate that these two clones represent two similar but distinct myosin heavy chain genes whose expression is responsible for ventricular myosin heavy chain isoforms alpha and beta, respectively. The derived amino acid sequences of both clones exhibit extensive homology (greater than 81%) with sequences obtained by direct analysis of adult rabbit skeletal muscle myosin heavy chain protein. The sequences corresponding to the subfragment 2 region are consistent with an alpha-helical conformation with a characteristic 7-residue periodicity in the linear distribution of nonpolar amino acids. Conversely, subfragment 1 sequences specified by pMHC beta 174 suggest a folded highly irregular structure.     

Isolating and localizing ATP-sensitive tryptophan emission in skeletal myosin subfragment 1

The fluorescence intensity difference between rabbit skeletal myosin subfragment 1 (S1) and nucleotide-bound or trapped S1 isolates ATP-sensitive tryptophans (ASTs) emission from the total tryptophan signal. Neutral (acrylamide) quenching of the ASTs is sensitive to the binding or trapping of nucleotide to the active site of S1. Anion (I(-)) quenching of the ASTs, sensitive to charge separation in the tryptophan micro environment, is negligible. These findings suggest the ASTs sense conformational change during ATPase from negatively charged surroundings. Specific chemical modifications of S1 identified the location of the ASTs. Trp131 was quenched by chemical modification, and its emission was isolated by taking the intensity difference between unmodified and modified S1. Trp131 fluorescence intensity and quenching constant do not distinguish among the bound or trapped nucleotides, suggesting that the vicinity of Trp131 does not change conformation during the ATPase cycle and eliminating Trp131 as an AST. Trp510 fluorescence was quenched by 5'-iodoacetamidofluorescein (5'IAF) modification of the reactive thiol (SH1) of S1. The tryptophan emission enhancement increment due to active site trapping decreases linearly with SH1 modification and extrapolates to 0 for 100% modification. These data identify Trp510 as the primary AST in skeletal S1 in agreement with observations from Dictyostelium (Batra and Manstein (1999) Biol. Chem. 380, 1017-1023) and smooth muscle S1 (Yengo et al. (2000) Biophys. J. 78, 242A). With Trp510 identified as the sole AST, fluorescence difference spectroscopy provides a novel means to monitor the concentration of myosin transient intermediates in ATP hydrolysis.     

Structure of the actin-myosin complex produced by crosslinking in the presence of ATP

The structure of the actin-myosin complex during ATP hydrolysis was studied by covalently crosslinking myosin subfragment 1 (S1) to F-actin in the presence of nucleotides (especially ATP) using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The fluorescence energy transfer was measured between N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine and 6-(iodoacetamide)fluorescein bound to the SH1 thiol of S1 and the Cys374 thiol of actin. The covalent acto-S1, produced by crosslinking in the absence of nucleotide or in the presence of ADP, showed transfer efficiency of 0.50 to 0.52 and intersite distance of 4.5 to 4.7 nm, which were equal to those obtained with non-crosslinked acto-S1 in the absence of nucleotide. However, the covalent acto-S1, produced by crosslinking in the presence of either 5'-adenylyl imidodiphosphate (AMPPNP) at high ionic strength or ATP, showed a significant decrease in the efficiency to 0.26 to 0.34 and hence an increase in the distance to 5.2 to 5.5 nm. These results suggest that AM-ATP and/or AM-ADP-P (formed during ATP hydrolysis) and AM-AMPPNP have a very different conformation from AM and AM-ADP (in which A is actin and M is myosin).     

Characterization of purine nucleotide metabolism in primary rat muscle cultures

The synthesis and metabolic fate of purine nucleotides were studied, employing labeled precursors, in primary rat muscle cultures. The cultures were found to produce purine nucleotides, by de novo and salvage pathways, both exhibiting dependence on cellular availability of substrate 5-phosphoribosyl-1-pyrophosphate (PPRibP). Depletion of cellular PPRibP decelerated the rate of purine synthesis, whereas increasing PPRibP generation by high Pi concentration in the incubation medium, accelerated purine synthesis. Ribose accelerated purine synthesis, indicating that ribose 5-phosphate availability in the cultured muscle is limiting for PPRibP synthesis. The study in the muscle cultures of the metabolic fate if IMP formed from [14C]formate and that of nucleotides formed from labeled purine bases, revealed that the main flow in the nucleotide interconversions pathways is from AMP to IMP. The flow from IMP to GMP and to AMP appeared to be of a lesser magnitude and virtually no flow could be detected from GMP to IMP. The greatest proportion of radioactivity of purine nucleotides following synthesis by either de novo or salvage pathways, accumulated in IMP, reflecting the relative rates of flows between the various nucleotides and probably also a relatively low, or inhibited activity of the IMP nucleotidase. The results suggest that primary muscle cultures are a plausible model for the study of the role of purine metabolism in muscle work.     

Investigations of equilibrium complexes of myoxin subfragment 1 with the manganous ion and adenosine diphosphate using magnetic resonance techniques.

Electron paramagnetic resonance spectroscopy and water proton relaxation rate (PRR) measurements were used to characterize a complex formed at the myosin subfragment 1 (S1) ATPase site with stoichiometric amounts of Mn(II) and ADP. In the absence of nucleotide, Mn(II) binding at the active site is very weak, although two other classes of sites for Mn(II) on subfragment 1 were identified which are not directly involved in the ATPase reaction. A high affinity Mn(II) site (termed L-site with KL = 3 muM) is associated with a region of the molecule which is susceptible to proteolysis (probably the LC2 light chain subunit) since its stoichiometry depends on the conditions employed for the preparation of subfragment 1 during the papain treatment of myosin. In addition there are a number of weak sites for Mn(II) (termed N-sites) probably associated with anionic groups on the surface of the molecule. In order to study the properties of Mn(II) and ADP bound at the active site by magnetic resonance techniques, subfragment 1 preparations virtually free of the L-site were used, since such an ancillary site competes for the available Mn(II). MnADP binds to subfragment 1 with an apparent dissociation constant, KT, of about 4 muM at 25 degrees. The resultant complex, S1-MnADP, has a low PRR enhancement factor (1.7 at 24.3 MHZ), and its frequency (magnetic field) dependence indicates that this is because there are no readily exchangeable water molecules within the first coordination sphere of Mn(II. Relaxation of the bulk solvent is mediated by protons bound transiently within the outer spheres (4 to 7 A) of the Mn(II). A nitroxide spin label attached to the reactive thiol group of subfragment 1 enhances the solvent PRR, and this property is sensitive to the binding of MgADP to the active site. However, no dipolar spin-spin interaction was detected between the nitroxide group and Mn(II) in the S1-MnADP complex, indicating that the metal ion and thiol group are well separated.     

Purine Nucleoside Phosphorylase Activity in Rat Cerebrospinal Fluid

The sequential hydrolysis of purines is present in rat CSF and generates nucleosides as inosine and guanosine that are usual substrates for purine nucleoside phosphorylase (PNP). PNP catalyzes phosphorolysis of the purine nucleosides and deoxynucleosides releasing purine bases. Here we investigated the presence of PNP in CSF of rats using: i) a specific chromophoric analogue of nucleosides, 2-amino-6-mercapto-7-methylpurine ribonucleoside (MESG), and ii) an inhibitor of PNP activity, immucillin-H. Additionally, we performed a preliminary kinetic characterization (K(M): Henry-Michaelis-Menten constant; V: maximal velocity) for MESG and inorganic phosphate (Pi). The values of K(M) and V for MESG (n = 3, mean+/-SD) were 142.5+/-29.5 microM and 0.0102+/-0.0006 U mg(-1), respectively. For Pi (n=3, mean+/-SD), the K(M) values and V were 186.8+/-43.7 microM and 0.0104+/-0.0016 U mg(-1), respectively. The results indicated that PNP is present in rat CSF and provided a preliminary kinetic characterization.     

Molecular arrangement of troponin-T in the thin filament.

1. Chymotrypsin cleaved troponin-T of skeletal muscle into two subfragments, i.e., troponin-T1 and -T2, each of which could be isolated by the use of DEAE-Sephadex. Troponin-T1 was a single subfragment with a molecular weight of 26,000 (chicken) or 22,000 (rabbit) daltons. Troponin-T2 consisted of two subfragments with molecular weights of about 13,000 daltons. Results obtained indicated that the smaller subfragment was formed by digestion of the larger subfragment of troponin-T2. 2. Antibodies against troponin-T1 and -T2 formed regular transverse striations along the whole length of thin filaments with 38 nm intervals, as was found reviously using antibodies against whole troponin complex as well as troponin components (Ohtsuki, I. et al., 1967; Ohtsuki, I. 1974 and 1975). 3. The first anti-troponin-T1 striation was situated 40 nm from the top of the filament. The first anti-troponin-T2 striation was 27 nm from the filament top and coincided with the first striations formed by antibodies against troponin-C or -I. 4. Troponin-T1 and the larger subfragment of troponin-T2 bound to tropomyosin which had been coupled to Sepharose, whereas the smaller subfragment of troponin-T2 did not.     

Conformation and dynamics of the SH1-SH2 helix in scallop myosin

Atomic structures of scallop myosin subfragment 1(S1) with the bound MgADP, MgAMPPNP, and MgADP.BeF(x) provide crystallographic evidence for a destabilization of the helix containing reactive thiols SH1 (Cys703) and SH2 (Cys693). A destabilization of this helix was not observed in previous structures of S1 (from chicken skeletal, Dictyostelium discoideum, and smooth muscle myosins), including complexes for which solution experiments indicated such a destabilization. In this study, the factors that influence the SH1-SH2 helix in scallop S1 were examined using monofunctional and bifunctional thiol reagents. The rate of monofunctional labeling of scallop S1 was increased in the presence of MgADP and MgATPgammaS but was inhibited by MgADP.V(i) and actin. The resulting changes in ATPase activities of S1 were symptomatic of SH2 and not SH1 modification, which was confirmed by mass spectrometry analysis. With bifunctional reagents of various lengths, cross-linking did not occur on a short time scale in the absence of nucleotides. In the presence of MgADP, cross-linking was greatly enhanced for all of the reagents. These reactions, as well as the formation of a disulfide bond between SH1 and SH2, were much faster in scallop S1.ADP than in rabbit skeletal S1.ADP and were rate-limited by the initial attachment of the reagent to scallop S1. The cross-linking sites were mapped by mass spectrometry to SH1 and SH2. These results reveal isoform-specific differences in the conformation and dynamics of the SH1-SH2 helix, providing a possible explanation for destabilization of this helix in some scallop S1 but not in other S1 isoform structures.     

Properties of porcine platelet myosin. I. Similarity between vertebrate smooth muscle and nonmuscle myosins in their binding properties with F-actin

The mechanism of the ATPase [EC 3.6.1.3] reaction of porcine platelet myosin and the binding properties of platelet myosin with rabbit skeletal muscle F-actin were investigated. The kinetic properties of the platelet myosin ATPase reaction, that is, the rate, the extent of fluorescence enhancement of myosin, the size of the initial P1 burst of myosin, and the amount of nucleotides bound to myosin during the ATPase reaction, were very similar to those found for other myosins. Strong binding of platelet myosin with rabbit skeletal muscle F-actin, as found for smooth muscle myosin, was suggested by the following results. The rate of the ATP-induced dissociation of hybrid actomyosin, reconstituted from platelet myosin and skeletal muscle F-actin, was very slow. The amount of ATP necessary for complete dissociation of hybrid actomyosin was 2 mol/mol of myosin, although skeletal muscle actomyosin is known to dissociate completely upon addition of 1 mol ATP per mol of myosin. Unlike skeletal muscle myosin, the EDTA(K+)-ATPase activity of platelet myosin was inhibited by skeletal muscle F-actin. These observations indicate that ATP hydrolysis by vertebrate nonmuscle myosin follows the same mechanism as with other myosins and that the binding properties of nonmuscle myosin with F-actin are similar to those of smooth muscle myosin but not to those of skeletal muscle myosin.     

Re-evaluation of the role of thiol groups in rabbit muscle aldolase A

Exposed thiol groups of rabbit muscle aldolase A were modified by 5,5'-dithiobis(2-nitrobenzoic) acid with concomittant loss of enzyme activity. When 5-thio-2-nitrobenzoate residues bound to enzyme SH groups were replaced by small and uncharged cyanide residues the enzyme activity was restored by more than 50%. The removal of a bulky C-terminal tyrosine residue from the active site of aldolase A resulted in enzyme which was inhibited by 5,5'-dithiobis(2-nitrobenzoic) acid only by 50% and its activity was nearly unchanged after modification of its thiol groups with cyanide. The results obtained show directly that rabbit muscle aldolase A does not possess functional cysteine residues and that the inactivation of the enzyme caused by sulfhydryl group modification reported previously can be attributed most likely to steric hindrance of a catalytic site by modifying agents.     

Characterization of the properties of ethenoadenosine nucleotides bound or trapped at the active site of myosin subfragment 1

The fluorescent nucleotide analogue of ADP, 1,N6-ethenoadenosine diphosphate (epsilon ADP), has been used to probe the active site of myosin subfragment 1 (SF1). The Mg complex of ADP was shown to be trapped stoichiometrically at the active site by a variety of thiol cross-linking agents having sulfur to sulfur spanning lengths of 2-14 A. Previous studies [Wells, J. A., & Yount, R. G. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 4966-4970] had suggested ADP was trapped by direct closure of a postulated active site cleft by cross-linking two activity critical thiols, SH1 and SH2. This model was tested by measuring the polarization of trapped and reversibly bound epsilon ADP, the off-rate of trapped epsilon ADP, and the solute quencher accessibility of trapped epsilon ADP on SF1 modified with thiol cross-linking agents of different spanning lengths. The lack of correlation of all of these properties with the length of the cross-linking span suggests that trapping occurs by indirect stabilization of a conformation favoring bound nucleotides rather than by sterically preventing the release of nucleotide. Measurement of the fluorescent properties of epsilon ADP bound to SF1 vs. epsilon ADP free gave a 20% increase in emission intensity, a 7-nm blue shift in the emission maximum, and a 70% increase in the absorbance at the excitation wavelength (330 nm). Trapping of epsilon ADP by the thiol cross-linking agent p-phenylenedimaleimide gave a further 24% increase in emission intensity. This change was shown to be the result of an increase in absorbance of trapped epsilon ADP at 330 nm rather than an increase in the quantum yield.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic studies on turtle pancreatic ribonuclease: a comparative study of the base specificities of the B2 and P0 sites of bovine pancreatic ribonuclease A and turtle pancreatic ribonuclease

Kinetic constants for the transesterification of eight dinucleoside phosphates CpX and UpX by bovine and turtle pancreatic ribonuclease were determined. Both ribonucleases have a preference for purine nucleotides at the position X. However, bovine ribonuclease, like other mammalian ribonucleases, prefers 6-amino bases at this site, while turtle ribonuclease prefers 6-keto bases. This difference in specificity at the B2 site may be explained by the substitution of glutamic acid at position 111 by valine in turtle ribonuclease. These results have been confirmed by inhibition studies with the four nucleoside triphosphates. Inhibition studies with pT and pTp showed that a cationic binding group (P0) for the 5'-phosphate of the pyrimidine nucleotides bound at the primary B1 site is present in turtle ribonuclease, although lysine at position 66 in bovine ribonuclease is absent in turtle ribonuclease. However, the side chain of lysine 122 in turtle ribonuclease is probably located in the correct position to take over the role as cationic P0 site.     

Transient kinetic and isotopic tracer studies of the myosin adenosine triphosphatase reaction

From transient kinetic studies of the Mg²⁺-dependent adenosine triphosphatase of myosin subfragment 1, prepared from rabbit skeletal muscle, a seven-step mechanism has been proposed. Features of this mechanism include two-step processes for ATP and ADP binding in which the binary complex isomerizes in addition to a rapid nucleotide association step. In the case of ATP a large negative standard free energy change is associated with the isomerization. An overall rate-limiting isomerization of the myosin-product complex prior to product release has been identified. Studies on the mechanism of cleavage of ATP bound to the active site indicate the process is readily reversible and can account for the observation that more than one oxygen of the product phosphate arises from water. This proposal has been substantiated by the finding that the oxygen atoms of the γ-phosphoryl group of bound ATP also undergo extensive exchange with water.     

Reaction mechanism of the magnesium ion-dependent adenosine triphosphatase of frog muscle myosin and subfragment 1.

The Mg2+-dependent ATPase (adenosine 5'-triphosphatase) mechanism of myosin and subfragment 1 prepared from frog leg muscle was investigated by transient kinetic technique. The results show that in general terms the mechanism is similar to that of the rabbit skeletal-muscle myosin ATPase. During subfragment-1 ATPase activity at 0-5 degrees C pH 7.0 and I0.15, the predominant component of the steady-state intermediate is a subfragment-1-products complex (E.ADP.Pi). Binary subfragment-1-ATP (E.ATP) and subfragment-1-ADP (E.ADP) complexes are the other main components of the steady-state intermediate, the relative concentrations of the three components E.ATP, E.ADP.Pi and E.ADP being 5.5:92.5:2.0 respectively. The frog myosin ATPase mechanism is distinguished from that of the rabbit at 0-5 degrees C by the low steady-state concentrations of E.ATP and E.ADP relative to that of E.ADP.Pi and can be described by: E + ATP k' + 1 in equilibrium k' - 1 E.ATP k' + 2 in equilibrium k' - 2 E.ADP.Pi k' + 3 in equilibrium k' - 3 E.ADP + Pi k' + 4 in equilibrium k' - 4 E + ADP. In the above conditions successive forward rate constants have values: k' + 1, 1.1 X 10(5)M-1.S-1; k' + 2 greater than 5s-1; k' + 3, 0.011 s-1; k' + 4, 0.5 s-1; k'-1 is probably less than 0.006s-1. The observed second-order rate constants of the association of actin to subfragment 1 and of ATP-induced dissociation of the actin-subfragment-1 complex are 5.5 X 10(4) M-1.S-1 and 7.4 X 10(5) M-1.S-1 respectively at 2-5 degrees C and pH 7.0. The physiological implications of these results are discussed.