Enzyme catalysis alcohols

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

Although quite reliable empirical rules exist for the enantioselectivity of hydrolases for secondary alcohols (see Section 4.2.1.2), such rules are not as developed for primary alcohols, partly because many hydrolases often show low enantioselectivity. With some exceptions, lipases from Pseudomonas sp. and porcine pancreas lipase (PPL) often display sufficient selectivity for practical use. The model described in Figure 4.3 has been developed for Pseudomonas cepacia lipase (reclassified as Burkholderia cepacia), and, provided that no oxygen is attached to the stereogenic center, it works well for this lipase in many cases [41]. However, as soon as primary alcohols are resolved by enzyme catalysis, independent proof of configuration for a previously unknown product is recommended. 

The NAD+-dependent alcohol dehydrogenase from horse liver contains one catalytically essential zinc ion at each of its two active sites. An essential feature of the enzymic catalysis appears to involve direct coordination of the enzyme-bound zinc by the carbonyl and hydroxyl groups of the aldehyde and alcohol substrates. Polarization of the carbonyl group by the metal ion should assist nucleophilic attack by hydride ion. A number of studies have confirmed this view. Zinc(II) catalyzes the reduction of l,10-phenanthroline-2-carbaldehyde by lV-propyl-l,4-dihy-dronicotinamide in acetonitrile,526 and provides an interesting model reaction for alcohol dehydrogenase (Scheme 45). The model reaction proceeds by direct hydrogen transfer and is absolutely dependent on the presence of zinc(II). The zinc(II) ion also catalyzes the reduction of 2- and 4-pyridinecarbaldehyde by Et4N BH4-.526 The zinc complex of the 2-aldehyde is reduced at least 7 x 105 times faster than the free aldehyde, whereas the zinc complex of the 4-aldehyde is reduced only 102 times faster than the free aldehyde. A direct interaction of zinc(II) with the carbonyl function is clearly required for marked catalytic effects to be observed. 

The occurred activated complex may induce formation of either epoxide or allyl alcohol. However, under current experimental conditions the biomimic possesses exclusive epoxi-dizing ability, and the catalytic cycle is completed at this stage. The number of cycles per perFTPhPFe(III)OH mole under optimal process conditions is 80 per hour [82], This example of the mimetic catalysis indicates the unity of acidic-basic and redox mechanisms typical of the enzyme catalysis. 

Recently, a controversial debate has arisen about whether the optimization of enzyme catalysis may entail the evolutionary implementation of chemical strategies that increase the probability of tunneling and thereby accelerate reaction rates [7]. Kinetic isotope effect experiments have indicated that hydrogen tunneling plays an important role in many proton and hydride transfer reactions in enzymes [8, 9]. Enzyme catalysis of horse liver alcohol dehydrogenase may be understood by a model of vibrationally enhanced proton transfer tunneling [10]. Furthermore, the double proton transfer reaction in DNA base pairs has been studied in detail and even been hypothesized as a possible source of spontaneous mutation [11-13]. 

Fourth, at lower temperatures nonreacting extra-index chains of atoms also undergo adsorption on the surface of catalysts. Experimental evidence of flat adsorption was obtained in the precatalytic range of the adsorption of alcohols on alumina. Plane adsorption allows explanation of both asymmetric catalysis and a number of facts of enzymic catalysis. 

To illustrate the use of RSSF spectroscopy to study enzyme catalysis, we give two systems detailed treatment herein, namely, horse liver alcohol dehydrogenase and Salmonella typhimurium tryptophan synthase. For an inclusive review of RSSF spectroscopy applications in the field of enzymology, see Brzovid and Dunn. ... 

Many of the important enzyme-catalyzed reactions are similar to the reactions we studied in the chapters on organic chemistry ester hydrolysis, alcohol oxidation, and so on. However, laboratory conditions cannot match what happens when these reactions are carried ont in the body enzymes canse snch reactions to proceed under mild pH and temperature conditions. In addition, enzyme catalysis within the body can accomplish in seconds reactions that ordinarily take weeks or even months under laboratory conditions. 

In this enzymatic reaction, the catalytic constant, ftcati is the rate-limiting step in the propan-2-ol —> acetone direction a plot of log fccat versus pH has a shape of a titration curve for a monobasic acid with pKa value of 7.0, suggesting that a histidine residue is involved in catalysis in the ternary complex enzyme-NAD" -alcohol (Maret Makinen, 1991). However, this assignment is not unambiguous, since carboxyl groups are known to have pK s as high as 8, and lysines to have pita s as low as 6. 

Low Molecular Weight Esters. Low molecular weight esters are important constituents of natural and artificial flavors and fragrances (56). Because of the mild reaction conditions required, enzyme catalysis is well suited for the production of these molecules. Lipase-catalyzed esterification and transesterification of terpene alcohols have been performed in solvents and in supercritical carbon dioxide. Esters of short-chain alcohols and acids were also produced in solvents and in gas/solid reactors. This last case, in which the enzyme catalyst is present in the solid form and the substrates and products are in the gas state, is especially appropriate for these highly volatile compounds (5). 

The type of reaction envisaged for enzyme catalysis is illustrated by that suggested for the chymotrypsin-catalysed hydrolysis of an ester shown in Fig. 7.4, where the groups involved in the catalysis are the alcohol group of serine and the imidazole of histidine. 

Interesterification is performed either by chemical or enzyme catalysis and involves at least two oils that have different fatty acid compositions. In chemical interesterification, alkali metals (sodium, potassium) and alkali-metal alcoholates (e.g., methylate, ethylate) are used as catalysts. Sodium methylate is the most widely used catalyst. The dried and deacidified oil is stirred at 80 to 100°C in the presence of alcoholate (0.1-0.3% of fat weight) and when the reaction is completed the catalyst is destroyed, by addition of water, and subsequently removed. The interesterified fat is recovered and then bleached and deodorized. Chemical interesterification progresses randomly with no regioselectivity (positional specificity) on the carbons of the glycerol moiety of the TAG (Gunstone, 1994). 

We suppose that the c ax value (i.e., the alcohol concentration, at which A has its maximum) always corresponds to the same optimum alcohol concentration in the aqueous (pseudo-) phase, at which the enzyme catalysis is at its maximum. If this is true, then the concentration of the alcohol incorporated in the micelles is given by = Cn x - Cmax> where is the optimum surfactant concentration at zero surfactant concentration. The alcohol partition coefficient can then be estimated as... 

Enzyme catalysis is specific, controlled, gives few by-products and is generally conducted in water under mild conditions of temperature and pressure. An ideal protocol for the polymer industry. Now, we have pioneers in the laboratory utilizing enzymes to produce addition polymers from vinyl monomers, condensation polymers from alcohols, amines and acids. One addition polymer is in commercial production in the UK utilizing specific enzyme condensation polymerization of primary alcohol groups in the... 

Warner, M. C., Nagendiran, A., Bogar, K., and Backvall, J.-E. (2012). Enantioselective route to ketones and lactones from exocyclic allylic alcohols via metal and enzyme catalysis. Org. Lett, 14,5094-5097. 

H. Groger, W. Hummel, S. Borchert, M. Krausser, Reduction of ketones and aldehydes to alcohols, in K. Drauz, H. Groger, O. May (Eds.) Enzyme Catalysis in Organic Synthesis, Vol. 2,3rd edition, Wiley-VGH/Verlag GmbH Go. KGaA, Weinheim, 2012, p>p. 1037-1110. 

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