Publisher Summary This chapter discusses the mitochondrial respiratory control and the polarographic measurement of ADP : O ratios. The polarographic oxygen electrode technique is used for measuring rapid changes in the rate of oxygen utilization by cellular and subcellular systems. Although the polarographic method measures changes in oxygen concentration of photosynthetic systems, yeast cells, and nerve, but the oxygen electrode technique is applied to a study the mitochondrial respiration and oxidative phosphorytation. The principle of the oxygen electrode has been summarized, and the design of the vibrating oxygen electrode for use with speetrophotometric studies is illustrated. The oxygen electrode apparatus can be calibrated in a number of ways. A more accurate calibration of oxygen content can be obtained by gas equilibration with various nitrogen-oxygen mixtures. When tightly coupled mitochondria are suspended in an isotonic buffer, a slow rate of oxygen uptake is measured in the presence of substrate and absence of ADP. Addition of ADP causes an immediate increase in the rate of oxygen utilization. The concentration of oxygen utilized is proportional to the amount of ADP phosphorylated to ATP. The type of oxygen electrode tracings is presented from which an ADP : O ratio (equivalent to a P : O ratio) can be directly calculated.

[7] Mitochondrial respiratory control and the polarographic measurement of ADP : O ratios

Microbial conversion of d-xylose to xylitol

Hemicellulosic sugars, especially d-xylose, are relatively abundant in agricultural and forestry residues. Moreover, they can be recovered from the hemicelluloses by acid hydrolysis more readily and in better yields than can d-glucose from cellulose. These factors favor hemicellulosic sugars as a feedstock for production of ethanol and other chemicals. Unfortunately, d-xylose is not so readily utilized as d-glucose for the production of chemicals by microorganisms. The reason may lie in the biochemical pathways used for pentose and hexose metabolism. Different pathways are employed by prokaryotes and eukaryotes in the initial stages of pentose assimilation. Transport and phosphorylation possibly limit the overall rate of d-xylose utilization. The intermediary steps of pentose metabolism are generally similar for both bacteria and fungi, but substantial variations exist. Phosphoketolase is present in some yeasts and bacteria able to use pentoses. Regulation of the oxidative pentose phosphate pathway occurs at d-glucose-6-phosphate dehydrogenase by the intracellular concentration of NADPH. Regulation of nonoxidative pentose metabolism is not well understood. In some yeasts and fungi, conversion of d-xylose to ethanol takes place under aerobic or anaerobic conditions with rates and yields generally higher in the former than in the latter. Xylitol and acetic acid are major byproducts of such conversions. Many yeasts are capable of utilizing d-xylose for the production of ethanol. Direct conversion of d-xylose to ethanol is compared with two-stage processes employing yeasts and d-xylose isomerase.

Utilization of xylose by bacteria, yeasts, and fungi

Metabolic engineering of Saccharomyces cerevisiae for ethanolic fermentation of xylose is summarized with emphasis on progress made during the last decade. Advances in xylose transport, initial xylose metabolism, selection of host strains, transformation and classical breeding techniques applied to industrial polyploid strains as well as modeling of xylose metabolism are discussed. The production and composition of the substrates — lignocellulosic hydrolysates — is briefly summarized. In a future outlook iterative strategies involving the techniques of classical breeding, quantitative physiology, proteomics, DNA micro arrays, and genetic engineering are proposed for the development of efficient xylose-fermenting recombinant strains of S. cerevisiae.

Metabolic engineering of Saccharomyces cerevisiae for xylose utilization.

Lignocellulosic materials containing cellulose, hemicellulose, and lignin as their main constituents are the most abundant renewable organic resource present on Earth The conversion of both cellulose and hemicellulose for production of fuel ethanol is being studied intensively with a view to develop a technically and economically viable bioprocess The fermentation of glucose, the main constituent of cellulose hydrolyzate, to ethanol can be carried out efficiently On the other hand, although bioconversion of xylose, the main pentose sugar obtained on hydrolysis of hemicellulose, to ethanol presents a biochemical challenge, especially if it is present along with glucose, it needs to be fermented to make the biomass-to-ethanol process economical A lot of attention therefore has been focussed on the utilization of both glucose and xylose to ethanol Accordingly, while describing the advancements that have taken place to get xylose converted efficiently to ethanol by xylose-fermenting organisms, the review deals mainly with the strategies that have been put forward for bioconversion of both the sugars to achieve high ethanol concentration, yield, and productivity The approaches, which include the use of (1) xylose-fermenting yeasts alone, (2) xylose isomerase enzyme as well as yeast, (3) immobilized enzymes and cells, and (4) sequential fermentation and co-culture process are described with respect to their underlying concepts and major limitations Genetic improvements in the cultures have been made either to enlarge the range of substrate utilization or to channel metabolic intermediates specifically toward ethanol These contributions represent real significant advancements in the field and have also been adequately dealt with from the point of view of their impact on utilization of both cellulose and hemicellulose sugars to ethanol

Simultaneous bioconversion of cellulose and hemicellulose to ethanol

The respiratory chain and the oxidative phosphorylation of rat brain mitochondria.

A new oxygen electrode model for the polarographic assay of cellular and mitochondrial respiration

Biochemistry and genetics of yeasts : pure and applied aspects : proceedings of the symposium held at the Universidade de São Paulo, Brazil, December 4-10, 1977

Respiration-rates and oxidative phosphorylation of heart sarcosomes in kwashiorkor.

Pentose metabolism in Candida. II. The diphosphopyridine nucleotide-specific polyol dehydrogenase of Candida utilis.

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