Enzymic synthesis hydrolysis

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

In both cases, the mixed anhydride is used to synthesize ATP from ADP. Hydrolysis of the anhydride liberates more energy than the hydrolysis of ATP to ADP and, therefore, can be linked to the enzymic synthesis of ATP from ADP. This may be shown mechanistically as a hydroxyl group on ADP acting as nucleophile towards the mixed anhydride, and in each case a new phosphoric anhydride is formed. In the case of succinyl phosphate, it turns out that GDP rather than ADP attacks the acyl phosphate, and ATP production is a later step (see Section 15.3). These are enzymic reactions therefore, the reaction and the nature of the product are closely controlled. We need not concern ourselves why attack should be on the P=0 rather than on the C=0. 

The examples discussed above constitute a selection of recent applications of the acid and basic hydrolysis of (3-lactams in synthesis. Hydrolysis and alcoholysis of (3-lactams can also be effected under roughly neutral reaction conditions when enzymes are the promoters [47]. The (3-lactamases catalyzed hydrolysis of (3-lactams is an efficient process for a broad spectrum of substrates, including those (3-lactams with base or acid sensitive groups [12-14]. This process proceeds through an acyl enzyme intermediate to give ring opened (3-amino acids. The class C (3-lactamases in particular, in Scheme 9, have the ability to catalyze the alcoholysis reaction and hence (3-amino esters are the products formed. 

This method represents a resolution by asymmetric enzymic synthesis (e.g. l-alanine, H2N,CH(CH3)-C02H, Expt 5.221). Related procedures involve other types of enzymic reactions (e.g. hydrolysis, oxidation, etc.). Asymmetric enzymic hydrolysis, for example, proceeds according to the following reaction sequence. 

A unique feature of the F/V/A-ATPases is that they are rotary molecular motor enzymes. This has been shown by experiment for members of the F-and V-ATPase subfamilies and is generally assumed to be true for the closely related A-ATPases as well. The two enzymatic processes, ATP synthesis/hydrolysis and ion translocation, are coupled via a rotational motion of a central domain of the complex (the rotor) relative to a static domain (the stator). The A-, F-, and V-ATPases represent the smallest rotary motors found in the living cell so far. Most of what we know about the structure and mechanism of these microscopic energy converters comes from studies conducted with the F-ATPase. In the following review, current structural knowledge for all three members of the family of F-, V-,... 

Fundamental knowledge of the structure, function and mechanism of DNA-modifying enzymes has been important not only in understanding how these enzymes perform a myriad of chemical reactions Ml vZvo but also for the development of the field of recombinant DNA technology. The functions of the major groups of enzymes in deoxyribonucleic acid synthesis, hydrolysis and modification are reviewed, as well as some structural and mechanistic aspects of the restriction endonucleases, ligases and polymerases. 

For use as a blood-plasma substitute, the dextran should have a molecular weight in the range of 50,000 to 100,000. This criterion has occasioned a concerted effort to produce dextrans in the correct range. Partial, acid hydrolysis of native dextran followed by fractionation with various solvents, or enzymic production of dextran of low molecular weight are methods which have been used. In addition, ultrasonics has been suggested as a means of depolymerizating native dextran to the correct size for clinical use. Introduction of alternative acceptors into the reaction mixture for enzymic synthesis has also been employed for this purpose it is described in an earlier Section of this Chapter. 

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... 

There are no significant differences between ethyl and methyl esters concerning synthesis or cleavage. Related protocols of the methyl esters (see Section 2.2.1.1.1.1) are, therefore, applied to the ethyl esters. The usefulness of ethyl esters is somewhat limited by the difficulties encountered in their saponification. Hydrolysis with alkali is feasible, but ethyl esters are less sensitive to nucleophilic attack than methyl esters. Aminolysis and hydrazinolysis as well as cleavage of the alkyl-oxygen bond with lithium iodide in pyridineb l proceed several times slower in the case of ethyl esters. Mild enzyme-catalyzed hydrolysis by trypsin and chymotrypsin,t 2° 2 1 or by carboxypeptidase remains an attractive alternative. 

In combination with chemical synthesis, glucansucrases (GTF-R from Streptococcus oralis and GTF-A from Lactobacillus reuteri) were used to produce thio-glucosides, which are tolerated by most biological systems but are less sensitive to acid/base or enzyme-mediated hydrolysis than G-glucosides [138]. 

All microorganisms producing D-aminoacylases commonly produce L-aminoacy-lases as well. Therefore, to reach high optical purity of the D-amino acids produced from the respective N-acetyl-D,L-amino acids, the D-aminoacylases have to be separated from the L-aminoacylases (Table 12.3-13). However, this is a disadvantage in view of an industrial application since additional purification steps lead to more expensive enzymes and thus add costs to the whole production process. This is one of several reasons why it is widely accepted today that the production of D-amino acids by enzyme-catalyzed hydrolysis of D,L-hydantoins seems to be more promising than the D-aminoacylase route via N-acetyl-D,L-amino acids. The enzyme-catalyzed synthesis of D-amino acids from the respective D,L-hydantoins is described in Chapter 12.4. 

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