Enzymes, membrane/enzyme oscillator

The earhest example of a membrane-enzyme oscillator was presented by Na-parstek, Caplan, and coworkers [52]. This oscillator consists of papain immobilized in a porous collodion membrane that is permeable to water, substrate, and ions. The membrane is cast as a thin film against a pH electrode and is exposed to an alkaline (pH 10) external solution of benzyl arginine ethyl ester (BAEE), which is a substrate... 

As illustrated by the examples above, the key to understanding the PI pathway is to comprehend the nature of the regulation of discrete subcellular microdomains of the lipids. Any changes in a specific plasma membrane signaling pool, whether a transient oscillation in PtdIns(4,5)P2 or a more sustained change in rate of flux through the pathway in response to a stimulus, will result in changes in cytoskeletal structure, membrane enzyme activity, and pump or channel activity within the microdomains where the lipid resides. 

In these equations, Pj and P2 are the two conformational states of the transport protein, and equilibrium constants (K) and rate constants (k) in an electric field are shown to be these constants in zero field multiplied by a nonlinear term that is the product of A Me and the electric field across the membrane, Em. The r in these equations is the apportionation constant and has a value between 0 and 1 (14). This property of a membrane protein has been explored, and a model called electroconformational coupling has been proposed to interpret data on the electric activation of membrane enzymes (13-17). A four-state membrane-facilitated transport model has been analyzed and shown to absorb energy from oscillating electric fields to actively pump a substrate up its concentration gradient (see the section entitled Theory of Electroconformational Coupling). 

Naparstek, A., D. Thomas S.R. Caplan. 1973. An experimental enzyme-membrane oscillator. Biochim. Biophys. Acta 323 643-6. 

Membrane potential oscillations, 3 in pancreatic p-cells, 13 see also Oscillations Metaphase arrest, 443,444,457 Michaelian enzyme kinetics, 73,74,368,... 

A comprehensive model of the hydrogel/enzyme oscillator will be complex, since it must take into account the presence and transport of several chemical species, and the distributed mechanical response of the hydrogel membrane. In an earlier analysis of the present system (21,22), a highly simplified, electromechanical relay-like model of the hydrogel was considered. This simple, heuristic model, lacked any elements of hydrogel physical chemistry or transport processes between the hyArogel and the reaction compartment (Cell II). 

Friboulet, A. and Thomas, D. (1982) Electrical excitability of artificial enzyme membranes. III. Hysteresis and oscillations observed with immobilized acetylcholinesterase membranes. Biophys. Chem., 16, 153-157. 

When mitochondria are disrupted into small vesicles or fragments with detergents, or by sonic oscillation, some enzymes and enzyme systems remain associated with the particles, while some others are recovered in the soluble phase. The cytochromes and the flavoproteins of the respiratory chain are exclusively recovered in the membrane fractions and seem to be firmly bound to the membrane. The ability to couple oxidation to phosphorylation is usually lost upon fragmentatioiu however, if the submitochondrial particles are prepared very carefully, they can... 

Abstract. The urea-urease system is a pH dependent enzymatic reaction that was proposed as a convenient model to study pH oscillations in vitro here, in order to determine the best conditions for oscillations, a two-variable model is used in which acid and substrate, urea, are supplied at rates kh and ks from an external medium to an enzyme-containing compartment. Oscillations were observed between pH 4 and 8. Thus the reaction appears a good candidate for the observation of oscillations in experiments, providing the necessary condition that kh > ks is met. In order to match these conditions, we devised an experimental system where we can ensure the fast transport of acid to the encapsulated urease, compared to that of urea. In particular, by means of the droplet transfer method, we encapsulate the enzyme, together with a suitable pH indicator, in a l-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid membrane, where differential diffusion of H+ and urea is ensured by the different permeability (Pm) of membranes to the two species. Here we present preliminary tests for the stability of the enzymatic reaction in the presence of lipids and also the successful encapsulation of the enzyme into lipid vesicles. 

Some of the main types of cellular regulation associated with rhythmic behavior are listed in Table III. Regulation of ion channels gives rise to the periodic variation of the membrane potential in nerve and cardiac cells [27, 28 for a recent review of neural rhythms see, for example, Ref. 29]. Regulation of enzyme activity is associated with metabolic oscillations, such as those that occur in glycolysis in yeast and muscle cells. Calcium oscillations originate... 

As indicated above, theoretical models for biological rhythms were first used in ecology to study the oscillations resulting from interactions between populations of predators and preys [6]. Neural rhythms represent another field where such models were used at an early stage The formalism developed by Hodgkin and Huxley [7] stiU forms the core of most models for oscillations of the membrane potential in nerve and cardiac cells [33-35]. Models were subsequently proposed for oscillations that arise at the cellular level from regulation of enzyme, receptor, or gene activity (see Ref. 31 for a detailed fist of references). 

When the same kind of electrode is introduced in a solution with a high pH (i.e., pH= 10) and a lower substrate concentration (first order kinetics), an oscillation in time of the measured pH inside the membrane spontaneously occurs. This enzyme, which has been extensively studied, does not give oscillation for any conditions of pH and substrate concentration. The period of oscillation is around one-half minute, and the oscillation is abolished by introducing an enzyme inhibitor. The phenomenon can be explained by the autocatalytic effect and by a feedback action of OH- diffusion in from the outside solution. The diffusion of this ion is quicker than the diffusion of the substrate. There is a qualitative agreement between the computer simulation and the experimental results. 

The absence of oscillation in the bulk solution was checked by using both a second pH electrode and a dye. The oscillation inside the membrane is not an interaction between polyelectrolyte and glass surface the effect is destroyed in the presence of an inhibitor of the enzyme activity. 

Where within the mitochondria are specific enzymes localized One approach to this question is to see how easily the enzymes can be dissociated from mitochondria. Some enzymes come out readily under hypotonic conditions. Some are released only upon sonic oscillation, suggesting that they are inside the matrix space. Others, including the cytochromes and the flavoproteins that act upon succinate and NADH, are so firmly embedded in the inner mitochondrial membranes that they can be dissociated only through the use of non-denaturing detergents. 

As a model to explain oscillations in an enzyme reaction Chay (1981) proposed a model based on a feedback mechanism of proton gradients across a membrane. The activity of the key enzyme in the reaction depends on the pH of the inner compartment. Oscillations predicted in pH as well as enzyme activity and substrate concentrations were observed. Chay and Cho (1982) extended the concept and computationally obtained similar results of oscillating elements of the enzyme reactions. 

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