5 - , enzymic synthesis isolation

The reasons for the increasing acceptance of enzymes as reagents rest on the advantages gained from utilizing them in organic synthesis Isolated or wholecell enzymes are efficient catalysts under mild conditions. Since enzymes are chiral materials, optically active molecules may be produced from prochiral or racemic substrates by catalytic asymmetric induction or kinetic resolution. Moreover, these biocatalysts may perform transformations, which are difficult to emulate by transition-metal catalysts, and they are environmentally more acceptable than metal complexes. 

This enzyme [EC 1.2.99.2], also known as acetyl-CoA synthase, catalyzes the reaction of carbon monoxide with water and an acceptor to produce carbon dioxide and the reduced acceptor. The cofactors of this enzyme include nickel and zinc ions as well as non-heme iron. Methyl viologen can act as the acceptor substrate. The enzyme is isolated from Clostridium sp. Interestingly, it also catalyzes an exchange reaction of carbon between Cl of acetyl-CoA and carbon monoxide. The protein participates in the synthesis of acetyl-CoA from carbon dioxide and hydrogen in the organisms. 

Physiological optimization of enzyme synthesis by variation of the culture parameters is usually required to enhance the catalytic activity of whole-cell biocatalysts to such a level that it can be apphed in a biocatalytic process. In addition, physiological conditions can influence the selectivity of the reaction, since enzymes with opposite selectivities can be differentially expressed. In some cases, genetic engineering is required to obtain biocatalysts with a desired selectivity that does not consume the product of choice (see 5.3.5). Alternatively, one may choose to isolate the desired activity from the culture in order to use the biocatalyst in an enzyme reactor. 

Foetal and neonatal livers contain exceptionally high levels of copper compared to the adult organ. Thus, the livers of new-born rats contain as much as 20 times the level of copper and zinc metallothionein as that found in 70-day-old rats.1150 Again, a metallothionein from foetal bovine liver contained eight copper and two zinc atoms per molecule of protein.1151 These proteins can only be isolated with difficulty under oxygen-free conditions. It appears then that large amounts of copper (and zinc) are stored in the liver bound to metallothionein, and are mobilized as required for enzyme synthesis after birth. 

E. coli uses nitrate as a terminal electron acceptor through a respiratory, dissimilatory nitrate reductase whose synthesis is induced when nitrate is provided, and which is repressed by oxygen. Nitrate reductase is discussed with other molybdoenzymes in Section 62.1.9, and catalyzes the reduction of nitrate to nitrite. The enzyme is isolated from the cytoplasmic membrane of E. coli, and contains three subunits (a, j8 and y) although the y-subunit may be absent in some preparations. The -y-subunit is a b-type cytochrome, and the a-subunit is reported to be the catalytic subunit. The enzyme contains a number of iron-sulfur clusters, including a HiPIP and at least two ferredoxins.1054,1437... 

Mode of isolation A. chemical synthesis B. enzymic synthesis C. degradation of oligosaccharide D. degradation of polysaccharide E. naturally occurring. 

Biocatalysis is becoming an established method to assist in the manufacture of synthetic targets and, in particular, chiral tine chemicals.13 The ability of enzymes (whether isolated or within an intact cell) to cause resolution or synthesis of chiral centers is without precedent, and there is consequently a strong drive to implement such processes in industry. 

In Azotobacter, the main enzyme of PolyP metabolism was shown to be polyphosphate kinase (Zaitseva and Belozersky, 1958,1960), which was capable of PolyP synthesis and of a reverse reaction. This enzyme was isolated and purified to a considerable extent (Zaitseva and Belozersky, 1960). 

DAHP synthetase, 251, 255 Dehydroquinase, 258 Dehydroshikimate riductase, 259 Dextrans, 341 acid hydrolysis of, 349 chain lengths of, 345, 346 cuprammonium complexes of, 355 electron microscope studies on, 349 enzymic synthesis of, 342, 345, 355 from sucrose, 342 flow birefringence studies on, 349 fructose-containing, 359 infrared absorption spectra of, 352 isolation of, 343... 

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