Citric add

Fool s Gold and the Reductive Citric Add Cycle—The First Metabolic Pathway ... 

The production of organic acids by micro-organisms, and especially citric add, is considered in detail in Chapter 4. In this section therefore we will only briefly consider dtric add production, from an energetics perspective. 

All of the compounds we shall study in this Chapter are primary metabolites though both phases of growth will be studied. For example, as we shall see, citric add is produced continuously at low levels during trophophase but only accumulates at high concentration during idiophase. 

Historically the production of titrate has been an important development in the pioneering of fermenter technology. It was shown back in 1893 by Wehmer that a fungus, Citromyces (now reclassified as a Penicilliutn spp.) would accumulate citric add in liquid culture. Wehmer in fact tried to scale up the process to an industrial level but there were two main problems. Firstly, the duration of the process under his conditions took far too long of the order of several weeks. Secondly, a problem was caused by Wehmer s incorrect belief that citric add only accumulated around neutral pH and lengthy incubation at this pH inevitably leads to contamination. 

The world demand for citric add around 1900 amounted to some 10,000 tonnes per annum. This was realised by pressing citrus fruits and precipitation of the citric add as calcium titrate. An Italian, government-led cartel had virtual monopoly of this process and as such the price of citric add was very high. 

A major breakthrough in the fermentation process came in 1916 - 1920 when it was found that Aspergillus niger grew well at pH values below 3.5, producing citric add in days rather than weeks. The faster incubation and highly add conditions (often below pH 2.0) also served to minimise potential problems caused by contamination. 

By the mid 1930 s over 80% of the world s citric add was produced by fermentation. At present virtually all of the world production comes from this process. By 1981 over 200,000 tonnes were produced annually (possibly as high as 300,000 tonnes) the industry in the United Kingdom at that time being worth some 20 million per annum, one tenth of the world s turnover. 

We know that citric add is formed from acetyl CoA and oxaloacetate by the reaction ... 

There are several apparent problems which we still need to resolve in this section before we have a better understanding of citric add production. 

Let us consider Figure 5.3 again. Both pyruvate kinase and dtrate synthase (enzymes III and V) are inhibited by elevated ATP concentrations. During citric acid production ATP concentrations are likely to arise (ATP produced in glycolysis) and either of these enzymes could, if inhibited, slow down the process. In fact all of the evidence suggests that both enzymes are modified or controlled in some way such that they are insensitive to other cellular metabolites during citric add production. 

Figure 5.4 summarises the changes occuring in A. niger in citric add production mode when compared to conventional metabolism. It is worth studying the Figure for some time because it explains some of the features necessary for a successful fermentation process. 

Insert the missing words from the list below into the following paragraph, which describes the mechanism of citric add accumulation in A. rtiger. 

In the presence of suffident metal ions such as zinc, phosphate defidency is known to inhibit growth and increase yields of dtric add. However, phosphate is added not only as a source of phosphorus but also as phosphoric add to addify the medium. Restricted growth but good citric add yield is also achieved by maintaining iron and zinc defidency hence low phosphate levels are not necessary. 

The process is usually completed within eight days after which a yield of 210 to 250 kg m 3 of citric add may be obtained, assuming a conversion ratio of 100 g glucose to 75 g dtric add. 

Conventional stirred reactors with working volumes of 50 to 150 m3 have been used routinely for citric add production whereas tower bioreactors, currently 200 m and larger (greater than 600 m3) are envisaged. 

Continuous culture is not considered suitable for citric add production the requirement for a multi-tank system to separate growth and product formation would make the process uneconomic. 

The answer is that the objective is to precipitate out all of the citric add as insoluble caldum dtrate. Magnesium dtrate is very soluble and would, therefore, be lost in the aqueous phase during the next separation. 

The pH has to be controlled - the acid which is produced has to be neutralised maintaining a pH in excess of 6.0. Below pH 3.0 the glucose oxidase is inactivated and in fungal systems low pH encourages citric add production. 

IR spectra, 2,469 protonation, 2, 465 zinc transport, 6, 672 Citric acid, fluoro-absolute configuration, 2, 478 Citric add, hydroxy-crystal structure, 2, 478 Clathrates amines, 2,25... 

Tetrabromophenolphthalein Ethyl Ester—Silver Nitrate—Citric Add (Duggan) 411... 

First, following a cleaning procedure, electrochemical deposition was carried out using an aqueous solution with PdS04 electrolyte, citric add, and boric acid (25 °C ... 

In stage three, the citric add (Krebs, or tricarboxylic acid [TCA]) cycle oxidizes acetyl CoA to COj- The energy released in this process is primarily conserved by reducing NAD to NADH or FAD to FADHj. 

Notice that none of the intermediates of the citric add cyde appear in this reaction, not as reactants or as products. This emphasizes an important (and frequently misunderstood) point about the cycle. It does not represent a pathway for the net conversion of acetyl CoA to citrate, to malate, or to any other intermediate of the cyde. The only fate of acetyl CoA in this pathway is its oxidation to CO,. Therefore, the dtric acid cycle does not represent a pathway by which there can be net synthesis of glucose from acetyl CoA... 

Pyruvate carboxylase is a mitochondrial enzyme requiring biotin. It is activated by acetyl CoA (fiom p oxidation). The product oxaloacetate (OAA), a citric add cyde intermediate, cannot leave the mitochondria but is reduced to malate that can leave via the malate shuttle. In the cytoplasm, malate is reoxidized to OAA. 

The citrate shuttle transports acetyl CoA groups from the mitochondria to the cytoplasm for fatty acid synthesis. Acetyl CoA combines with oxaloacetate in the mitochondria to form citrate, but rather than continuing in the citric add cycle, citrate is transported into the cytoplasm. Factors that indirectly promote this process indude insuKn and high-energy status. 

Figure 1-17-3 presents a diagram of pathways in which selected amino acids are converted to citric add cyde intermediates (and glucose) or to acetyl CoA (and ketones). Important genetic defidendes are identified on the diagram. 

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