Metabolic consequences of phosphotransferase (PTS) mutation in a phenylalanine-producing recombinant Escherichia coli

E. coli strain PPA305, which has a wild-type PTS system, and PPA316, which utilizes a proton-galactose symport system for glucose uptake, were used as host strains to harbor a phenylalanine overproduction plasmid pSY130-14 and to study the effects of using different glucose uptake systems on phenylalanine production. The non-PTS strain (PPA316/pSY130-14) produced much less phenylalanine, ranging from 0 to 67% of that produced by the PTS strain (PPA305/pSY130-14) depending on cultivation conditions used. The non-PTS strain PPA316/pSY130-14 had an intracellular PEP concentration only one-sixth that of the PTS strain, PPA305/pSY130-14. Additionally, PPA316/pSY130-14 had a substantially lower energy state in terms of the size of the pool of high-energy phosphate compounds and the magnitude of the pH difference across the cytoplasmic membrane. The non-PTS strain consumed oxygen at a higher rate, attained lower biomass concentration, and produced no acetate and phenylalanine during fermentation, suggesting more carbon was oxidized to CO2, most likely through the TCA cycle. Analysis of intracellular fluxes through the central carbon pathways was performed for each strain utilizing exponential phase data on extracellular components and assuming quasi-steady state for intermediate metabolites. The non-PTS strain had a higher flux through pyruvate kinase (PYK) and TCA cycle which, in agreement with the observed higher oxygen uptake rate, suggests that more carbon was oxidized to CO2 through the TCA cycle. Further analysis using rate expression data for PYK and NMR data for the intracellular metabolites identified the regulatory properties of PYK as the probable cause for lower intracellular PEP levels in PPA316/pSY130-14.     

Numerical investigation of hydrodynamics and mass transfer for in-line fiber arrays in laminar cross-flow at low Reynolds numbers

In this work, mass transfer at the shell side of an in-line hollow fiber array subjected to cross-flow is simulated by applying the domain decomposition method combined with orthogonal grid generation. Two-dimensional Navier-Stokes equations written in stream function-vorticity variables, were separately solved along with a species conservation equation for different arrays. The main factors influencing the concentration fields, local mass transfer rates and global mass transfer rates in the laminar flow of Re=10-200 and Pe=10-300 with pitch to tube diameter ratios of 1.45, 1.50, 1.75, 1.85 and 2.00 are discussed in detail. Mass transfer correlations obtained from the numerical simulations show good agreement with typical empirical correlations proposed earlier.     

Modulation of cardiac sodium current by alpha1-stimulation and volatile anesthetics

BACKGROUND: Alpha1-adrenoceptor stimulation is known to produce electrophysiologic changes in cardiac tissues, which may involve modulations of the fast inward Na+ current (I(Na)). A direct prodysrhythmic alpha1-mediated interaction between catecholamines and halothane has been demonstrated, supporting the hypothesis that generation of halothane-epinephrine dysrhythmias may involve slowed conduction, leading to reentry. In this study, we examined the effects of a selective alpha1-adrenergic receptor agonist, methoxamine, on cardiac I(Na) in the absence and presence of equianesthetic concentrations of halothane and isoflurane in single ventricular myocytes from adult guinea pig hearts. METHODS: I(Na) was recorded using the standard whole-cell configuration of the patch-clamp technique. Voltage clamp protocols initiated from two different holding potentials (V(H)) were applied to examine state-dependent effects of methoxamine in the presence of anesthetics. Steady state activation and inactivation and recovery from inactivation were characterized using standard protocols. RESULTS: Methoxamine decreased I(Na) in a concentration- and voltage-dependent manner, being more potent at the depolarized V(H). Halothane and isoflurane interacted synergistically with methoxamine to suppress I(Na) near the physiologic cardiac resting potential of -80 mV. The effect of methoxamine with anesthetics appeared to be additive when using a V(H) of -110 mV, a potential where no Na+ channels are in the inactivated state. Methoxamine in the absence and presence of anesthetics significantly shifted the half maximal inactivation voltage in the hyperpolarizing direction but had no effect on steady-state activation. CONCLUSION: The present results show that methoxamine (alpha1-adrenergic stimulation) decreases cardiac Na+ current in a concentration- and voltage-dependent manner. Further, a form of synergistic interaction between methoxamine and inhalational anesthetics, halothane and isoflurane, was observed. This interaction appears to depend on the fraction of Na+ channels in the inactivated state. (Key words: Anesthetics, volatile: halothane; isoflurane; methoxamine. Patch clamp: whole-cell configuration; sodium current; ventricular guinea pig myocytes.)     

Disruption of syntaxin-mediated protein interactions blocks neurotransmitter secretion

The membrane protein syntaxin participates in several protein-protein interactions that have been implicated in neurotransmitter release. To probe the physiological importance of these interactions, we microinjected into the squid giant presynaptic terminal botulinum toxin C1, which cleaves syntaxin, and the H3 domain of syntaxin, which mediates binding to other proteins. Both reagents inhibited synaptic transmission yet did not affect the number or distribution of synaptic vesicles at the presynaptic active zone. Recombinant H3 domain inhibited the interactions between syntaxin and SNAP-25 that underlie the formation of stable SNARE complexes in vitro. These data support the notion that syntaxin-mediated SNARE complexes are necessary for docked synaptic vesicles to fuse.     

A model for the performance analysis and design of waveguide p-i-n photodetectors

A model to investigate the frequency response of waveguide (WG) p-i-n photodetectors (PDs) is presented. The model is based on the 2-D position-dependent distribution of carriers in the device and the results show that the contribution to the total current mainly comes from a small length of the PD measured from its input end. The effect of carrier trapping at a heterointerface has also been considered to study the frequency dependence of the photocurrent at low-bias voltages. The frequency response and bandwidth obtained from the model are in good agreement with published experimental results. A simplified and approximate relation for the fiber-to-WG coupling efficiency has also been used to calculate the overall quantum-efficiency of WGPDs. Then, the effect of WG geometry on the bandwidth-quantum efficiency product has been analyzed and some results on the optimum design of a WG p-i-n PD for maximum bandwidth-quantum efficiency product have been tabulated.     

A general noise and S-parameter deembedding procedure for on-waferhigh-frequency noise measurements of MOSFETs

A general deembedding procedure using one “OPEN” and two “THRU” dummy structures for noise and scattering parameter deembedding based on cascade configurations is presented in this paper. This technique does not require any equivalent-circuit modeling of probe pads or interconnections. This deembedding procedure is valid for designs having interconnections with any kinds of geometries and for devices operated at frequencies of several tens of gigahertz