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Biomining is defined as biotechnology for metal recovery from minerals, and is promoted by the concerted effort of a consortium of acidophile prokaryotes, comprised of members of the Bacteria and Archaea domains. Ferroplasma acidiphilum and Leptospirillum ferriphilum are the dominant species in extremely acid environments and have great use in bioleaching applications; however, the role of each species in this consortia is still a subject of research. The hypothesis of this work is that F. acidiphilum uses the organic matter secreted by L. ferriphilum for growth, maintaining low levels of organic compounds in the culture medium, preventing their toxic effects on L. ferriphilum. To test this hypothesis, a characterization of Ferroplasma acidiphilum strain BRL-115 was made with the objective of determining its optimal growth conditions. Subsequently, under the optimal conditions, L. ferriphilum and F. acidiphilum were tested growing in each other's supernatant, in order to define if there was exchange of metabolites between the species. With these results, a mixed culture in batch cyclic operation was performed to obtain main specific growth rates, which were used to evaluate a mixed metabolic model previously developed by our group. It was observed that F. acidiphilum, strain BRL-115 is a chemomixotrophic organism, and its growth is maximized with yeast extract at a concentration of 0.04% wt/vol. From the experiments of L. ferriphilum growing on F. acidiphilum supernatant and vice versa, it was observed that in both cases cell growth is favorably affected by the presence of the filtered medium of the other microorganism, proving a synergistic interaction between these species. Specific growth rates were obtained in cyclic batch operation of the mixed culture and were used as input data for a Flux Balance Analysis of the mixed metabolic model, obtaining a reasonable behavior of the metabolic fluxes and the system as a whole, therefore consolidating the model previously developed. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1390-1396, 2016.

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The first manually curated genome-scale metabolic model for Salinispora tropica strain CNB-440 was constructed. The reconstruction enables characterization of the metabolic capabilities for understanding and modeling the cellular physiology of this actinobacterium. The iCC908 model was based on physiological and biochemical information of primary and specialised metabolism pathways. The reconstructed stoichiometric matrix consists of 1169 biochemical conversions, 204 transport reactions and 1317 metabolites. A total of 908 structural open reading frames (ORFs) were included in the reconstructed network. The number of gene functions included in the reconstructed network corresponds to 20% of all characterized ORFs in the S. tropica genome. The genome-scale metabolic model was used to study strain-specific capabilities in defined minimal media. iCC908 was used to analyze growth capabilities in 41 different minimal growth-supporting environments. These nutrient sources were evaluated experimentally to assess the accuracy of in silico growth simulations. The model predicted no auxotrophies for essential amino acids, which was corroborated experimentally. The strain is able to use 21 different carbon sources, 8 nitrogen sources and 4 sulfur sources from the nutrient sources tested. Experimental observation suggests that the cells may be able to store sulfur. False predictions provided opportunities to gain new insights into the physiology of this species, and to gap fill the missing knowledge. The incorporation of modifications led to increased accuracy in predicting the outcome of growth/no growth experiments from 76 to 93%. iCC908 can thus be used to define the metabolic capabilities of S. tropica and guide and enhance the production of specialised metabolites.

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The oxidation process of sulfide minerals in natural environments is achieved by microbial communities from the Archaea and Bacteria domains. A metabolic reconstruction of two dominant species, Leptospirillum ferriphilum and Ferroplasma acidiphilum, which are always found together as a mixed culture in this natural environments, was made. The metabolic model, composed of 152 internal reactions and 29 transport reactions, describes the main interactions between these species, assuming that both use ferrous iron as energy source, and F. acidiphilum takes advantage of the organic compounds secreted by L. ferriphilum for chemomixotrophic growth. A first metabolic model for a mixed culture used in bacterial leaching is proposed in this article, which pretends to represent the characteristics of the mixed culture in a simplified manner. It was evaluated with experimental data through flux balance analysis (FBA) using as objective function the maximization of biomass. The growth yields on ferrous iron obtained for each microorganism are consistent with experimental data, and the flux distribution obtained allows understanding of the metabolic capabilities of both microorganisms growing together in a bioleaching process. The model was used to simulate the growth of F. acidiphilum on different substrates, to determine in silico which compounds maximize cell growth, and which are essential. Knockout simulations were carried out for L. ferriphilum and F. acidiphilum metabolic models, predicting key enzymes of central metabolism. The results of this analysis are consistent with experimental data from literature, showing a robust behavior of the metabolic model.

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Macroalgae have high potential to be an efficient, and sustainable feedstock for the production of biofuels and other more valuable chemicals. Attempts have been made to enable the co-fermentation of alginate and mannitol by Saccharomyces cerevisiae to unlock the full potential of this marine biomass. However, the efficient use of the sugars derived from macroalgae depends on the equilibrium of cofactors derived from the alginate and mannitol catabolic pathways. There are a number of strong metabolic limitations that have to be tackled before this bioconversion can be carried out efficiently by engineered yeast cells. An analysis of the redox balance during ethanol fermentation from alginate and mannitol by Saccharomyces cerevisiae using metabolic engineering tools was carried out. To represent the strain designed for conversion of macroalgae carbohydrates to ethanol, a context-specific model was derived from the available yeast genome-scale metabolic reconstructions. Flux balance analysis and dynamic simulations were used to determine the flux distributions. The model indicates that ethanol production is determined by the activity of 4-deoxy-l-erythro-5-hexoseulose uronate (DEHU) reductase (DehR) and its preferences for NADH or NADPH which influences strongly the flow of cellular resources. Different scenarios were explored to determine the equilibrium between NAD(H) and NADP(H) that will lead to increased ethanol yields on mannitol and DEHU under anaerobic conditions. When rates of mannitol dehydrogenase and DehRNADH tend to be close to a ratio in the range 1-1.6, high growth rates and ethanol yields were predicted. The analysis shows a number of metabolic limitations that are not easily identified through experimental procedures such as quantifying the impact of the cofactor preference by DEHU reductase in the system, the low flux into the alginate catabolic pathway, and a detailed analysis of the redox balance. These results show that production of ethanol and other chemicals can be optimized if a redox balance is achieved. A possible methodology to achieve this balance is presented. This paper shows how metabolic engineering tools are essential to comprehend and overcome this limitation.

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AIMS: Cloning, expression and characterization of a new cold-adapted protease with potential biotechnological application, isolated from Antarctic bacteria. METHOD AND RESULTS: A subtilisin-like gene was isolated from several Antarctic bacterial genus using CODPEHOP-designed primers and a genome walking method. This gene encodes a precursor protein, which undergoes an autocatalytic cleavage resulting in a 34.6 kDa active cold-adapted protease with a maximum activity at 25-35°C and optimum pH of 8.0-9.0. It showed a higher catalytic efficiency at lower temperatures compared to its mesophilic counterpart. Heat-induced inactivation resulted in a very low melting point. Local packing analysis using the homology model indicated Ala284 as an important cold-adaptation determinant, which was corroborated by the site-directed mutagenesis. CONCLUSIONS: A new thermolabile subtilisin-like protease has been successfully cloned and analysed, and an important hot spot in the evolution of the cold adaptation and substrate specificity of this enzyme was identified and tested. SIGNIFICANCE AND IMPACT OF THE STUDY: This work reports a new cold-adapted protease with a vast representation amongst Antarctic genus, suggesting therefore its evolutionary success in this cold environment. Likewise, important sites for genetic potentiation have been identified, which are extrapolated to other enzymes of the same kind.

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A continuous model of a metabolic network including gene regulation to simulate metabolic fluxes during batch cultivation of yeast Saccharomyces cerevisiae was developed. The metabolic network includes reactions of glycolysis, gluconeogenesis, glycerol and ethanol synthesis and consumption, the tricarboxylic acid cycle, and protein synthesis. Carbon sources considered were glucose and then ethanol synthesized during growth on glucose. The metabolic network has 39 fluxes, which represent the action of 50 enzymes and 64 genes and it is coupled with a gene regulation network which defines enzyme synthesis (activities) and incorporates regulation by glucose (enzyme induction and repression), modeled using ordinary differential equations. The model includes enzyme kinetics, equations that follow both mass-action law and transport as well as inducible, repressible, and constitutive enzymes of metabolism. The model was able to simulate a fermentation of S. cerevisiae during the exponential growth phase on glucose and the exponential growth phase on ethanol using only one set of kinetic parameters. All fluxes in the continuous model followed the behavior shown by the metabolic flux analysis (MFA) obtained from experimental results. The differences obtained between the fluxes given by the model and the fluxes determined by the MFA do not exceed 25% in 75% of the cases during exponential growth on glucose, and 20% in 90% of the cases during exponential growth on ethanol. Furthermore, the adjustment of the fermentation profiles of biomass, glucose, and ethanol were 95%, 95%, and 79%, respectively. With these results the simulation was considered successful. A comparison between the simulation of the continuous model and the experimental data of the diauxic yeast fermentation for glucose, biomass, and ethanol, shows an extremely good match using the parameters found. The small discrepancies between the fluxes obtained through MFA and those predicted by the differential equations, as well as the good match between the profiles of glucose, biomass, and ethanol, and our simulation, show that this simple model, that does not rely on complex kinetic expressions, is able to capture the global behavior of the experimental data. Also, the determination of parameters using a straightforward minimization technique using data at only two points in time was sufficient to produce a relatively accurate model. Thus, even with a small amount of experimental data (rates and not concentrations) it was possible to estimate the parameters minimizing a simple objective function. The method proposed allows the obtention of reasonable parameters and concentrations in a system with a much larger number of unknowns than equations. Hence a contribution of this study is to present a convenient way to find in vivo rate parameters to model metabolic and genetic networks under different conditions.

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Little is known about the HIV testing behaviours among Caribbean youth. The purpose of this study was to determine the prevalence of and the factors associated with HIV testing in Caribbean youth. Data were used from population-based surveys in three Caribbean countries: Guyana, Haiti and the Dominican Republic. Chi-square tests and multiple logistic regression models were fitted to examine the associations between independent correlates and ever being tested of HIV. The proportion of youth reporting HIV testing ranged from 17% in Haiti to 48% in the Dominican Republic. Across all countries increased odds of ever being tested for HIV was associated with ever being married, having five or more lifetime partners, and knowing someone who was living with or had died from HIV. Male gender and rural residence were correlated with decreased odds of HIV testing. Low levels of HIV testing were observed among sexually experienced youth in three Caribbean countries. Men, younger and never married youth were less likely to be have been tested for HIV. Outreach and public health messages targeting these youth should be implemented. Programmes that assist youth in accurately assessing their risk behaviours are also required to improve HIV testing.

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OBJECTIVES: Human Immunodeficiency Virus (HIV) testing is the gateway to treatment and care of HIV infection, however, little is known about the HIV testing behaviours among Caribbean youth. The objective of this study was to determine the prevalence of HIV testing and to examine associations of HIV testing with sociodemographic characteristics and risk behaviours. Methods:Data were used from nationally representative surveys in three Caribbean countries: Guyana AIDS Indicator Survey 2005- 2006; Haiti Demographic and Health Survey 2005-2006 and the Dominican Republic Demographic and Health Survey 2007. Youth 15-24 years who had ever heard of AIDS and ever had sex were selected, yielding samples of875 in Guyana, 4199 in Haiti and 12 418 in the Dominican Republic. Bivariate tests were conducted to examine the associations between sociodemographic characteristics, risk behaviours and being tested for HIV. RESULTS: The proportion of youth reporting HIV testing ranged from 17% in Haiti to 48% in the Dominican Republic. About 54% of youth in Haiti and less than one-third in the Dominican Republic initiated HIV testing. A greater proportion of females than males had ever tested in each country, ranging from 68% in Guyana to 82% in Haiti. Higher rates ofHIV testing were observed among ever married youth and among youth with 2-4 lifetime sexual partners. CONCLUSIONS: Males, rural and never married youth were less likely to be tested. Outreach at individual and community levels and public health messages targeting these youth should be implemented. There is also a need to mainstream gender into the design ofprogrammes aimed at increasing uptake ofHIV testing. Programmes which assist youth in accurately assessing their risk behaviours are also required to improve HIV testing.

OBJETIVOS: La prueba del virus de la inmunodeficiencia humana (VIH) es la vía de acceso al tratamiento y cuidado de los casos de infección por VIH. Sin embargo, poco se sabe acerca de las conductas en relación con la prueba del VIH entre los jóvenes del Caribe. El objetivo de este estudio fue determinar la prevalencia de la prueba de VIH y examinar las asociaciones del VIH con las características sociodemográficas y los comportamientos de riesgo. MÉTODOS: Se usaron datos de encuestas nacionalmente representativas de tres países caribeños: la Encuesta de Indicadores del SIDA en Guyana, 2005-2006; Encuesta Demográfica y de Salud de Haití, 2005-2006; y la Encuesta Demográfica y de Salud de República Dominicana, 2007. Se seleccionaron jóvenes de 15-24 años que habían oído hablar alguna vez de SIDA y alguna vez habían tenido sexo, produciéndose muestras de 875 en Guyana, 4199 en Haití y 12418 en República Dominicana. Se llevaron a cabo pruebas bivariantes para examinar las asociaciones entre las características sociodemográficas, los comportamientos de riesgo, y las pruebas de VIH. RESULTADOS: La proporción de jóvenes que reportan haber tenido pruebas de VIH, fluctuó de 17% en Haití a 48% en República Dominicana. Aproximadamente el 54% de los jóvenes de Haití y menos de un tercio en la República Dominicana reportaron haberse iniciado en las pruebas de VIH. Una proporción mayor de hembras que de varones se había hecho la prueba alguna vez en estos países, fluctuando de 68% en Guyana a 82% en Haití. Se observaron proporciones más altas de pruebas de VIH entre los jóvenes casados y entre los que habían tenido 2-4 parejas sexuales durante su vida. CONCLUSIÓN: Los jóvenes varones, casados y de áreas rurales mostraron una menor probabilidad de haberse hecho la prueba. Deben implementarse actividades de extensión individual y comunitaria, así como mensajes de salud pública dirigidos a estos jóvenes. Es también necesario incorporar regularmente consideraciones de género en el diseño de los programas dirigidos a aumentar la disposición a recibir pruebas de VIH. Asimismo, se requieren programas que ayuden a la juventud a evaluar con exactitud sus comportamientos de riesgo, a fin de lograr una mejor participación en la prueba de VIH.

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OBJECTIVES: Human Immunodeficiency Virus (HIV) testing is the gateway to treatment and care of HIV infection, however little is known about the HIV testing behaviours among Caribbean youth. The objective of this study was to determine the prevalence of HIV testing and to examine associations of HIV testing with sociodemographic characteristics and risk behaviours. METHODS: Data were used from nationally representative surveys in three Caribbean countries: Guyana AIDS Indicator Survey 2005-2006; Haiti Demographic and Health Survey 2005-2006 and the Dominican Republic Demographic and Health Survey 2007. Youth 15-24 years who had ever heard of AIDS and ever had sex were selected, yielding samples of 875 in Guyana, 4199 in Haiti and 12 418 in the Dominican Republic. Bivariate tests were conducted to examine the associations between sociodemographic characteristics, risk behaviours and being tested for HIV. RESULTS: The proportion of youth reporting HIV testing ranged from 17% in Haiti to 48% in the Dominican Republic. About 54% of youth in Haiti and less than one-third in the Dominican Republic initiated HIV testing. A greater proportion of females than males had ever tested in each country, ranging from 68% in Guyana to 82% in Haiti. Higher rates of HIV testing were observed among ever married youth and among youth with 2-4 lifetime sexual partners. CONCLUSIONS: Males, rural and never married youth were less likely to be tested. Outreach at individual and community levels and public health messages targeting these youth should be implemented. There is also a need to mainstream gender into the design of programmes aimed at increasing uptake of HIV testing. Programmes which assist youth in accurately assessing their risk behaviours are also required to improve HIV testing.

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The rational selection of optimal protein purification sequences, as well as mathematical models that simulate and allow optimization of chromatographic protein purification processes have been developed for purification procedures such as ion-exchange, hydrophobic interaction and gel filtration chromatography. This paper investigates the extension of such analysis to affinity chromatography both in the selection of chromatographic processes and in the use of the rate model for mathematical modelling and simulation. Two affinity systems were used: Blue Sepharose and Protein A. The extension of the theory developed previously for ion-exchange and HIC chromatography to affinity separations is analyzed in this paper. For the selection of operations two algorithms are used. In the first, the value of η, which corresponds to the efficiency (resolution) of the actual chromatography and, Σ, which determines the amount of a particular contaminant eliminated after each separation step, which determines the purity, have to be determined. It was found that the value of both these parameters is not generic for affinity separations but will depend on the type of affinity system used and will have to be determined on a case by case basis. With Blue Sepharose a salt gradient was used and with Protein A, a pH gradient. Parameters were determined with individual proteins and simulations of the protein mixtures were done. This approach allows investigation of chromatographic protein purification in a holistic manner that includes ion-exchange, HIC, gel filtration and affinity separations for the first time.

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A metabolic model for Leptospirillum ferrooxidans was developed based on the genomic information of an analogous iron oxidizing bacteria and on the pathways of ferrous iron oxidation, nitrogen and CO(2) assimilation based on experimental evidence for L. ferrooxidans found in the literature. From this metabolic reconstruction, a stoichiometric model was built, which includes 86 reactions describing the main catabolic and anabolic aspects of its metabolism. The model obtained has 2 degrees of freedom, so two external fluxes were estimated to achieve a determined and observable system. By using the external oxygen consumption rate and the generation flux biomass as input data, a metabolic flux map with a distribution of internal fluxes was obtained. The results obtained were verified with experimental data from the literature, achieving a very good prediction of the metabolic behavior of this bacterium at steady state.

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Retinopathy of prematurity is an emerging cause of blindness in Latin America. In 2007, the authors examined 88 consecutive infants in Guatemala City, Guatemala, who met the screening criteria for retinopathy of prematurity; 14 (16%) had either "plus" or stage V disease. The incidence of retinopathy of prematurity in Guatemala City is likely high, and a formal screening program is warranted.

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OBJECTIVE: To clarify the process of end-of-life decision-making in culturally different neonatal intensive care units (NICUs). STUDY DESIGN: Review of medical files of newborns >22 weeks gestation who died in the delivery room (DR) or the NICU during 12 months in 4 NICUs (Chicago, Milwaukee, Montreal, and Groningen). We categorized deaths using a 2-by-2 matrix and determined whether mechanical ventilation was withdrawn/withheld and whether the child was dying despite ventilation or physiologically stable but extubated for neurological prognosis. RESULTS: Most unstable patients in all units died in their parents' arms after mechanical ventilation was withdrawn. In Milwaukee, Montreal, and Groningen, 4% to 12% of patients died while receiving cardiopulmonary resuscitation. This proportion was higher in Chicago (31%). Elective extubation for quality-of-life reasons never occurred in Chicago and occurred in 19% to 35% of deaths in the other units. The proportion of DR deaths in Milwaukee, Montreal, and Groningen was 16% to 22%. No DR deaths occurred in Chicago. CONCLUSIONS: Death in the NICU occurred differently within and between countries. Distinctive end-of-life decisions can be categorized separately by using a model with uniform definitions of withholding/withdrawing mechanical ventilation correlated with the patient's physiological condition. Cross-cultural comparison of end-of-life practice is feasible and important when comparing NICU outcomes.

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The HEK293 cell line has been used for the production of adenovirus vectors to be used in the potential treatment of alcoholism using a gene therapy strategy. Culture optimization and scale-up has been achieved by first adapting the cells to serum-free media and secondly by growing them in suspension. Adenovirus production after infection was increased, resulting in higher specific glucose consumption and lactate accumulation rates compared to the growth phase. We applied media design tools and Metabolic Flux Analysis (MFA) to compare the metabolic states of cells during growth and adenovirus production and to optimize culture media according to the metabolic demand of the cells in terms of glucose and glutamine concentrations. This allowed obtaining a higher maximum cell concentration and increased adenovirus production by minimizing the production of metabolites that can have an inhibitory effect on cell growth. We have proposed a stoichiometric equation for adenovirus synthesis. MFA results allowed determination of how these changes in composition affected the way cells distribute their nutrient resources during cell growth and virus production. Virus purification was successfully achieved using chromatography and Aqueous Two-Phase Systems (ATPS).

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To achieve a high level of purity in the purification of recombinant proteins for therapeutic or analytical application, it is necessary to use several chromatographic steps. There is a range of techniques available including anion and cation exchange, which can be carried out at different pHs, hydrophobic interaction chromatography, gel filtration and affinity chromatography. In the case of a complex mixture of partially unknown proteins or a clarified cell extract, there are many different routes one can take in order to choose the minimum and most efficient number of purification steps to achieve a desired level of purity (e.g. 98%, 99.5% or 99.9%). This review shows how an initial 'proteomic' characterization of the complex mixture of target protein and protein contaminants can be used to select the most efficient chromatographic separation steps in order to achieve a specific level of purity with a minimum number of steps. The chosen methodology was implemented in a computer- based Expert System. Two algorithms were developed, the first algorithm was used to select the most efficient purification method to separate a protein from its contaminants based on the physicochemical properties of the protein product and the protein contaminants and the second algorithm was used to predict the number and concentration of contaminants after each separation as well as protein product purity. The application of the Expert System approach was experimentally tested and validated with a mixture of four proteins and the experimental validation was also carried out with a supernatant of Bacillus subtilis producing a recombinant beta-1,3-glucanase. Once the type of chromatography is chosen, optimization of the operating conditions is essential. Chromatographic elution curves for a three-protein mixture (alpha-lactoalbumin, ovalbumin and beta-lactoglobulin), carried out under different flow rates and ionic strength conditions, were simulated using two different mathematical models. These models were the Plate Model and the more fundamentally based Rate Model. Simulated elution curves were compared with experimental data not used for parameter identification. Deviation between experimental data and the simulated curves using the Plate Model was less than 0.0189 (absorbance units); a slightly higher deviation [0.0252 (absorbance units)] was obtained when the Rate Model was used. In order to optimize operating conditions, a cost function was built that included the effect of the different production stages, namely fermentation, purification and concentration. This cost function was also successfully used for the determination of the fraction of product to be collected (peak cutting) in chromatography. It can be used for protein products with different characteristics and qualities, such as purity and yield, by choosing the appropriate parameters.

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A stoichiometric model of Acidithiobacillus ferrooxidans based on the sequenced genome from strain ATCC 23270 is derived and parameterized using genome/pathway databases. The model describes the main aspects of catabolism and anabolism. By the construction and utilization of the mathematical determination of the network, metabolic flux analysis is performed for such a bacterium for the first time and results are successfully verified by comparison to literature values. This first metabolic model of A. ferrooxidans is able to simulate the main aspects of metabolism and will be useful for further investigation and improvement of bioleaching procedures.

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This paper describes the use of a discrete mathematical model to represent the basic mechanisms of regulation of the bacteria E. coli in batch fermentation. The specific phenomena studied were the changes in metabolism and genetic regulation when the bacteria use three different carbon substrates (glucose, glycerol, and acetate). The model correctly predicts the behavior of E. coli vis-à-vis substrate mixtures. In a mixture of glucose, glycerol, and acetate, it prefers glucose, then glycerol, and finally acetate. The model included 67 nodes; 28 were genes, 20 enzymes, and 19 regulators/biochemical compounds. The model represents both the genetic regulation and metabolic networks in an inrtegrated form, which is how they function biologically. This is one of the first attempts to include both of these networks in one model. Previously, discrete mathematical models were used only to describe genetic regulation networks. The study of the network dynamics generated 8 (2(3)) fixed points, one for each nutrient configuration (substrate mixture) in the medium. The fixed points of the discrete model reflect the phenotypes described. Gene expression and the patterns of the metabolic fluxes generated are described accurately. The activation of the gene regulation network depends basically on the presence of glucose and glycerol. The model predicts the behavior when mixed carbon sources are utilized as well as when there is no carbon source present. Fictitious jokers (Joker1, Joker2, and Repressor SdhC) had to be created to control 12 genes whose regulation mechanism is unknown, since glycerol and glucose do not act directly on the genes. The approach presented in this paper is particularly useful to investigate potential unknown gene regulation mechanisms; such a novel approach can also be used to describe other gene regulation situations such as the comparison between non-recombinant and recombinant yeast strain, producing recombinant proteins, presently under investigation in our group.

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Elution curves in ionic exchange chromatography (IEC) for a three-protein mixture (alpha-lactoalbumin, ovalbumin, and beta-lactoglobulin), carried out under different flow rates and ionic strength conditions, were simulated using two different mathematical models. These models were the Plate Model and the more fundamentally based Rate Model. Relatively low protein concentrations were used to avoid protein-protein interactions. Simulated elution curves were compared with experimental data not used for parameter identification. Deviation between experimental data and the simulated curves using the Plate Model was less than 0.0189 (absorbance units); a slightly higher deviation [0.0252 (absorbance units)] was obtained when the Rate Model was used. A cost function was built that included the effect of the different production stages, namely fermentation, purification, and concentration. These considered the effect on the performance of IEC; yield, purity, concentration and the time needed to accomplish the separation. Operational conditions in the IEC such as flow rate, ionic strength gradient and the operational time can be selected using this model in order to find the minimum cost for the protein production process depending on the characteristics of the final product desired such as purity and yield. This cost function was successfully used for the selection of the operational conditions as well as the fraction of the product to be collected (peak cutting) in IEC. It can be used for protein products with different characteristics and qualities, such as purity and yield, by choosing the appropriate parameters.

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Correlations to describe the effect of surface hydrophobicity and charge of proteins with their partition coefficient in aqueous two-phase systems were investigated. Polyethylene glycol (PEG) 4000/phosphate, sulfate, citrate, and dextran systems in the presence of low (0.6% w/w) and high (8.8% w/w) levels of NaCl were selected for a systematic study of 12 proteins. The surface hydrophobicity of the proteins was measured by ammonium sulfate precipitation as the inverse of their solubility. The hydrophobicity values measured correlated well with the partition coefficients, K, obtained in the PEG/salt systems at high concentration of NaCl (r = 0.92-0.93). In PEG/citrate systems the partition coefficient correlated well with protein hydrophobicity at low and high concentrations of NaCl (r = 0.81 and 0.93, respectively). The PEG/citrate system also had a higher hydrophobic resolution than other systems to exploit differences in the protein's hydrophobicity. The surface charge and charge density of the proteins was determined over a range of pH (3-9) by electrophoretic titration curves; PEG/salt systems did not discriminate well between proteins of different charge or charge density. In the absence of NaCl, K decreased slightly with increased positive charge. At high NaCl concentration, K increased as a function of positive charge. This suggested that the PEG-rich top phase became more negative as the concentration of NaCl in the systems increased and, therefore, attracted the positively charged proteins. The effect of charge was more important in PEG/dextran systems at low concentrations of NaCl. In the PEG/dextran systems at lower concentration of NaCl, molecular weight appeared to be the prime determinant of partition, whereas no clear effect of molecular weight could be found in PEG/salt systems.

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An attempt has been made to adopt a different approach to evaluate the effect of a protein's charge on its partitioning behaviour in PEG/salt aqueous two-phase systems (ATPS). This has been done using a computer methodology (DelPhi) that allows the calculation of the electrostatic solvation energy that charged proteins present in a particular media such as aqueous polymer-salt systems. This calculation was done for the protein in each of the phases and a correlation was investigated that related the electrostatic energy difference of the protein in each of the phases and its partition coefficient in ATPS. Such correlation resulted in a statistical model that also included the effect of molecular weight and a shape factor at each particular pH. A global correlation which included the effect of pH was also found. All the correlations were statistically evaluated and gave good results.

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