The effects of solvent viscosity on the kinetic parameters of myosin and heavy meromyosin ATPase

Kramers’ Theory and the Dependence of Enzyme Dynamics on Trehalose-Mediated Viscosity

The disaccharide trehalose is accumulated in the cytoplasm of some organisms in response to harsh environmental conditions. Trehalose biosynthesis and accumulation are important for the survival of such organisms by protecting the structure and function of proteins and membranes. Trehalose affects the dynamics of proteins and water molecules in the bulk and the protein hydration shell. Enzyme catalysis and other processes dependent on protein dynamics are affected by the viscosity generated by trehalose, as described by the Kramers’ theory of rate reactions. Enzyme/protein stabilization by trehalose against thermal inactivation/unfolding is also explained by the viscosity mediated hindering of the thermally generated structural dynamics, as described by Kramers’ theory. The analysis of the relationship of viscosity–protein dynamics, and its effects on enzyme/protein function and other processes (thermal inactivation and unfolding/folding), is the focus of the present work regarding the disaccharide trehalose as the viscosity generating solute. Finally, trehalose is widely used (alone or in combination with other compounds) in the stabilization of enzymes in the laboratory and in biotechnological applications; hence, considering the effect of viscosity on catalysis and stability of enzymes may help to improve the results of trehalose in its diverse uses/applications.

Sucrose increases the activation energy barrier for actin-myosin strong binding

To determine the mechanism by which sucrose slows in vitro actin sliding velocities, V, we used stopped flow kinetics and a single molecule binding assay, SiMBA. We observed that in the absence of ATP, sucrose (880mM) slowed the rate of actin-myosin (A-M) strong binding by 71±8% with a smaller inhibitory effect observed on spontaneous rigor dissociation (21±3%). Similarly, in the presence of ATP, sucrose slowed strong binding associated with Pi release by 85±9% with a smaller inhibitory effect on ATP-induced A-M dissociation, kT (39±2%). Sucrose had no noticeable effect on any other step in the ATPase reaction. In SiMBA, sucrose had a relatively small effect on the diffusion coefficient for actin fragments (25±2%), and with stopped flow we showed that sucrose increased the activation energy barrier for A-M strong binding by 37±3%, indicating that sucrose inhibits the rate of A-M strong binding by slowing bond formation more than diffusional searching. The inhibitory effects of sucrose on the rate of A-M rigor binding (71%) are comparable in magnitude to sucrose's effects on both V (79±33% decrease) and maximal actin-activated ATPase, kcat, (81±16% decrease), indicating that the rate of A-M strong bond formation significantly influences both kcat and V.

Effects of ethylene glycol on the kinetics of contraction on flash photolysis of caged ATP in rat psoas muscle fibres

ATP (1-1.2 mM) was photoreleased from caged ATP (5 mM) in skinned fibres from rat psoas muscle at 15-17 degrees C, to examine the effects of ethylene glycol (EG; 20% in solvent) on the kinetics of isometric contraction. Muscle fibres were stretched by 0.5-2% before photolysis, so that force just before photolysis was almost equal to the steady-state force after photolysis. At the phase of steady-state contraction, force and 500 Hz-stiffness in the presence of EG were 50% and 70% of the controls, respectively, resulting in a higher stiffness-to-force with EG, as reported previously. Following photolysis, force fell before rising to a steady-state plateau. The estimated rate constant of the force decay was approximately 90 s-1, and in the presence of EG was 80-85% of the control. This suggested a small effect of EG on the crossbridge detachment induced by ATP. The rate of force redevelopment was approximately 70 s-1, and EG decreased this rate to 50% of the control. This suggested that EG greatly slows the transition of the crossbridges from the detached state to the reattached force-producing state. The time course of the stiffness signals was consistent with this interpretation. The high stiffness-to-force ratio with EG indicated that EG not only reduces the rate constants which were directly examined in this study but also modifies other aspects of the crossbridge reaction.

The dynamic basis of energy transduction in enzymes

The most important idea underlying our treatment herein is the unity of the enzyme molecule and the medium. Appreciation of this relationship is vital, if enzymology is to graduate from its present reductionistic status to a more holistic posture. Enzymes are biological entities firstly, and isolated objects of physicochemical analysis secondly. Perhaps the most crucial 'biological lesson', particularly apropos of enzymes in intermediary metabolism, concerns the 'cytosociology' of enzyme action in vivo [94,128]. The natural habitat of many enzymes in the living cell is far different from that in bulk aqueous solution in vitro. In order to obtain a real grasp of the nature of enzyme function, one must ultimately couch enzymology in concepts emerging from contemporary cell biology [95]. Notwithstanding, analysis precedes synthesis; and one must needs begin with the individual enzyme molecule. The trenchant efforts of the physical chemist and the organic chemist have produced a wealth of information on the nature of the binding and catalytic events at the enzyme active site. While it is not yet possible to explain precisely the complete sequence of events in the catalytic process, nevertheless, the basic mechanisms by which enzymes effect catalysis (i.e., reduce activation energy) now seem apparent [81,129]. The new frontier is to be found, in exploring the dynamic role of the protein matrix [17]. Not only does the protein provide the 3-D scaffolding for active-site processes, but, more importantly, it serves as the local solvent for the bound chemical subsystem. Thus, the dynamical aspects of enzyme catalysis (for thermally based systems) must arise from the fluctuational properties of the protein molecule. This notion is the common denominator in all of the models in subsection IIC. It is the anisotropic nature of this fluctuational behavior, which would characterize the energy-transduction phenomenon leading to localized catalytic events at the active-site. In Section III we attempted to show that all of the various enzyme models contribute pieces to a single, all-embracing jig-saw puzzle. Some models focus on the dynamical properties of the protein per se, whereas others deal with the stochastic aspects of protein-solvent interaction. The two approaches are complementary, as are mutually interlocking pieces of a puzzle. The ultimate picture depicted by this 'jig-saw puzzle' is still somewhat vague--owing to the present paucity of empirical information on protein motions.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of crossbridge states by ethylene glycol in insect flight muscle

Substitution of ethylene glycol for part of the solvent water changes the mechanical properties, structure and nucleotide binding of glycerol-extracted flight muscle fibres from the waterbug Lethocerus. On addition of ethylene glycol the rigor tension falls, rapidly and reversibly. With increasing glycol concentration the effect saturates at a non-zero tension. The isotonic stiffness is unchanged on adding ethylene glycol. Adding MgAMPPNP (adenylylimidodiphosphate) to a muscle fibre in 50% ethylene glycol causes a further rapid tension fall; above 100 microM AMPPNP the tension reaches zero. The isotonic stiffness of restretched muscle is then close to that of a relaxed fibre. Removal of MgAMPPNP from the bathing medium has no immediate mechanical effect. After several hours the isotonic stiffness rises to some extent; on removal of the glycol both tension and stiffness rise to rigor values within one minute. 3H-Labelled AMPPNP binds to muscle fibres in 50% ethylene glycol in a similar amount to the number of myosin heads present. The binding is tighter than that in aqueous solution and the nucleotide is only released very slowly. Upon removal of the ethylene glycol nucleotide is rapidly released. X-ray diffraction of muscle in 50% ethylene glycol reveals a highly ordered structure, in which both the 14 nm and the 38 nm layer lines are sharply sampled and are of intermediate values between rigor and relaxation. The two inner equatorial peaks are also of intermediate values. On adding MgAMPPNP the pattern resembles that of relaxed muscle. Upon removal of the nucleotide the pattern does not revert towards rigor but on removal of glycol it does. These results are interpreted in terms of changes within the myosin heads and their array within the filament lattice.