Varying molecular interactions explain aspects of crowder-dependent enzyme function of a viral protease

Biochemical processes in cells, including enzyme-catalyzed reactions, occur in crowded conditions with various background macromolecules occupying up to 40% of cytoplasm's volume. Viral enzymes in the host cell also encounter such crowded conditions as they often function at the endoplasmic reticulum membranes. We focus on an enzyme encoded by the hepatitis C virus, the NS3/4A protease, which is crucial for viral replication. We have previously found experimentally that synthetic crowders, polyethylene glycol (PEG) and branched polysucrose (Ficoll), differently affect the kinetic parameters of peptide hydrolysis catalyzed by NS3/4A. To gain understanding of the reasons for such behavior, we perform atomistic molecular dynamics simulations of NS3/4A in the presence of either PEG or Ficoll crowders and with and without the peptide substrates. We find that both crowder types make nanosecond long contacts with the protease and slow down its diffusion. However, they also affect the enzyme structural dynamics; crowders induce functionally relevant helical structures in the disordered parts of the protease cofactor, NS4A, with the PEG effect being more pronounced. Overall, PEG interactions with NS3/4A are slightly stronger but Ficoll forms more hydrogen bonds with NS3. The crowders also interact with substrates; we find that the substrate diffusion is reduced much more in the presence of PEG than Ficoll. However, contrary to NS3, the substrate interacts more strongly with Ficoll than with PEG crowders, with the substrate diffusion being similar to crowder diffusion. Importantly, crowders also affect the substrate-enzyme interactions. We observe that both PEG and Ficoll enhance the presence of substrates near the active site, especially near catalytic H57 but Ficoll crowders increase substrate binding more than PEG molecules.

ScrepYard: An online resource for disulfide-stabilized tandem repeat peptides

Receptor avidity through multivalency is a highly sought-after property of ligands. While readily available in nature in the form of bivalent antibodies, this property remains challenging to engineer in synthetic molecules. The discovery of several bivalent venom peptides containing two homologous and independently folded domains (in a tandem repeat arrangement) has provided a unique opportunity to better understand the underpinning design of multivalency in multimeric biomolecules, as well as how naturally occurring multivalent ligands can be identified. In previous work, we classified these molecules as a larger class termed secreted cysteine-rich repeat-proteins (SCREPs). Here, we present an online resource; ScrepYard, designed to assist researchers in identification of SCREP sequences of interest and to aid in characterizing this emerging class of biomolecules. Analysis of sequences within the ScrepYard reveals that two-domain tandem repeats constitute the most abundant SCREP domain architecture, while the interdomain "linker" regions connecting the functional domains are found to be abundant in amino acids with short or polar sidechains and contain an unusually high abundance of proline residues. Finally, we demonstrate the utility of ScrepYard as a virtual screening tool for discovery of putatively multivalent peptides, by using it as a resource to identify a previously uncharacterized serine protease inhibitor and confirm its predicted activity using an enzyme assay.

Non-Conserved Amino Acid Residues Modulate the Thermodynamics of Zn(II) Binding to Classical ββα Zinc Finger Domains

Classical zinc fingers domains (ZFs) bind Zn(II) ion by a pair of cysteine and histidine residues to adopt a characteristic and stable ββα fold containing a small hydrophobic core. As a component of transcription factors, they recognize specific DNA sequences to transcript particular genes. The loss of Zn(II) disrupts the unique structure and function of the whole protein. It has been shown that the saturation of ZFs under cellular conditions is strictly related to their affinity for Zn(II). High affinity warrants their constant saturation, while medium affinity results in their transient structurization depending on cellular zinc availability. Therefore, there must be factors hidden in the sequence and structure of ZFs that impact Zn(II)-to-protein affinities to control their function. Using molecular dynamics simulations and experimental spectroscopic and calorimetric approaches, we showed that particular non-conserved residues derived from ZF sequences impact hydrogen bond formation. Our in silico and in vitro studies show that non-conserved residues can alter metal-coupled folding mechanisms and overall ZF stability. Furthermore, we show that Zn(II) binding to ZFs can also be entropically driven. This preference does not correlate either with Zn(II) binding site or with the extent of the secondary structure but is strictly related to a reservoir of interactions within the second coordination shell, which may loosen or tighten up the structure. Our findings shed new light on how the functionality of ZFs is modulated by non-coordinating residues diversity under cellular conditions. Moreover, they can be helpful for systematic backbone alteration of native ZF ββα scaffold to create artificial foldamers and proteins with improved stability.

Current View on the Mechanisms of Alcohol-Mediated Toxicity

Alcohol is a psychoactive substance that is widely used and, unfortunately, often abused. In addition to acute effects such as intoxication, it may cause many chronic pathological conditions. Some of the effects are very well described and explained, but there are still gaps in the explanation of empirically co-founded dysfunction in many alcohol-related conditions. This work focuses on reviewing actual knowledge about the toxic effects of ethanol and its degradation products.

Biomimetic Design of a 3 D Transition Metal/Carbon Dyad for the One-Step Hydrodeoxygenation of Vanillin

Enzyme catalysts always show an excellent catalytic selectivity, which is important in biochemistry, especially in catalytic synthesis and biopharming. This selectivity is achieved by combining the binding effect induced by the electrostatic effect of the enzyme to attract a specific substrate and then the prearrangement of the substrates inside the enzyme pocket. Herein, we report a proof-of-concept application of an interfacial electrostatic field induced by constructing Schottky heterojunctions to mimic the electrostatic catalysis of an enzyme. In combination with the 3 D structure, a transition metal/carbon dyad was designed by nanoconfinement methods to promote the differential binding effect and the space-induced organization of the reaction intermediate (vanillyl alcohol) to develop a new one-step hydrogenolysis of vanillin for the production of 2-methoxy-4-methylphenol with a remarkably high selectivity (>99 %).

Transition Metal Ion Interactions with Disordered Amyloid-β Peptides in the Pathogenesis of Alzheimer's Disease: Insights from Computational Chemistry Studies

Monomers and oligomers of the amyloid-β peptide aggregate to form the fibrils found in the brains of Alzheimer's disease patients. These monomers and oligomers are largely disordered and can interact with transition metal ions, affecting the mechanism and kinetics of amyloid-β aggregation. Due to the disordered nature of amyloid-β, its rapid aggregation, as well as solvent and paramagnetic effects, experimental studies face challenges in the characterization of transition metal ions bound to amyloid-β monomers and oligomers. The details of the coordination chemistry between transition metals and amyloid-β obtained from experiments remain debated. Furthermore, the impact of transition metal ion binding on the monomeric or oligomeric amyloid-β structures and dynamics are still poorly understood. Computational chemistry studies can serve as an important complement to experimental studies and can provide additional knowledge on the binding between amyloid-β and transition metal ions. Many research groups conducted first-principles calculations, ab initio molecular dynamics simulations, quantum mechanics/classical mechanics simulations, and classical molecular dynamics simulations for studying the interplay between transition metal ions and amyloid-β monomers and oligomers. This review summarizes the current understanding of transition metal interactions with amyloid-β obtained from computational chemistry studies. We also emphasize the current view of the coordination chemistry between transition metal ions and amyloid-β. This information represents an important foundation for future metal ion chelator and drug design studies aiming to combat Alzheimer's disease.

Purification and characterization of a novel glutamate dehydrogenase from Geotrichum candidum with higher alcohol and amino acid activity

Crude enzyme from Geotrichum candidum S12 exhibited high activity towards hexanol at pH 4.0, distinguishing it from currently known enzymes. To identify the dominant enzyme contributing to this activity, the crude enzyme extract was separated into different fractions by ammonium sulfate precipitation, MonoQ anion-exchange chromatography, and Sephacryl S-200 gel filtration chromatography. Afraction with high activity towards hexanol at pH 4.0 was obtained, exhibiting 38-fold improved purity and a specific activity of 3802.7 U/mg. After electrophoretic analysis, the fraction showed a molecular weight of 223 kDa by Native-PAGE and 51.4 kDa by SDS-PAGE. The purified fraction was identified as a glutamate dehydrogenase (GDH) by peptide mass fingerprinting data. This fraction showed high activity towards glutamate, α-ketoglutarate, hexanol, and isoamyl alcohol with a Km value of 41.74, 4.01, 20.37, and 19.37 mM, respectively, but with no activity towards methanol, ethanol, 1-propanol, and isobutanol. As a comparison, the GDH from yeast had no activity towards hexanol and other alcohols. Kinetic analysis revealed that glutamate and hexanol served as competitive inhibitors to each other for the purified GDH. The GDH showed the highest activity towards hexanol at pH 4.0 and 30 °C, and was the most stable at pH 2.2-7.0 and ≤40 °C. The presence of ADP, Fe²⁺, K⁺, and Zn²⁺ increased the enzymatic activity towards hexanol and EDTA, Pb²⁺, Mn²⁺, ATP, and DTT decreased the activity. These novel characteristics expand the reported properties of GDH and suggest the newly characterized GDH has unique potential for practical application.

Apocynin Alleviates Renal Ischemia/Reperfusion Injury Through Regulating the Level of Zinc and Metallothionen

The purpose of this research was to evaluate the protective effects of apocynin on renal ischemia/reperfusion (I/R) injury (RI/RI) in rats. Rats preconditioned with apocynin were subjected to renal I/R. Zinc levels in serum and renal tissues, blood urea nitrogen (BUN), and serum creatinine (Scr) were detected. We further measured the activity of superoxide dismutase (SOD); the content of malondialdehyde (MDA), IL-4, IL-6, IL-10, and TNF-α; and the expression of metallothionein (MT) in the renal tissues. Results indicated that the levels of MDA, IL-4, IL-6, IL-10, TNF-α, and MT in the kidney tissue and serum BUN and Scr levels in RI/RI group were significantly higher than those in sham-operated group, while the levels of serum Zn and kidney Zn and SOD were reduced in RI/RI group. Apocynin treatment further decreased the levels of MDA, IL-6, TNF-α, and serum BUN and Scr, whereas it significantly increased the levels of Zn, SOD, IL-4, IL-10, and MT in the kidney tissue and serum Zn. These findings suggest that apocynin might play a protective role against RI/RI in rats through regulating zinc level and MT expression involving in oxidative stress.

A theoretical and experimental study of calcium, iron, zinc, cadmium, and sodium ions absorption by aspartame

Aspartame (L-Aspartyl-L-phenylalanine methyl ester) is a sweet dipeptide used in some foods and beverages. Experimental studies show that aspartame causes osteoporosis and some illnesses, which are similar to those of copper and calcium deficiency. This raises the issue that aspartame in food may interact with cations and excrete them from the body. This study aimed to study aspartame interaction with calcium, zinc, iron, sodium, and cadmium ions via molecular dynamics simulation (MD) and spectroscopy. Following a 480-ns molecular dynamics simulation, it became clear that the aspartame is able to sequester Fe²⁺, Ca²⁺, Cd²⁺, and Zn²⁺ ions for a long time. Complexation led to increasing UV-Vis absorption spectra and emission spectra of the complexes. This study suggests a potential risk of cationic absorption of aspartame. This study suggests that purification of cadmium-polluted water by aspartame needs a more general risk assessment.

Addition of Zn to the ternary Mg-Ca-Sr alloys significantly improves their antibacterial property

Most of the magnesium (Mg) alloys possess excellent biocompatibility, mechanical property and biodegradability in orthopedic applications. However, these alloys may suffer from bacterial infections due to their insufficient antibacterial capability. In order to reduce the post-surgical infections, a series of biocompatible Mg-1Ca-0.5Sr-xZn (x=0, 2, 4, 6) alloys were fabricated with the addition of antibacterial Zn with variable content and evaluated in terms of their biocompatibility and antibacterial property. The in vitro corrosion study showed that Mg-1Ca-0.5Sr-6Zn alloys exhibited a higher hydrogen evolution volume after 100 h immersion and resulted in a higher pH value of the immersion solution. Our work indicated that Zn-containing Mg alloys exhibited good biocompatibility with high cell viability. The antibacterial studies reveal that the number of bacteria adhered on all of these Mg alloy samples diminished remarkably compared to the Ti-6Al-4V control group. We also found that the proliferation of the bacteria was inhibited by these Mg alloys extracts. Among the prepared alloys, Mg-1Ca-0.5Sr-6Zn alloy not only exhibited a strong antibacterial effect, but also promoted the proliferation of MC3T3-E1 osteoblasts, suggesting that it is a promising alloy with both good antibacterial property and good biocompatibility for use as an orthopedic implant.

Adhesin competence repressor (AdcR) from Streptococcus pyogenes controls adaptive responses to zinc limitation and contributes to virulence

Altering zinc bioavailability to bacterial pathogens is a key component of host innate immunity. Thus, the ability to sense and adapt to the alterations in zinc concentrations is critical for bacterial survival and pathogenesis. To understand the adaptive responses of group A Streptococcus (GAS) to zinc limitation and its regulation by AdcR, we characterized gene regulation by AdcR. AdcR regulates the expression of 70 genes involved in zinc acquisition and virulence. Zinc-bound AdcR interacts with operator sequences in the negatively regulated promoters and mediates differential regulation of target genes in response to zinc deficiency. Genes involved in zinc mobilization and conservation are derepressed during mild zinc deficiency, whereas the energy-dependent zinc importers are upregulated during severe zinc deficiency. Further, we demonstrated that transcription activation by AdcR occurs by direct binding to the promoter. However, the repression and activation by AdcR is mediated by its interactions with two distinct operator sequences. Finally, mutational analysis of the metal ligands of AdcR caused impaired DNA binding and attenuated virulence, indicating that zinc sensing by AdcR is critical for GAS pathogenesis. Together, we demonstrate that AdcR regulates GAS adaptive responses to zinc limitation and identify molecular components required for GAS survival during zinc deficiency.

Protective effect of grape seed and skin extract against high-fat diet-induced liver steatosis and zinc depletion in rat

BACKGROUND

Obesity is a tremendous public health problem, characterized by ectopic deposition of fat into non-adipose tissues as liver generating an oxidative stress that could lead to steato-hepatitis. Grape seed and skin extract (GSSE) is a complex mixture of polyphenolics exhibiting robust antioxidative properties.

AIM

We hypothesize that GSSE could protect the liver from fat-induced lipotoxicity and have a beneficial effect on liver function.

METHODS

Hepatoprotective effect of GSSE was measured by using an experimental model of fat-induced rat liver steatosis. Male rats were fed a standard diet or a high-fat diet (HFD) during 6 weeks and treated or not with 500 mg/kg bw GSSE. Lipid deposition into the liver was assessed by triglyceride, cholesterol and phospholipid measurements. Fat-induced lipoperoxidation, carbonylation, depletion of glutathione and of antioxidant enzyme activities were used as oxidative stress markers with a special emphasis on transition metal distribution.

RESULTS

HFD induced liver hypertrophy and inflammation as assessed by high liver transaminases. HFD also induced an oxidative stress characterized by increased lipid and protein oxidation, a drop in glutathione and antioxidant enzyme activities as glutathione peroxidase and superoxide dismutase and a drastic depletion in liver zinc. Importantly, GSSE prevented all the deleterious effects of HFD treatment.

CONCLUSIONS

Data suggest that GSSE could be used as a safe preventive agent against fat-induced liver lipotoxicity which could also have potential applications in other non-alcoholic liver diseases.

Comparison of the effects of Mg-6Zn and Ti-3Al-2.5V alloys on TGF-β/TNF-α/VEGF/b-FGF in the healing of the intestinal tract in vivo

To evaluate the different effects of Mg-6Zn alloy and Ti-3Al-2.5V alloy implants in intestinal tract healing, we compared these two different alloys with respect to their effect on a rat's intestinal tract, using serum magnesium, radiology, pathology and immunohistochemistry in vivo. It was found using the scanning electron microscope that the Mg-6Zn alloy began to degrade during the first week and that the Ti-3Al-2.5V alloy was non-degradable throughout the process. The Mg-6Zn alloy did not have an impact on serum magnesium. Superior to the Ti-3Al-2.5V alloy, the Mg-6Zn alloy enhanced the expression of transforming growth factor-β1 in healing tissue, and promoted the expression of both the vascular endothelial growth factor and the basic fibroblast growth factor, which helped angiogenesis and healing. The Mg-6Zn alloy reduced the expression of the tumor necrosis factor (TNF-α) at different stages and decreased inflammatory response, which may have been related to the zinc inhibiting TNF-α. In general, the Mg-6Zn alloy performed better than Ti-3Al-2.5V at promoting healing and reducing inflammation. The Mg-6Zn alloy may be a promising candidate for use in the pins of circular staplers for gastrointestinal reconstruction in medicine.

Comparison of degradation behavior and the associated bone response of ZK60 and PLLA in vivo

The aim of this study was to investigate whether ZK60, an extruded magnesium alloy, reacts in vivo with an appropriate host response, and to investigate how microarc oxidation treatment influences this in vivo corrosion behavior. Twelve cylinders were machined from as-extruded ZK60, with six cylinders treated with MAO and six left untreated; poly-l-lactic acid pins were used as a control to compare biocompatibility. These cylinders were implanted into the right distal femur of mice along the transepicondylar axis from the medial condyle. Microcomputerized tomography was used to quantitatively analyze corrosion in a nondestructive manner in vivo and the corrosion rate was calculated based on the volume measurements of the residual implants. The physiological response of the rats postimplantation was obtained by clinical observation and blood biochemical analysis. Histological analyses of the soft tissue around the implants were used to assess bone response in relation to the implants. The results obtained clearly indicate that the untreated ZK60 alloy showed high degradation rates in vivo, and that MAO treatment had a significant but unsatisfying effect on protecting the implant from further corrosion. Compared with PLLA, the ZK60 alloy showed good osteoconductivity and osteoinductivity, and, according to biochemical indicators, had good biocompatibility in vivo.

Joint toxicity of alkoxyethanol mixtures: contribution of in silico applications

Exposure to chemicals occurs often as mixtures. Presented in this paper is information on alkoxyethanols and the impact they might have on human health in combination with some commonly found aliphatic and aromatic compounds. Our studies to evaluate the joint toxicity of these chemicals among themselves and in combination with other chemicals reveal a variety of possible outcomes depending on the exposure scenario. The interactions are predominantly based on metabolic pathways and are common among several solvents and organic compounds. Quantitative structure activity relationship (QSAR) analysis can be used with high confidence to identify chemicals that will interact to influence overall joint toxicity. Potential human exposure to a combination of alkoxyethanol, toluene and substituted benzenes may increase reproductive and developmental disease conditions. Inheritable gene alterations result in changes in the enzyme function in different subpopulations causing variations in quantity and/or quality of particular isoenzymes. These changes are responsible for differential metabolism of chemicals in species, genders, and life stages and are often the basis of a population's susceptibility. Unique genotypes introduced as a function of migration can alter the genetic makeup of any given population. Hence special consideration should be given to susceptible populations while conducting chemical health risk assessments.

Modeling Structural Coordination and Ligand Binding in Zinc Proteins with a Polarizable Potential

As the second most abundant cation in human body, zinc is vital for the structures and functions of many proteins. Zinc-containing matrix metalloproteinases (MMPs) have been widely investigated as potential drug targets in a range of diseases ranging from cardiovascular disorders to cancers. However, it remains a challenge in theoretical studies to treat zinc in proteins with classical mechanics. In this study, we examined Zn(2+) coordination with organic compounds and protein side chains using a polarizable atomic multipole based electrostatic model. We find that polarization effect plays a determining role in Zn(2+) coordination geometry in both matrix metalloproteinase (MMP) complexes and in zinc-finger proteins. In addition, the relative binding free energies of selected inhibitors binding with MMP13 have been estimated and compared with experimental results. While not directly interacting with the small molecule inhibitors, the permanent and polarizing field of Zn(2+) exerts a strong influence on the relative affinities of the ligands. The simulation results also reveal the polarization effect on binding is ligand dependent and thus difficult to be incorporated into fixed-charge models implicitly.

Purification and characterization of a hexanol-degrading enzyme extracted from apple

An enzyme having activity toward n-hexanol was purified from apple, and its biochemical characteristics were analyzed. The purification steps consisted of sedimentation with ammonium sulfate, DEAE Sepharose Fast Flow ion exchange chromatography, and Sephadex G-100 column. The obtained enzyme had a yield of 16.00% with a specific activity of 18879.20 U/mg protein and overall purification of 142.77-fold. The enzyme showed activity to isoamylol, 1-propanol, n-hexanol, and isobutanol but not toward methanol and ethanol. With n-hexanol as a substrate, the optimum conditions were pH 4.0 and 30 °C for enzyme activity and pH 3.0-4.0 and temperatures below 40 °C for enzyme stability. The enzyme activity was increased significantly by adding l-cysteine and Fe(2+) at all tested concentrations and slightly by Zn(2+) at a high concentration but decreased by additions of EDTA, Ga(2+), K(+), Mg(2+), sodium dodecyl sulfate (SDS), sodium aluminum sulfate (SAS), dithiothreitol (DTT), and glutathione (GSH). The enzyme activities toward n-hexanol and n-hexanal were increased by NADH but decreased by NAD(+), in contrast to a decrease toward n-hexane by addition of both NAD(+) and NADH.

Biodegradable Magnesium Alloys: A Review of Material Development and Applications

Magnesium based alloys possess a natural ability to biodegrade due to corrosion when placed within aqueous substances, which is promising for cardiovascular and orthopedic medical device applications. These materials can serve as a temporary scaffold when placed in vivo, which is desirable for treatments when temporary supportive structures are required to assist in the wound healing process. The nature of these materials to degrade is attributed to the high oxidative corrosion rates of magnesium. In this review, a summary is presented for magnesium material development, biocorrosion characteristics, as well as a biological translation for these results.

Aggregation of amyloids in a cellular context: modelling and experiment

Amyloid-related diseases are a group of illnesses in which an abnormal accumulation of proteins into fibrillar structures is evident. Results from a wide range of studies, ranging from identification of amyloid-β dimers in the brain to biophysical characterization of the interactions between amyloidogenic peptides and lipid membranes during fibril growth shed light on the initial events which take place during amyloid aggregation. Accounts of fibril disaggregation and formation of globular aggregates due to interactions with lipids or fatty acids further demonstrate the complexity of the aggregation process and the difficulty to treat amyloid-related diseases. There is an inherent difficulty in generalizing from studies of aggregation in vitro, but the involvement of too many cellular components limits the ability to follow amyloid aggregation in a cellular (or extracellular) context. Fortunately, the development of experimental methods to generate stable globular aggregates suggests new means of studying the molecular events associated with amyloid aggregation. Furthermore, simulation studies enable deeper understanding of the experimental results and provide useful predictions that can be tested in the laboratory. Computer simulations can nowadays provide molecular or even atomistic details that are experimentally not available or very difficult to obtain. In the present review, recent developments on modelling and experiments of amyloid aggregation are reviewed, and an integrative account on how isolated interactions (as observed in vitro and in silico) combine during the course of amyloid-related diseases is presented. Finally, it is argued that an integrative approach is necessary to get a better understanding of the protein aggregation process.

Transcriptional response of Streptococcus pneumoniae to Zn2+) limitation and the repressor/activator function of AdcR

Zinc (Zn(2+)) is an important trace metal ion that has been shown to regulate the expression of several (virulence) genes in streptococci. Previously, we analyzed the genome-wide response of S. pneumoniae to Zn(2+)-stress. In this work, we have performed a transcriptomic analysis to identify genes that are differentially expressed under intracellular Zn(2+) limitation. This revealed a number of genes that are highly upregulated in the absence of extracellular Zn(2+), amongst which the genes belonging to the regulon of the Zn(2+)-responsive repressor AdcR, like adcBCA, encoding a Zn(2+)-dependent ABC-uptake system, adcAII, encoding a Zn(2+)-binding lipoprotein, and also virulence genes belonging to the Pht family (phtA, phtB, phtD and phtE). Using transcriptome analysis, lacZ-reporter studies, in vitro DNA binding experiments, and in silico operator predictions, we show that AdcR directly represses the promoters of adcRCBA, adcAII-phtD, phtA, phtB and phtE in the presence of Zn(2+). AdcR can also function as an activator, since in the presence of Zn(2+) it directly induces expression of adh that encodes a Zn(2+)-containing alcohol dehydrogenase. In conclusion, the genome-wide transcriptional response of S. pneumoniae to Zn(2+) limitation was established, which is mainly mediated via direct regulation by the Zn(2+)-dependent regulator AdcR.

Frozen-in defects in bismuth in relation to its magnetoresistivity and thermoelectric power

An experimental study is presented of effects on the electrical properties of bismuth single crystals of quenching from about 4 K below the melting point (544 K ) into liquid nitrogen. Each of the twelve coefficients that describe the low field, magnetoresistivity tensor and the components of the thermoelectric power tensor has been compared in the annealed and quenched sample conditions at 77 K and at room temperature. Pronounced changes in the tensor coefficients at 77 K have been observed following quenching; the anisotropy ratio P 11 /p 33 zero resistivity in verts from 0.9446 before quenching to 1.030 after. Q uenching effects anneal out on heating to below room temperature. Results are analysed in terms of a two-band, multivalley Fermi surface model to obtain carrier densities and mobilities, the tilt angle of the electron ellipsoids and the Fermi energies and band overlap. Carrier densities at 77 K in both bands are 4.4 x 10 23 m -3 in the carefully annealed state. Quenching increases both the electron and hole densities, the former to 4.6 x 17 23 m -3 and the latter to 5.3 x 1023 m -3 ; the separation between the Fermi level and the valence band edge is increased markedly (27 %), as is the band overlap (13 %). The quenched-in defects show predominantly acceptor-like character. Carrier mobilities in the xy plane are reduced after quenching but those along the z axis are slightly increased.