Critical review of the swinging crossbridge theory and of the cardinal active role of water in muscle contraction

Contraction-Induced Changes in Hydrogen Bonding of Muscle Hydration Water

Protein-water interaction plays a crucial role in protein dynamics and hence function. To study the chemical environment of water and proteins with high spatial resolution, synchrotron radiation-Fourier transform infrared (SR-FTIR) spectromicroscopy was used to probe skeletal muscle myofibrils. Observing the OH stretch band showed that water inside of relaxed myofibrils is extensively hydrogen-bonded with little or no free OH. In higher-resolution measurements obtained with single isolated myofibrils, the water absorption peaks were relatively higher within the center region of the sarcomere compared to those in the I-band region, implying higher hydration capacity of thick filaments compared to the thin filaments. When specimens were activated, changes in the OH stretch band showed significant dehydrogen bonding of muscle water; this was indicated by increased absorption at ∼3480 cm-1 compared to relaxed myofibrils. These contraction-induced changes in water were accompanied by splitting of the amide I (C=O) peak, implying that muscle proteins transition from α-helix to β-sheet-rich structures. Hence, muscle contraction can be characterized by a loss of order in the muscle-protein complex, accompanied by a destructuring of hydration water. The findings shed fresh light on the molecular mechanism of muscle contraction and motor protein dynamics.

Evaluation of the sliding distance in shortening muscles and in polymerizing actin from Hill's force-velocity equation

The relationship derived earlier between the sliding distance, Deltal(m), and a/P(0), the characteristic parameter of Hill's force-velocity equation for muscle contraction, was re-formulated in order to get a more general relationship which can be applied also to other biological mechano-chemical energy converters: alpha x Deltal(m)=phi (0)(a/P(0))Deltal(m)=-Deltag where Deltag is the free energy change accompanying the hydrolysis of one ATP molecule while alpha and phi (0) are, respectively, the average forces developed by a myosin head-actin complex which are responsible for shortening and for isometric tension generation. These two molecular forces are different in magnitude and in nature and it is demonstrated that alpha , not phi (0), is the true contractile force. The values of alpha and of phi (0) have been calculated for three muscles. The equation has been successfully applied to actin polymerization-based motility. The value of Deltag in different muscles under different environmental conditions can be easily determined from this equation with the value of Deltal(m) derived experimentally.

The alpha-helix, an overlooked molecular motor

At first sight the alpha-helix appears as a rigid scaffold braced by hydrogen bonds nearly parallel to the helix axis. Looked at more closely it turned out to be highly dynamic and able to transform chemical into mechanical energy. The hydrogen bonds are fairly weak and compliant bonds. Their length, usually between 0.267 and 0.291 nm (mean value, 0.28 nm), depends on the interaction of the side chains. The most important strong interaction is the electrostatic repelling force between equally charged side chains (Glu-, Asp-, Lys+, Arg+), well known by experiments with polyamino acids. In proteins with different amino acids, repelling forces between charged side chains work in the axial direction and stretch the hydrogen bonds. Extreme shortening of the hydrogen bonds occurs when ions, e.g., Ca2+, H+, or PO3-, are added and discharge side chains. This means a cooperative pitch decrease of the alpha-helix (pitch range between 0.52 and more than 0.55 nm; mean value, 0.54 nm). This pitch change is absolutely connected by steric reasons with torque generation and torsional rotations, as demonstrated by molecular and tubular alpha-helix models. Thus, charged alpha-helices are molecular motors propelled by the electrostatic energy of added ions. The motor effect is most striking with highly charged alpha-helical coiled coils, e.g., tropomyosin, myosin, and alpha-actinin that can rotate actin filaments by winding and unwinding. For example, the shortening of muscle depends on the sliding (drilling) motion of the Ca2+-activated helical actin filaments into the cross-bridges of the A-band. Here, models are presented for the in vitro sliding of actin filaments and for cytoplasmic streaming by winding and unwinding of myosin chains, and for membrane proteins that contain nonhelical domains between membrane-penetrating alpha-helices. They may transport molecules by the described torsional rotations if they perform supercoiling. Winding and supercoiling can lead to displacement of bound ions and to a feed-back-regulated oscillation between two different coiling stages E1 and E2 that explain "eversion". The models need the torque for 1-2 rotations. They explain active and passive transports, the driving-effects of ion gradients, ATP hydrolysis by unwinding, ATP synthesis by winding up of the supercoils, etc.

The role of aqueous interfaces in the cell

The cell is rich with biopolymeric surfaces. Yet, the role of these surfaces and attendant surface-water interfaces has received little attention among biologists, most of whom consider water as a neutral carrier. This review aims to begin bridging the gap between biology and interface science-to show that a surface-oriented approach has power to bring fresh insights into an otherwise impenetrably complex maze. In this approach the cell is treated as a polymer gel. If the cell is a gel, then a logical approach to the understanding of cell function is through an understanding of gel function. Great strides have been made recently in understanding the principles of polymer-gel dynamics, and particularly the role of the polymer-water interface. It has become clear that a central mechanism in biology is the phase-transition-a major structural change prompted by a subtle change of environment. Phase-transitions are capable of doing work and such work could be responsible for much of the work of the cell. Here, we pursue this approach. We set up a polymer-gel-based foundation for cell behavior, and explore the extent to which this foundation explains how the cell achieves its everyday tasks.

Is the cell a gel--and why does it matter?

That the cell is a gel is broadly acknowledged. Textbooks begin with this assertion-and then proceed with great abandon to derive mechanisms based on free diffusion, as though the gel concept were groundless and cell was an aqueous solution. This disconnect emerges in part because the behavior of gels is not well understood, particularly among most biologists. Recently, great strides have been made in the understanding of gel behavior. It has become clear, for example, that a central mechanism in gel function is the phase-transition-a qualitative structural change prompted by a subtle change of environment, not unlike the transition from ice to water. Phase-transitions are capable of doing work. If the cell is a gel, then a logical approach to understanding cell function is to understand gel function-especially whether some role may be played by the phase-transition. Here we pursue this approach. We first consider the dichotomy of the cell as a gel and the cell as an aqueous solution. We then set up a gel-based foundation for cell behavior, in which the gels' physical chemical features are used to explore how the cell achieves its everyday tasks. If there is a common underlying mechanism of cell function, it appears that the polymer gel phase-transition could well be a candidate.

Muscle force arises by actin filament rotation and torque in the Z-filaments

Actin filament rotation in skeletal muscle is studied by a mechanical model that simulates structure and tension. The four anchoring Z-filaments are twisted around and change the structure of the Z-lattice. The "small square" without twist represents the resting stage of muscle. Torque causes contraction by clockwise rotation (as seen from the Z-band), drilling into the A-band and transition of the "small square" to "basket weave" by increasing the twist and decreasing the torque. Release decreases the torque ("force-depression") by passive clockwise rotation. Stretch causes increased torque ("stretch activation") by passive counterclockwise rotation. Torque arises during Ca(2+)-activation by a conformational change in the highly charged coiled-coils: The four alpha-actinin Z-filaments generate strong torque for the isometric tension. Quick release experiments show that less than one rotation reduces this torque to zero. The 5-12 rotations necessary for isotonic shortening result from torque-generation in the two long tropomyosin coiled-coils. Myosin controls the velocity of active and passive rotations.

Effects of exercise on muscle transverse relaxation determined by MR imaging and in vivo relaxometry

The purpose of this study was to determine the effects of intense exercise on the proton transverse (T(2)) relaxation of human skeletal muscle. The flexor digitorium profundus muscles of 12 male subjects were studied by using magnetic resonance imaging (MRI; 6 echoes, 18-ms echo time) and in vivo magnetic resonance relaxometry (1,000 echoes, 1.2-ms echo time), before and after an intense handgrip exercise. MRI of resting muscle produced a single T(2) value of 32 ms that increased by 19% (P < 0.05) with exercise. In vivo relaxometry showed at least three T(2) components (>5 ms) for all subjects with mean values of 21, 40, and 137 ms and respective magnitudes of 34, 49, and 14% of the total magnetic resonance signal. These component magnitudes changed with exercise by -44% (P < 0.05), +52% (P < 0.05), and +23% (P < 0.05), respectively. These results demonstrate that intense exercise has a profound effect on the multicomponent T(2) relaxation of muscle. Changes in the magnitudes of all the T(2) components synergistically increase MRI T(2), but changes in the two shortest T(2) components predominate.

Implications of quantal motor action in biological systems

We demonstrate in this paper that quantal behavior is a central feature of biological motile and contractile systems. Step-like behavior has been demonstrated in the interaction between single molecules and filaments both in the kinesin-microtubule system and in the myosin-actin filament system. We show here that the step-like molecular features appear also in the single intact sarcomere. We studied single sarcomeres of single bumble-bee myofibrils, both in the unactivated and activated states. Myofibril-length changes induced by a motor-imposed ramp were accompanied by corresponding sarcomere-length changes. However, the sarcomere-length changes were stepwise. Computer analysis of the stepwise shortening patterns revealed a step-size distribution containing multiple peaks. In the activated state, the peaks were separated by 2.7 nm per half-sarcomere which is the linear actin-subunit spacing. Thus, translation steps are an integer multiple of the actin-subunit spacing. This result parallels the one observed in the kinesin-tubulin spacing, where step size is a multiple of the tubulin-subunit spacing. In the muscle system, however, the steps are preserved on a macroscopic scale, implying high synchrony. The quantal steps are easily explained by a model in which the actin filament propels itself over stationary cross-bridges: if actin binds to the cross-bridges between steps, then the observed quantal result is inevitable. As probes of contractile phenomena approach the molecular level, the discrete unitary events underlying contraction begin to emerge. Thus, step-like behavior is observed as the single kinesin molecule translates along the microtubule, as the single myosin molecule translates over the actin filament, and as the single isolated titin molecule is stretched.

Do the bacterial flagellar motor and ATP synthase operate as water turbines?

Despite much progress in the study of the rotary motors ATP synthase (F0F1) and the bacterial flagellar motor, we still cannot answer the most basic and simple question, how the random thermal movement of H+ (or Na+) ions down a pH (or Na+) gradient spins the rotors. I suggest consideration of the possibility that the motors operate like water turbines, i.e., rotation is the outcome of water mini-jets impinging tangentially on the rotors. The vectorial jets are formed when the cations lose part or all of their hydration water upon interacting with a properly positioned site on a protein component which is part of the rotor or is located on a peripheral protein.

Are rotors at the heart of all biological motors?

Biological motors are generally divided into two classes: 1) rotary motors. These include ATP synthase (F0-F1) and the bacterial flagellar motor which are driven by proton and Na+ gradients., 2) linear motors. Myosin, kinesin and dynein are considered to be such motors, F-actin and microtubules serving as passive "tracks". However, data is presented which suggests that the actin filaments rotate in shortening muscle. Microtubules also have been reported to rotate upon interacting with kinesin and dynein. Axial protein rotation thus appears to be a common fundamental characteristic of actin- and of microtubule-based motility systems, in addition to F0-F1 and the bacterial motor. An analysis is carried out of the way ATP hydrolysis and randomly moving protons can induce rotation. It is concluded that all four engines are driven by water jets, thus operating like water turbines.