PubMed · 13857525
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1960-04-23. KREBS cycle.. https://pubmed.ncbi.nlm.nih.gov/13857525/
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It is of interest to calculate equilibrium compositions of systems of biochemical reactions at specified concentrations of coenzymes because these reactants tend to be in steady states. Thermodynamic calculations under these conditions require the definition of a further transformed Gibbs energy G" by use of a Legendre transform. These calculations are applied to the pyruvate dehydrogenase reaction plus the citric acid cycle, but steady-state concentrations of CoA, acetyl-CoA and succinyl-CoA cannot be specified because they are involved in the conservation of carbon atoms. These calculations require the use of linear algebra to obtain further transformed Gibbs energies of formation of reactants and computer programs to calculate equilibrium compositions. At specified temperature, pH, ionic strength and specified concentrations of several coenzymes, the equilibrium composition depends on the specified concentrations of the coenzymes and the initial amounts of reactants.
We investigated oxidative processes in mitochondria of Saccharomyces cerevisiae grown on ethanol in the course of chronological aging. We elaborated a model of chronological aging that avoids the influence of exhaustion of medium, as well as the accumulation of toxic metabolites during aging. A decrease in total respiration of cells and, even more, of the contribution of respiration coupled with ATP-synthesis was observed during aging. Aging is also related with the decrease of the contribution of malonate-insensitive respiration. Activities of citrate-synthase (CS), alpha-ketoglutarate dehydrogenase (KGDH) and malate dehydrogenase (MDH) were threefold decreased. The activity of NADP-dependent isocitrate dehydrogenase (NADP-ICDH) decreased more significantly, while the activity of NAD-dependent isocitrate dehydrogenase (NAD-ICDH) fell even greater, being completely inactivated on the third week of aging. In contrast, succinate dehydrogenase (SDH), enzymes of glyoxylate cycle (GCL) (isocitrate lyase (ICL) and malate synthase (MLS)), and enzymes of ethanol oxidation (alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ACDH)), were activated by 50% or more. The behavior of oxidative enzymes and metabolic pathways are apparently inherent to a more viable, long-lived cells in population, selected in the course of chronological aging. This selection allows cells to reveal the mechanism of their higher viability as caused by shunting of complete Krebs cycle by glyoxylate cycle, with a concomitant increased rate of the most efficient energy source, namely succinate formation and oxidation. Thiobarbituric-reactive species (TAR species) increased during aging. We supposed that to be the immediate cause of damage of a part of yeast population. These data show that a greater succinate contribution to respiration in more active cells is a general property of yeast and animal tissues.
Saccharomyces cerevisiae is able to switch from fermentation to respiration (diauxic shift) with major changes in metabolic activity. This phenomenon has been previously studied on the transcriptional level. Here we present a parallel analysis of the yeast mitochondrial proteome and the corresponding transcriptional activity in cells grown on glucose (fermentation) and glycerol (respiration). A two-dimensional reference gel for this organelle proteome was established (available at www.biochem.oulu.fi/proteomics/), which contains about 800 intense spots. From 459 spots 253 individual proteins were identified, among them low abundant and hydrophobic proteins, and 37 proteins previously deemed hypothetical, with partially unknown cellular localization. After the diauxic shift, mitochondrial levels of only 18 proteins were changed (17 increased, with 1 decreased), among them proteins involved in the tricarboxylic acid cycle (Sdh1p, Sdh2p, and Sdh4p) and the respiratory chain (Cox4p, Cyb2p, and Qcr7p), proteins contributing to other respiratory pathways (Ach1p, Adh2p, Ald4p, Cat2p, Icl2p, and Pdh1p), and two proteins with unknown function (Om45p and Ybr230p). Apart from an overall increase in mitochondrial protein mass, the mitochondrial proteome remains remarkably constant, even in a major metabolic adaptation. This seemingly disagrees with results of the DNA microarray analyses, where a rather heterogenous up- or down-regulation of genes encoding mitochondrial proteins implies large changes in the proteome. We propose that the discrepancy between proteome and transcriptional regulation, apart from different translation efficiency, indicates a changed turnover rate of proteins in different physiological conditions.