The reaction of myosin with N-ethylmaleimide in the presence of ADP

Myosin reacted with N-ethylmaleimide in the presence of ADP lost its ability to be activated by actin. Subfragment 1 behaved similarly. About 2 moles of N-ethylmaleimide per mole of Subfragment 1 were required to eliminate actin activation of the Mg²⁺-ATPase activity. At the point at which actin activation was lost the K⁺-EDTA-ATPase activity was also lost, but the Ca²⁺-activated ATPase activity was increased. Kinetic measurements indicated that the labelling with N-ethylmaleimide in the presence of ADP reduced V (the ATPase activity at infinite actin concentration) but did not effect K_app (which is related to the dissociation constant of the actin-Subfragment 1 complex). The Mg²⁺-activated activity of the reacted myosin alone remained unaltered and the ability to bind actin was retained. We propose that the N-ethylmaleimide labelling blocked the actin activation by preventing the accelerated release of hydrolysis products from the myosin.     

Comparison of the reaction of N-ethylmaleimide with myosin and heavy meromyosin subfragment 1

Myosin reacted at low ionic strength with NEM forms an actomyosin which is Ca⁺⁺ insensitive. With HMM S-1 the reaction with NEM causes a marked loss of the actin activated ATPase activity and the Ca⁺⁺ sensitivity is reduced but not eliminated. The presence of actin during the sulfhydryl reaction does not significantly alter this result. HMM S-1 prepared from myosin previously desensitized by NEM regains Ca⁺⁺ sensitivity. These results indicate that the conformations of myosin and HMM S-1 are different and could reflect a difference between insoluble (filamentous) myosin and myosin, or its fragments, in solution.     

Dynamic interaction between actin and myosin subfragment 1 in the presence of ADP

The equilibrium and dynamics of the interaction between actin, myosin subfragment 1 (S1), and ADP have been investigated by using actin which has been covalently labeled at Cys-374 with a pyrene group. The results are consistent with actin binding to S1.ADP (M.D) in a two-step reaction, A + M.D K1 equilibrium A-M.D K2 equilibrium A.M.D, in which the pyrene fluorescence only monitors the second step. In this model, K1 = 2.3 X 10(4) M-1 (k+1 = 4.6 X 10(4) M-1 s-1) and K2 = 10 (k+2 less than or equal to 4 s-1); i.e., both steps are relatively slow compared to the maximum turnover of the ATPase reaction. ADP dissociates from both M.D and A-M.D at 2 s-1 and from A.M.D at greater than or equal to 500 s-1; therefore, actin only accelerates the release of product from the A.M.D state. This model is consistent with the actomyosin ATPase model proposed by Geeves et al. [(1984) J. Muscle Res. Cell Motil. 5, 351]. The results suggest that A-M.D cannot break down at a rate greater than 4 s-1 by dissociation of ADP, by dissociation of actin, or by isomerizing to A.M.D. It is therefore unlikely to be significantly occupied in a rapidly contracting muscle, but it may have a role in a muscle contracting against a load where the ATPase rate is markedly inhibited. Under these conditions, this complex may have a role in maintaining tension with a low ATP turnover rate.     

Location of the sites of reaction of N-ethylmaleimide in papain and chymotryptic fragments of the gizzard myosin heavy chain

The thiol of the gizzard myosin heavy chain, which reacts most rapidly with N-ethylmaleimide (MalNEt), has been located in the subfragment 2 region of myosin rod by fragmentation of [14C]-MalNEt-labeled myosin with papain and chymotrypsin. MalNEt reacts more slowly with thiols present in the 70- and 25-kilodalton (kDa) papain fragments of subfragment 1. The reaction of MalNEt with thiols present in these regions is increased on addition of ATP by factors of 2 and 10, respectively, when myosin is modified in 0.45 M NaCl where it is present in the extended, 6S conformation. The rate of increase of Mg2+-activated adenosinetriphosphatase (ATPase) activity, which reflects the loss of ability of myosin to assume the folded, 10S conformation, and the rate of loss of K+-EDTA-activated activity produced by MalNEt are both accelerated 5- to 10-fold on addition of ATP. The rates at which ATPase activities change agree closely to the reaction rates of MalNEt with the 25-kDa region of subfragment 1; therefore, the changes in these activities can be attributed to modification of a thiol of the 25-kDa segment. An increase in actin-activated ATPase activity produced by reaction of myosin with MalNEt in 0.45 M NaCl is accelerated by ATP by a factor of at least 4. Reaction with [14C]MalNEt in the presence of MgATP and 0.2 M NaCl, where myosin is in the 10S form, inhibits the incorporation of radioactive MalNEt into the 25-kDa papain fragment of subfragment 1. It also prevents the increase in actin-activated ATPase activity and preserves the ability of myosin to assume the 10S form.     

Calorimetric studies of the interaction of myosin with ADP.

A calorimetric titration method was used to study ADP binding to native myosin. Data were analyzed by assuming that the myosin molecule has n independent and identical sites for ADP binding. The enthalpy change (deltaH), the binding constant (K), and n were determined. In 0.5 M KCl, 0.01 M MgCl2, and 0.02 M Tris/HCl (pH 7.8), we found: at 0 degrees, deltaH = -57.1 +/- 3.2 kJ-mol-1, log K = 6.42 +/- 0.13, n = 1.49 +/- 0.07; at 12 degrees, deltaH = 73.1 +/- 3.2 kJ-mole-1, log K = 6.08 +/- 0.13, and n = 1.74 +/- 0.07. The average heat capacity change on ADP binding to myosin between 0 and 12 degrees is thus -1.4 +/- 0.4 kJ-mol-1-K-1. Reasonably consistent results were obtained at 25 degrees, suggesting ADP binding to myosin is as strongly exothermic as at lower temperatures, although further interpretation of this result seems unwarranted, mainly because of the instability of myosic at this temperature. The number of protons released on binding of ADP to myosin was determined in separate experiments. The value was 0.19 +/- 0.02 at both 0 and 12 degrees. The reaction of protons with Tris thus contributes about -9.5 kJ-mol-1 to the observed heat on ADP binding.     

Effect of N-ethylmaleimide on Ca-inhibition of Physarum myosin

We have established a quick method for preparing Physarum myosins whose actin-activated ATPase activities are inhibited by microM levels of Ca2+ (from plasmodial stage: Kohama, K. & Kendrick-Jones, J. (1986) J. Biochem. 99, 1433-1446; and from amoebal stage: Kohama, K., Takano-Ohmuro, H., Tanaka, T., Yamaguchi, Y., & Kohama, T. (1986) J. Biol. Chem. 261, 8022-8027). N-Ethylmaleimide alkylates sulfhydryl (SH) groups on the heavy chains in the heads of the plasmodial myosin. The actin-activated ATPase activity of the modified myosin was significantly decreased when assayed in low Ca2+ concentrations. Moreover, the activity remained low even when the Ca2+ concentrations was increased, i.e., the myosin was desensitized. For complete desensitization, about 4 mol SH per mol myosin (500,000 Mr) must be modified. These residues are probably the "reactive thiols" which have been predicted from primary structure studies to be conserved among myosins of higher and lower eukaryotes. Ultraviolet absorption spectra of the modified and intact myosins showed a peak at 277 nm. The height of this peak in intact myosin was reduced when the Ca2+ concentration was increased. This Ca-induced reduction was hardly detectable in the modified myosin although Ca-binding activity to myosin did not appear to be affected by the modification. We interprete these results that Ca2+ may change the conformation of the myosin heavy chain by binding to myosin and speculate that impairment of this process upon modification could cause the desensitization to Ca2+ in the ATPase activity.     

The mechanism of skeletal muscle myosin ATPase. Interaction of myosin active center with ATP and with ADP

The kinetics of the fluorescence enhancement and the transient release of H+ caused by the binding of ADP to the active center of myosin has been compared to that caused by myosin-ATP interaction. The results show that both the time courses of the fluorescence enhancement and the transient H+ release caused by ADP binding, like that caused by ATP hydrolysis in the initial burst, are monophasic exponential processes. The fact that the rates of these two processes are also equal suggests that they both reflect the same mechanistic event in the mechanism of ADP binding. The kinetics of ADP binding as measured by the fluorescence enhancement and the H+ release is different from that of ATP. This is in agreement with our previous finding that the enhancement of fluorescence and the transient release of H+, in the case of ATP, reflect the initial burst of ATP hydrolysis, whereas in the case of ADP, they represent a conformational change in the myosin-ADP complex. The magnitude of the H+ transient caused by the initial burst is approximately equal to that caused by ADP binding. The amplitude of the fluorescence enhancement caused by ADP binding is equal to one-third of that caused by the initial burst.     

The reaction of bovine glutamate dehydrogenase with periodate-oxidised ADP

1. The reactive analogue oADP produced by periodate oxidation of ADP has been studied as a potential affinity label for the enzyme bovine glutamate dehydrogenase, using circular dichroism (CD) difference spectroscopy to monitor specific binding. 2. The analogue binds stoichiometrically, rapidly and reversibly to the adenine nucleotide binding site with Kd approximately equal to 12 microM (20 degrees C, pH 7) with characteristic intensification of the adenine nucleotide CD at 260 nm. 3. This complex is unstable and decays with a half-life of about 1.5 h; the analogue then becomes attached as a Schiff base to a number of subsidiary sites, including the enzyme active site, with partial inactivation of the enzyme. 4. Depending upon initial concentration of oADP, the enzyme activity is progressively lost during the slow reaction; following borohydride reduction, up to four molecules of analogue are bound/subunit. 5. Protection against loss of enzyme activity is afforded by the coenzyme NAD+ plus glutarate or L-hydroxyglutarate (an effective inhibitor), or by glutarate alone, but not by NAD+ alone. 6. Spectroscopic and protection studies indicate that after the decay of the specific CD signal, the enzyme retains the capacity to bind ADP, but that this is progressively lost in parallel with decay of enzymic activity. 7. The results are consistent with proximity or functional interaction between the adenine nucleotide site and the coenzyme binding portion of the active site. 8. Thus oADP does not act as a true affinity label for the adenine nucleotide binding site, but the reaction subsequent to binding at that site shows some specificity directed towards the active site.     

The binding of Mn2+ and ADP to myosin

The metal ion requirement of myosin-ADP binding was investigated by use of Mn²⁺. Mn²⁺ binds to two sets of noninteracting sites on myosin which are characterized by affinity constants of 10⁶ and 10³, M⁻¹ at 0.016 M KCl concentration. The maximum number of sites is 2 for the high affinity and 20–25 for the low affinity set. Binding of Mn²⁺ to the high affinity sites increases the affinity of ADP binding to myosin. F-actin inhibits ADP binding (Kiely, B., and Martonosi, A., Biochim. Biophys. Acta 172: 158–170 [1969]), but even at F-actin concentrations much higher than that required to saturate the actin binding sites of myosin or its proteolytic fragments, significant ADP binding remained. The actin insensitive portion of ADP binding was inhibited by 10⁻⁴ M inorganic pyrophosphate or ATP. The results are discussed on the basis of a model in which actin and ADP bind to myosin at distinct but interacting sites.     

ADP inhibition of myosin V ATPase activity

The kinetic mechanism of myosin V is of great interest because recent evidence indicates that the two-headed myosin V molecule functions as a processive motor, i.e., myosin V is capable of moving along an actin filament for many catalytic cycles of the motor without dissociating. Three recent publications assessing the kinetics of single-headed myosin V provide different conclusions regarding the mechanism, particularly the rate-limiting step of the cycle. One study (, Proc. Natl. Acad. Sci. USA. 96:13726-13731) identifies ADP release as the rate-limiting step and provides a kinetic explanation for myosin V processivity. The others (, J. Biol. Chem. 274:27448-27456;, J. Biol. Chem. 275:4329-4335) do not identify the rate-limiting step but conclude that it is not ADP release. We show experimental and simulated data demonstrating that the inconsistencies in the reports may be due to difficulties in the measurement of the steady-state ATPase rate. Under standard assay conditions, ADP competes with ATP, resulting in product inhibition of the ATPase rate. This presents technical problems in analyzing and interpreting the kinetics of myosin V and likely of other members of the myosin family with high ADP affinities.     

The reaction of N-ethylmaleimide at the active site of succinate dehydrogenase.

Since 1938 mammalian succinate dehydrogenase has been thought to contain thiol groups at the active site. This hypothesis was questioned recently, because irreversible inhibition by bromopyruvate and N-ethylmaleimide appeared not to satisfy the requisite criteria for reaction at the active site. These recent observations of incomplete inactivation of succinate dehydrogenase by N-ethylmaleimide and incomplete protection by substrates can, however, be explained adequately by the presence of oxalacetate and other strong competitors of the inactivation process in the enzyme used in these studies. Substrates, competitive inhibitors, and anions which activate succinate dehydrogenase protect the enzyme from inhibition by N-ethylmaleimide. Inhibition of succinate dehydrogenase by N-ethylmaleimide involves at least two second order reactions which are pH dependent, with pKa values of 8.0 to 8.2. This pH dependence, the known reactivity of N-ethylmaleimide toward thiols, and the protection by substrate and competitive inhibitors indicate that sulfhydryl residues are required for catalytic activity and perform an essential, not secondary, role in the catalysis. Just as the presence of tightly bound oxalacetate prevents inhibition by N-ethylmaleimide, alkylation of the sulfhydryl residue(s) at the active site prevents the binding of [14C]oxalacetate. Thus, these thiol groups at the active site also may be the site of tight binding of oxalacetate during the activation-deactivation cycle.