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Biomedical subjects

F Oosawa

Publications and source records attributed to F Oosawa.

At least 19 recordsLinked to original sources
PubMed2001

Spontaneous signal generation in living cells.

Living cells often generate signals spontaneously in the absence of external stimuli. Those signals play an important role in their tactic behaviors. This paper presents a theoretical treatment on the mechanism of spontaneous signal generation. The mechanism consists of two steps: (1) production of the basic fluctuation of the intracellular electric potential due to the open-closed fluctuation of the gates of ion channels and (2) generation of a spike-like fluctuation of potential depending on the positive shift of the basic fluctuation. The first step is described by an equation of the Langevin type, where the random force is proportional to the circulating ion current across the membrane; the average of the square of the random force is proportional to the rate of free-energy consumption by the current. The second step is described by a rate equation of transition of field-sensitive channel gates which contains the fluctuating electric field in the exponential term. There, the fluctuation has a nonlinear effect. Such a two-step process may work in various kinds of living cells. The presence of circulating ion current in the resting state is a most important key. Some cells may be quiet and some cells may be active to generate spontaneous signals.

Animals↗
PubMed2000

The loose coupling mechanism in molecular machines of living cells.

Living cells have molecular machines for free energy conversion, for example, sliding machines in muscle and other cells, flagellar motors in bacteria, and various ion pumps in cell membranes. They are constructed from protein molecules and work in the nm (nanometer), pN (piconewton) and ms (millisecond) ranges, without inertia. In 1980s, a question was raised of whether the input-output or influx-efflux coupling in these molecular machines is tight or loose, and an idea of loose coupling was proposed. Recently, the long-distance multistep sliding of a single myosin head on an actin filament, coupled with the hydrolysis of one ATP molecule, was observed by Yanagida's group using highly developed techniques of optical microscopy and micromanipulation. This gave direct evidence for the loose coupling between the chemical reaction and the mechanical event in the sliding machine. In this review, I will briefly describe a historical overview of the input-output problem in the molecular machines of living cells.

Actins↗
PubMed1995

Sliding and ATPase.

Sliding machines composed of F-actin and myosin or microtubules and kinesin or dynein convert the free energy of ATP hydrolysis into sliding movements and mechanical work. Development of optical microscopy with micromanipulation techniques has made possible direct observation of single events of sliding exhibited by single sliding machines. The experimental data and theoretical consideration suggest that the influx-efflux coupling in these machines may be loose. Specific characters of sliding machines are summarized and the problems we have for understanding of the coupling mechanism are discussed.

Actins↗
PubMed1993

Physical chemistry of actin: past, present and future.

History of actin research is reviewed with special emphasis on dynamics of the G-F transformation and flexibility or intrafilamentous mobility of F-actin. Good correlation was found between the flexibility of F-actin and its activity in cell motility. In molecular machines such as the flagellar motor and the sliding machine of F-actin and myosin, the coupling between influx and efflux seems to be loose. F-actin would assume multiple active states during sliding on myosin with hydrolysis of ATP. Recently, the three-dimensional structure of actin molecule in crystals has been determined. Actin research is expected to give an answer to the question on the physiological significance of internal mobility of protein molecules and their assemblies and the structural origin of such mobility.

Actins↗
PubMed1990

Protein motors and Maxwell's demons: does mechanochemical transduction involve a thermal ratchet?

This paper represents a preliminary effort in considering how protein motors could harness thermal fluctuations to generate force and movement. The initial premise for this model is the thermal motor described by Feynman which consists of a ratchet and an interdigitating, spring-loaded pawl. By analogy, one can imagine that biological motors interact weakly with their filament subunit substrate and that thermal fluctuations displace the motor to adjacent subunits on the filament. Unidirectional motion ensues if ATP energy either changes the energy of a spring-like component in the system or asymmetrically alters the energy barrier to displacement. Although a simple thermal ratchet model can account for the maximal forces and velocities produced by biological systems, it does not adequately explain the force produced and the energy expended by muscle as a function of its velocity of shortening. To explain these phenomena, we propose that the energy barrier of the thermal ratchet changes as a function of load. A load or velocity-dependency in the transition of the motor from a weak to a strong binding state could produce this effect. The thermal ratchet model for energy transduction can also explain many of the observations of filament translocation along motor-covered surfaces in the in vitro motility assay. Furthermore, unlike the rotating cross-bridge model which predicts a large conformational change in the motor, unidirectional motion and force production with a thermal ratchet motor could be accomplished through small structural alterations in the motor head and the filament subunit. In general, models are useful if they formulate a set of predictions that serve as guides for future experimentation. First of all, it should be instructive to examine more critically the contact between motors and filaments and to determine the quantal episodes of displacement and force. Furthermore, it will be important to inspect the filament, in addition to the motor, for conformational changes that occur throughout the ATPase cycle. Along this line, the ATP hydrolysis reaction should be reexamined to determine if a large number of energy states of the motor protein and the filament occur during the hydrolysis cycle. Furthermore, our model makes predictions regarding the relationships between the energy barrier height (related perhaps to motor-filament binding affinity) and velocity or force. Whether the weak to strong binding state transition is dependent upon load or velocity is another unique prediction of our model that could be experimentally probed.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphatases↗
PubMed1986

The loose coupling mechanism in molecular machines of living cells.

For a bacterial flagellar motor driven by a proton flux, a loose coupling mechanism has been proposed in which the movement of the proton is indirectly and loosely coupled with the rotation of the motor. This mechanism assures the efficient and smooth conversion of both electrical and chemical potential energies of the proton of the same order as the energy of the thermal fluctuation. Loose coupling has been also assumed for the proton ATPase. A proton flux produces a rotational movement of protein molecules and this movement promotes the synthesis of ATP. In the proposed mechanism, the number of protons necessary for the synthesis of one ATP molecule is not an integer but varies depending on the environmental condition. In the case of muscle, the coupling between the hydrolysis of ATP and the shortening was found to be extremely loose. It is likely that molecular machines in living cells often adopt a loose coupling mechanism in which the chemical reaction and the physical cycle have not always a definite one-to-one correspondence.

Adenosine Triphosphate↗
PubMed1985

Amiloride-sensitive Na+-H+ antiporter in Escherichia coli.

In everted vesicles of Escherichia coli, delta pH caused by H+ efflux through the Na+/H+ antiporter was measured by using a fluorescent dye. Amiloride inhibited the activity of the Na+/H+ antiporter. Kinetic studies showed that amiloride competed with Na+. The inhibition constant of 40 microM was obtained.

Amiloride↗
PubMed1984

Ca2+-dependent regulation of beat frequency of cilia in Paramecium.

Triton-extracted models of Paramecium cells prepared in the presence of Mg2+ and EGTA showed Ca2+ sensitivity not only in the direction of beat but also in the beat frequency of cilia. The beat frequency increased over a range of concentration of Ca2+ from 2 X 10(-7) to 4 X 10(-7) M, in which the model swam forwards. The frequency increased also above 10(-6)M-Ca2+, in which the model swam backwards. The increase in frequency was inhibited by calmodulin antagonists. Intracellular injection of a Ca2+ buffer giving a free Ca2+ concentration of 2 X 10(-7) to 5 X 10(-7) M in intact cells induced an increase in the beat frequency. Therefore, it is very likely that the beat frequency of cilia is regulated by the intracellular concentration of Ca2+.

Adenosine Triphosphate↗
PubMed1983

Studies on conformation of F-actin in muscle fibers in the relaxed state, rigor, and during contraction using fluorescent phalloidin.

F-actin in a glycerinated muscle fiber was specifically labeled with fluorescent phalloidin-(fluorescein isothiocyanate) FITC complex at 1:1 molar ratio. Binding of phalloidin-FITC to F-actin affected neither contraction of the fiber nor its regulation by Ca2+. Comparison of polarized fluorescence from phalloidin-FITC bound to F-actin in the relaxed state, rigor, and during isometric contraction of the fiber revealed that the changes in polarization accompanying activation are quantitatively as well as qualitatively different from those accompanying transition of the fiber from the relaxed state to rigor. The extent of the changes of polarized fluorescence during isometric contraction increased with decreasing ionic strength, in parallel with increase in isometric tension. On the other hand, polarized fluorescence was not affected by addition of ADP or by stretching of the fiber in rigor solution. It is concluded from these observations that conformational changes in F-actin are involved in the process of active tension development.

Actins↗
PubMed1982

Asymmetry of fluctuation with respect to time reversal in steady states of biological systems.

The asymmetry of fluctuation with respect to time reversal which is expected in an energy-consuming steady state is discussed with special attention to biological systems. The necessary condition for asymmetry of fluctuation of an observed quantity is given. To show the usefulness of the experimental analysis of asymmetry of fluctuation, some calculations are carried out on two simple examples of three-state reactions. In one of them, the two-point time correlation function of the observed quantity has an oscillatory component, while in the other the function is nearly exponential, but in both cases, the fluctuation has a pronounced asymmetry. A method to estimate the degree of asymmetry of fluctuation is proposed, and the application of the present method to investigation of the molecular events in biological systems such as muscle is discussed.

Animals↗
PubMed1979

Crystallization and preliminary crystallographic data of chicken gizzard G-actin . DNase I complex and Physarum G-actin . DNase I complex.

Smooth muscle G-actin from chicken gizzard and Physarum plasmodium G-actin both interact with DNase I and form 1 : 1 complexes. These complexes were crystallized by using polyethylene glycol 6000 as a precipitant. Both crystals belong to the same orthorhombic space group P2(1)2(1)2(1). The cell dimensions of chicken gizzard G-actin.DNase I complex are a=42.00 +/- 0.07 A, b=225.3 +/- 0.4 A, and c=77.4 +/- 0.1 A, while those of Physarum G-actin.DNase I complex are a=42 A, b=221 A, and c=77 A.

Actins↗