Carbohydrates, breakdown

Steps 1-2 of Figure 29.5 Acyl Transfers The starting material for fatty-acid synthesis is the thioesteT acetyl CoA, the ultimate product of carbohydrate breakdown, as we ll see in Section 29.6. The synthetic pathway begins with several priming reactions, which transport acetyl CoA and convert it into more reactive species. The first priming reaction is a nucleophilic acyl substitution reaction that converts acetyl CoA into acetyl ACP (acyl carrier protein). The reaction is catalyzed by ACP transacyla.se. 

Compounds are discussed in terms of their structure. For example, fatly acids are discussed as aliphatic compounds, even though their presence is the result of carbohydrate breakdown. 

Carbohydrate metabolism in the organism tissues encompasses enzymic processes leading either to the breakdown of carbohydrates (catabolic pathways), or to the synthesis thereof (anabolic pathways). Carbohydrate breakdown leads to energy release or intermediary products that are necessary for other biochemical processes. The carbohydrate synthesis serves for replenishment of polysaccharide reserve or for renewal of structural carbohydrates. The effectiveness of various routes of carbohydrate metabolism in tissues and organs is defined by the availability of appropriate enzymes in them. 

The wall of the small intestine is permeable to water and to small molecules such as the amino acids produced by protein breakdown and sugars produced by carbohydrate breakdown so this system is a reactor-separator combination, a membrane reactor. Finally the undigested food passes into the large intestine, where more water is removed through the permeable wall before exiting the reactor. 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Parasitic stages, on the other hand, generally do not use oxygen as the final electron acceptor but use fermentative processes to obtain most of their ATP. For these stages, an uneconomical energy metabolism is not detrimental, as the host provides the nutrients. Most adult flatworms inside the final host produce end products of a fermentative carbohydrate breakdown, such as succinate, acetate, propionate and lactate. These end products are formed via malate dismutation, a fermentative pathway, which is present in all types of parasitic worms (flatworms as well as many nematodes), but which is also present in animals like freshwater snails, mussels, oysters and other marine organisms. Malate dismutation is linked to a specially... 

The demonstration of the importance of inorganic phosphate in carbohydrate breakdown, the discovery of ATP (1928-1929) and the successful elucidation of the mechanisms by which Pj was incorporated into glycolytic intermediates profoundly influenced interpretation of mitochondrial and photosynthetic phosphorylation (see Ord and Stocken, 1995 Whatley, 1997). The importance of the proton motive force was aggressively disputed for several years by traditional biochemists (see Ferguson, 1997). 

Table 5.4. End-products of carbohydrate breakdown in cestodes, besides CO 2. (Data from Barrett, 1981 Rahman Met trick, 1982 Pampori et al. 1984a,b McManus Sterry, 1982J...

The characteristic feature of carbohydrate breakdown in cestodes is the production of a range of complex end-products, usually organic acids, even under aerobic conditions (Table 5.4). This contrasts with predominantly aerobic organisms, such as most free-living metazoa, where the end-product of glycolysis is almost exclusively lactic acid formed from pyruvic acid. Lactic acid is produced as a result of rapid muscular contraction carried out essentially under anaerobiosis and its production ensures a rapid expenditure of energy without the limitation due to the rate of diffusion of oxygen. The anaerobic phase is followed by an aerobic phase, where pyruvic acid is metabolised to acetyl-coenzyme A which is in turn oxidised completely to... 

They rejected the oxygen context for the selective advantage of the protomitochondrial symbionts to their host based on increased efficiency in ATP production through respiratory carbohydrate breakdown. 

Stitt, M. and ap Rees, T. 1980. Carbohydrate breakdown by chloroplasts of Pisum sativum. 627,131-143. 

So, when there is a change from anaerobic to aerobic conditions, there is a diminution in the consumption of D-glucose. This is termed the Pasteur effect. The action of oxygen in diminishing carbohydrate breakdown, and in decreasing the accumulation of the products of anaerobic metabolism, was first described by Pasteur,270 who found271 that, in the presence of air, the ratio of weight of yeast... 

The solution came, in part, by the application of biotransformations. These can be defined as biological processes that modify organic compounds via simple chemical reactions (oxidations, reductions) by means of enzymes contained in microbial, plant, or even animal cells. The aim is usually a one-step reaction to a recoverable product in a sequence of steps in which the majority of conversions are chemical steps (i.e., synthesis). In fermentation processes the whole sequence of reactions is carried out by microorganisms, be it a carbohydrate breakdown to alcohol (or other solvents), the production of antibiotics, or even enzymes. 

Phosphoenolpyruvate is produced in the next-to-last step in the metabolic pathway called glycolysis, which is the first stage of carbohydrate breakdown. In the final reaction of glycolysis the phosphoryl group from phosphoenolpyruvate is transferred to adenosine diphosphate (ADP). The reaction produces ATP, the major energy currency of the cell. 

Explain the significance of thioester formation in the metabolic pathways involved in fatty acid and carbohydrate breakdown. 

The biochemical mechanism by which carbohydrates spare proteins is not known. Carbohydrate breakdown yields ATP, NADH, and amino acid precursors—all compounds needed for amino acid or protein synthesis, and each of them could alone be responsible for the sparing effect. To assume that in the presence of excess carbohydrates the cell selects carbohydrates rather than protein as an energy source is obviously an oversimplified interpretation of the sparing effect. Yet, glucose and fructose administration reverses urea formation and glycine catabolism. 

In ruminant animals, considerable amounts of propionate are produced from carbohydrate breakdown in the rumen. The acid then passes across the rumen wall, where a little is changed to lactate. The remainder is carried to the liver, where it is converted... 

Microbiota keeps an important role in human metabolism. It must be considered a metabolizing organ, with impact oti endo- and xenobiotic metabolism, beyond its metabolic relevance for vitamin B12 synthesis, and carbohydrate breakdown, between other important functions. 

The best-known glycolytic pathway is that studied especially in muscle tissue and yeast cells. This pathway, sometimes known as the Embden-Meyerhof-Parnas (EMP) scheme, is shown in Fig. 8. The reactions take place under anaerobic conditions. In yeast the end-products are ethanol and carbon dioxide, whereas in muscle tissue the end-product is L-lactic acid. The EMP scheme is operative in a great many tissues and organisms and apparently represents the major pathway of carbohydrate breakdown. 

This table is an attempt to place in a chronological sequence the events leading up to carbohydrate breakdown. It should be noted that this scheme only applies to work done on Proctor barley, imbibed and germinated under non-malting conditions. The times should be taken as being very approximate since the literature drawn upon to devise this scheme is often equivocal. Related events for which the published chronology is very inconsistent are marked with asterisks... 

Thiamine was the first vitamin to have its precise biochemical functions determined. In the form of its pyrophosphate, thiamine participates in several very important enzyme systems namely (1) pyruvate dehydrogenase (page 232) which converts pyruvate to acetyl-CoA and carbon dioxide in the course of carbohydrate breakdown (2) the reaction of the citrate cycle in which oxoglutarate is oxidatively decarboxylated to succinyl-CoA (page 242) (3) the transketolase reaction of the pentose phosphate pathway of glucose breakdown (page 233). 

Shifting the metabolic machinery of the body to excessive utilization of fats instead of carbohydrates or a balance of fats and carbohydrates results In the buildup of ketone bodies— acetoacetate, beta-hydroxybutyrate, and acetone—in the blood and their appearance in the urine. This condition is referred to as ketosis, and outwardly noted by the sweetish, acetone odor of the breath. Three circumstances can cause ketosis (1) high dietary intake of fat but low carbohydrate intake as in ketogenic diets (2) diminished carbohydrate breakdown and high mobilization of fats as in starvation or (3) disorders in carbohydrate metabolism as in diabetes melli-tus. Unless ketosis goes unchecked and results in acidosis, it is a normal metabolic adjustment. 

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