摘要:

AbstractEnergetic analysis was applied to reduced product formation following perturbation of ethanol‐ and propionate‐fed methanogenic continuous stirred tank reactors (CSTRs). Formation and dissipation of longer‐chainedn‐carboxylic acids corresponded with the variation in Gibbs free energy change associated with beta‐oxidation reactions. Formation appeared to occur from acetate and propionate by reductive back‐reactions, made energetically favorable by elevated hydrogen partial pressure (P H 2), and possibly mediated by biosynthetic enzymes. The formed longer‐chained acids dissipated when theP H 2fell and equilibrium shifted to favor beta‐oxidations.n‐Propanol was found to be produced from propionate in a coupled ethanol oxidation/propionate reduction reaction, mediated by ethanol‐oxidizing organisms during high rates of ethanol utilization and elevatedP H 2. WhenP H 2declined,n‐propanol was oxidized back to its precursor propionate. Both reaction energetics and intracellular diffusion of the electron carrier may effect transie

ISSN:0006-3592    

DOI:10.1002/bit.260340702

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractEquilibrium adsorption studies onEscherichia coliandSaccharomycessp. revealed the capacity and affinity of these organisms for the surfaces of powdered charcoal and nickel. In simple salt solutions both organisms readily adsorbed to each solid with an affinity and maximum loading capacity individual to each cell–solid combination. In the presence of common growth media (lab‐lemco, nutrient broth, peptone, and yeast extract, individually at a concentration of 1.3%), each medium substantially inhibited adsorption. Each medium contained a protein‐aceous constituent as determined by ultraviolet (UV) analysis. The degree of inhibition was relative to medium concentration present during assay. Cell wall extracts from whole‐yeast cells also effectively inhibited adsorption. Cells adsorbed in the presence of sodium chloride solutions were susceptible to subsequent desorption by nutrien

ISSN:0006-3592    

DOI:10.1002/bit.260340703

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractExperiments were performed to evaluate, qualitatively and quantitatively, the adaptation ofEscherichia colito plasmid maintenance and cloned gene expression. Experimental findings indicate that the metabolic response to low plasmid levels is an increase of the biosynthetic capacity of both transcription and translation. At high copy number levels the gene‐specific transcription rate continues to increase but the stability of plasmid‐derived mRNA drops sharply. Protein levels are maintained, but translation efficiency decreases. These results indicate that cellular biosynthetic capacity may not be limiting productivity in recombinant systems. If macromolecular stability is the bottleneck, then current efforts to increase gene expression that focus on enhancing synthesis rates will be ineffect

ISSN:0006-3592    

DOI:10.1002/bit.260340704

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractA novel means of extracting carboxylic acids from aqueous solutions is described which involves the use of a lipolytic enzyme to convert the acid to a more hydrophobic ester. A water‐immiscible long‐chain alcohol serves both as a reactant in esterifying the acid, and as a solvent for extraction of the ester. Radiochemical tracer studies of the underlying reaction and extraction equilibria established that a low equilibrium concentration of ester (0.04–0.5mM) in the presence of water is counterbalanced by the high distribution coefficient of the ester (220–3380, wt. basis). The net result is a substantial (4‐ to 15‐fold) increase in the apparent distribution coefficient of the acid; 80%–95% of the extracted acid is in the esterified form. The method is applicable to a variety of alcoh

ISSN:0006-3592    

DOI:10.1002/bit.260340705

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractTraditionally, enzyme populations have been treated as if they were either homogenous, or heterogeneous with distinct and separable subpopulations. The microheterogeneity model, however, assumes that there is a continuous distribution of properties in the population. In the area of enzyme deactivation kinetics, this model describes the heterogeneous population as having a continuous distribution of activation energy of deactivation. This distribution is characterized by mean activation energy, and a standard deviation of activation energy. The microheterogeneity model contains two parameters, ϵ0and σ. Parameter ϵ0is the mean value of ϵ for a heterogeneous enzyme population; ϵ is the activation energy divided by absolute temperature and the ideal gas constant. Parameter σ is the standard deviation of the Gaussian distribution of ϵ values in the population. If the population is homogeneous, then ϵ = ϵ0for all enzyme molecules and σ = 0. There are certain ratios which are independent of ϵ0and dependent upon σ. Two important ratios aret1/4/t1/2andt1/2/t 1/2′, wheret 1/2′representst1/2for a homogeneous enzyme population with the same mean ϵ (ϵ0), as the heterogeneous population. If there is experimental deactivation data for the heterogeneous population which is well behaved, the first ratio,t1/4/t1/2, can be determined by estimating the time in minutes at which the enzyme has lost 25% of its activity (t1/4), and the time in minutes at which the enzyme has lost 50% of its activity (t1/2), and then taking the ratiot1/4/t1/2. The corresponding value of σ can be estimated from a graph. The ratiot1/2/t 1/2′can be found directly as a function oft1/4/t1/2, and can be estimated from another graph. The value of ϵ0can then be calculated from the f

ISSN:0006-3592    

DOI:10.1002/bit.260340706

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThis article proposes a simple steady‐state method for measuring the effective diffusion coefficient of oxygen (De) in gel beads entrapping viable cells. We applied this method to the measurement ofDein Ca‐ and Ba‐alginate gel beads entrappingSaccharomyces cerevisiaeandPseudomonas ovalis.The diffusivity of oxygen through gel beads containing viable cells was measured within an accuracy of ±7% and found not to be influenced by cell density (0–30 g/L gel), cell type, and cell viability in gel beads. The oxygen diffusivity in the Ca‐alginate gel beads was superior to that of the Ba‐alginate gel beads, and theDein the Ca‐alginate gel beads nearly equalled the molecular diffusion coefficient in the liquid containing the gel beads. The oxygen concentration profile in a single Ca‐alginate gel bead was calculated and compared to the distribution of mycelia ofAspergillus awamorigrown in that gel bead. This procedure indicated that the oxygen concentration profile is useful for the estimation of the thickness of the cell layer in a gel bead. Numerical investigation revealed that high effectiveness factors, greater than 0.8, could be obtained using microgel beads with a r

ISSN:0006-3592    

DOI:10.1002/bit.260340707

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractAlthough higher initial rates of phenylacetyl carbinol formation were observed in fermentations containing a high starting benzaldehyde level, a massive reduction in yeast viability was observed resulting in early cessation of production formation. Pulse feeding to maintain lower benzaldehyde concentrations resulted in a lower initial reaction rate, but prolonged yeast viability and the biotransformation. This resulted in higher overall product tilers. As benzaldehyde concentration was increased, yeast growth rate was reduced (0.5 g/L), inhibited (1–2 g/L), or cell viability reduced (3 g/L). Benzaldehyde appeared to alter the cell permeability barrier to substrates and products. Reductions in yeast biomass levels and especially protein and lipid content were observed during the biotransformation. The effects of benzaldehyde and reaction products on yeast pyruvate decarboxylase and alcohol dehydrogenase stability were determined. Homogenized yeast cells produced similar phenylacetyl carbinol levels to whole yeast only if supplemented with thiamine pyrophosphate and magnesiu

ISSN:0006-3592    

DOI:10.1002/bit.260340708

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractLipase fromRhizopussp. has been immobilized covalently on tresyl activated silica. Three different coupling media were evaluated: aqueous buffer,n‐hexane, and a microemulsion based onn‐hexane, aqueous buffer, and the nonionic surfactant triethylene glycol monododecyl ether. In addition, coupling via a very long, hydrophilic spacer arm, polyethylene glycol 1500 (PEG 1500), was compared with attachment to the silica via a short silane bridge only. The enzyme preparations were tested in hydrolysis and transesterification reactions. In the hydrolysis no marked differences in activity were found between the coupling media used. In the transesterification, on the other hand, the choice of immobilization medium had a very large effect on lipase activity, the preparation from microemulsion being the most active one. The use of the hydrophilic spacer had a large effect on activity in the hydrolysis reaction. Whereas direct coupling gave an activity of immobilized lipase of 26–34% of that of free enzyme, depending on the reaction medium, lipase bound via the spacer exhibited 56–67% activity. The latter values are considerably higher than previously reported in the literature for covalently immobilized lipase. The hydrophilic spacer had no effect on enzyme activity in the transesterification, however, a fact which is attributed to the hydrophobic medium of this reaction. The spacer is incompatible with the reaction medium and will, therefore, adsorb on the particles rather than stretch out into the bulk phase. The stability of the bound lipase was extremely good, no loss in activity being observed after a period of three weeks in aqueous solution

ISSN:0006-3592    

DOI:10.1002/bit.260340709

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractUrea hydrolysis by urease immobilized onto ion exchange resins in a fixed‐bed reactor has been studied. A modified Michaelis–Menten rate expression is used to describe the pH‐dependent, substrate‐ and product‐inhibited kinetics. Ionic equilibria of product and buffer species are included to account for pH changes generated by reaction. An isothermal, heterogeneous plug‐flow reactor model has been developed. An effectiveness factor is used to describe the reaction–diffusion process within the particle phase. The procedure for covalent immobilization of urease onto macroporous cation exchangers is described. Urea conversion data are used to estimate kinetic parameters by a simplex optimization method. The best‐fitted parameters are then used to predict the outlet conversions and pH values for systems with various inlet pH values, inlet urea and ammonia concentrations, buffers, particle sizes, and spacetimes. Very good agreement is obtained between experimental data and model predictions. This immobilized urease system exhibits quite different kinetic behavior from soluble urease because the pH near the enzyme active sites is different from that of the pore fluid. This effect results in a shift of the optimal pH value of theVmax(pH) curve from 6.6 (soluble urease) to ca. 7.6 in dialysate solution, and ca. pH 8.0 in 20mMphosphate buffer. The reactor model is especially useful for estimating intrinsic kinetic parameters of immobilized enzymes and for designing urea

ISSN:0006-3592    

DOI:10.1002/bit.260340710

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractDiffusion of oxygen through aqueous solutions is of great importance in biological systems. In this work, three models for the diffusion of oxygen through aqueous salt solutions are compared. One model uses mole fraction as the driving force (Fick's Law) and another uses chemical potential. The third model uses the gradient in oxygen activity as the driving force. This new model was chosen because of the availability of oxygen electrodes which directly measure oxygen activity in aqueous solution. These models have been used to reevaluate the technique of measuring O2diffusivities. We show that Pick's Law diffusion coefficients do not vary strongly with salt concentration as was erroneously reported in the literature. In addition, we compare the predicted O2fluxes of the three models over a wide range in O2concentrations. For oxygen concentrations of biological interest, the three models give identical predictions of the flux.

ISSN:0006-3592    

DOI:10.1002/bit.260340711

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY