Energy metabolism system

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

Purines such as ATP and adenosine play a central role in the energy metabolism of all life forms. This fact probably delayed recognition of other roles for purines as autocrine and paracrine substances and neurotransmitters. Today it is recognized that purines are released from neurons and other cells and that they produce widespread effects on multiple organ systems by binding to purinergic receptors located on the cell surface. The principal ligands for... 

Sokoloff, L. Relationships among local functional activity, energy metabolism, and blood flow in the central nervous system. Fed. Proc. 40 2311-2316,1981. 

Normal biochemical events surrounding the maintenance and functions of the nervous system centers around energy metabolism, biosynthesis of macromolecules, and neurotransmitter synthesis, storage, release, uptake, and degradation. Measurement of these events is complicated by the sequenced nature of the components of the nervous system and the transient and labile nature of the moieties involved. Use of measurements of alterations in these functions as indicators of neurotoxicity is further complicated by our lack of a complete understanding of the normal operation of these systems and by the multitude of day-to-day occurrences (such as diurnal cycle, diet, temperature, age, sex, and endocrine status) which are constantly modulating the baseline system. For detailed discussions of these difficulties, the reader is advised to see Damstra and Bondy (1980, 1982). 

At the point where amphiphiles were recruited to provide the precursors to cell membranes, stable lipid vesicles could have evolved [141] to enclose autocatalytic chiral hypercycles. Credible models for the subsequent evolution of vesicles containing self-replicating chiral molecules have appeared in the literature. [193,194] These vesicles could then emerge from the feldspar spaces [134,192] as micron-sized self-reproducing, energy-metabolizing vesicular systems protobacteria ready to face the hydrothermal world on their own terms. 

In summary, methanophenazine (10) is the first phenazine whose involvement in the electron transport of biologic systems could be established. The experiments indicate that its role in the energy metabolism of methanogens corresponds to that of ubiquinones in mitochondria and bacteria. 

The objective of this chapter is to place energy metabolism within the context of the biochemistry that takes place in the whole body, in the tissnes and in biochemical pathways or processes. In all of these systems, the generation and ntilisation of ATP are central. Consequently, the final dis-... 

Using the transport systems in the membranes, cells regulate their volume, internal pH value, and ionic environment. They concentrate metabolites that are important for energy metabolism and biosynthesis, and exclude toxic substances. Transport systems also serve to establish ion gradients, which are required for oxidative phosphorylation and stimulation of muscle and nerve cells, for example (see p. 350). 

An index of the phospho anhydride (i.e.,P—O—P) bond content of the adenine nucleotides of a cell, based on a hypothetical modeP that attempts to explain the metabolic basis for control of ATP utilization and regeneration. Later studies demonstrated that the energy charge model is overly simplistic and that its principles are unlikely to constitute a useful model for the control of energy metabolism within biological systems. 

Fluoroacetate produces its toxic action by inhibiting the citric acid cycle. The fluorine-substituted acetate is metabolized to fluoroci-trate that inhibits the conversion of citrate to isocitrate. There is an accumulation of large quantities of citrate in the tissue, and the cycle is blocked. The heart and central nervous system are the most critical tissues involved in poisoning by a general inhibition of oxidative energy metabolism. ... 

Y. Handa, T. Kubota, A. Tsuchida, M. Kaneko, H. Caner, H. Kobayashi and T. Kubota, Effect of systemic hypotension on cerebral energy metabolism during chronic cerebral vasospasm in primates. /. Neurosurg., 1993, 78,112-119. 

The oxidative decarboxylation of pyruvate and a-ketoglutaraie I which plays a key role in energy metabolism of most cells, is parn ularly important in tissues of the nervous system. In thiaminel deficiency, the activity of these two dehydrogenase reactions <1 decreased, resulting in a decreased production of ATP and, fxsi I... 

A characteristic of this nonequilibrium or irreversible thermodynamics is that time is explicitly introduced. Furthermore, open systems, in which materials and energy flow into and out of the system, are considered. Clearly, a living organism is an open system not a closed one of classical thermodynamics. Because of the flow of materials concentration gradients are set up and transport phenomena often become of primary importance. Articles and books that provide an introduction to nonequilibrium thermodynamics and to the literature in the field include the following.10 26 28 34 Whether these methods can be applied in a practical way to metabolic systems has been debated.35 36... 

Of central importance to the energy metabolism of all cells is the adenylate system which consists of adenosine 5 -triphosphate (ATP), adenosine 5-diphosphate (ADP), and adenosine 5 -monophosphate (AMP) together with inorganic phosphate (P ), pyrophosphate (PP ), and magnesium ions. Remember that P refers to the mixture of ionic forms of phosphoric acid present under experimental conditions. Between pH 4 and pH 10 this will be mainly LLPO,... 

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