Enzymatic Conversions

Another interesting example of resolution through formation of diastereo-mers is the isolation of four stereoisomers of 3-amino-2-methyl-3-trifluoro-methyl butanoic acid [55]. In this process, the chemical-enzymatic method by the combination of chemical and enzymatic reaction is a very convenient. At first, -phenylacetyl derivatives 61a and 61b were prepared in excellent isolated yields via the Schotten-Baumann procedure. After these materials were hydrolysed with penicillin acylase (EC 3.5.1.11) from Escherichia coli until attainment of 50% conversion, enzymatically unconverted -phenylacetyl derivatives 62 a and 62 b (organic layer) and amino acids 63 b and 63 d (aqueous layer) were separated. Acidic hydrolysis of unconverted materials produced other stereoisomers 63 a and 63 c in high optical pure form. 

In Chap. 6, biological supermolecules are explained and classified by function. Artificial supramolecular systems that mimic biological ones are also described. Biomimetic chemistry, which mimics the essence of a biosystem and then develops an artificial system that is better than the biological one, is widely used in this field. Fimctional developments, such as molecular transport, information transmission and conversion, energy conversion and molecular conversion (enzymatic functionaUty) based on biomimetic chemistry are described. New methodologies such as combinatorial chemistry and in vitro selection mimic evolutionary processes in nature. We leave this topic until the end of the book because we want to show that there is still lots to do in supramolecular chemistry, and that supramolecular chemistry has huge future potential. 

As opposed to applications where high viscosity must be maintained, some applications require starches with low viscosity. These thinned starches are produced by a variety of depolymerizing techniques. They include oxidation, thermomechanical conversion, enzymatic depolymerization, and acid- modification. A special depolymerized form of starch, dextrins, are distinct and will be discussed in a following section. Starches can be depolymerized to... 

A biorefinery is the integral upstream, midstream, and downstream processing of biomass into a range of produas. In the classification system lEA Bioenergy Task 42 (described in the next chapter) has differentiated between mechanical pretreatments (extraction, fractionation, separation), thermochemical conversions, chemical conversions, enzymatic conversions, and microbial (fermentation both aerobic, anaerobic) conversions. 

Watanabe, T. Potential of cellulosic ethanol. In Lignocellulose conversion Enzymatic and microbial tools for bioethanol production, Faraco, V, Ed., Springer Verlag Berlin, 2013, pp. 1-20. 

In keeping with its biogenetic origin m three molecules of acetic acid mevalonic acid has six carbon atoms The conversion of mevalonate to isopentenyl pyrophosphate involves loss of the extra carbon as carbon dioxide First the alcohol hydroxyl groups of mevalonate are converted to phosphate ester functions—they are enzymatically phosphorylated with introduction of a simple phosphate at the tertiary site and a pyrophosphate at the primary site Decarboxylation m concert with loss of the terti ary phosphate introduces a carbon-carbon double bond and gives isopentenyl pyrophos phate the fundamental building block for formation of isoprenoid natural products... 

Biochemistry resulted from the early elucidation of the pathway of enzymatic conversion of glucose to ethanol by yeasts and its relation to carbohydrate metaboHsm in animals. The word enzyme means "in yeast," and the earfler word ferment has an obvious connection. Partly because of the importance of wine and related products and partly because yeasts are relatively easily studied, yeasts and fermentation were important in early scientific development and stiU figure widely in studies of biochemical mechanisms, genetic control, cell characteristics, etc. Fermentation yeast was the first eukaryote to have its genome elucidated. 

Fructose—Dextrose Separation. Emctose—dextrose separation is an example of the appHcation of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C H 2Dg, fmctose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high" fmctose com symp by the enzymatic conversion of cornstarch. Because fmctose has about double the sweetness index of dextrose, the separation of fmctose from this mixture and the recycling of dextrose for further enzymatic conversion to fmctose is of commercial interest (see Sugar Sweeteners). 

Enzymatic Conversion of Cholesterol. A decrease of cholesterol in meat products in the future may be possible through the conversion of cholesterol [57-88-5] to coprosterol [560-68-9] which is not absorbed readily in the intestine. Cholesterol reductase can be isolated from alfalfa leaves and cucumber leaves (53). Treatment of meat animals might involve an injection of this ensyme immediately prior to slaughter, allowing for the conversion of a portion of the membrane-bound cholesterol into coprostanol. 

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

Enzymatic Method. L-Amino acids can be produced by the enzymatic hydrolysis of chemically synthesized DL-amino acids or derivatives such as esters, hydantoins, carbamates, amides, and acylates (24). The enzyme which hydrolyzes the L-isomer specifically has been found in microbial sources. The resulting L-amino acid is isolated through routine chemical or physical processes. The D-isomer which remains unchanged is racemized chemically or enzymatically and the process is recycled. Conversely, enzymes which act specifically on D-isomers have been found. Thus various D-amino acids have been... 

The conversion of cholesterol (2) to pregnenolone [145-13-1] (78) is accomplished primarily through enzymatic systems in the adrenocortical and gonadal mitochondria. This conversion appears to be rate-limiting and therefore is regarded as the control point for the entire steroid hormone... 

Folic acid is synthesized both in microorganisms and in plants. Guanosine-5-ttiphosphate (GTP) (33), -aminobenzoic acid (PABA), and L-glutamic acid are the precursors. Reviews are available for details (63,64). The sequence of reactions responsible for the enzymatic conversion of GTP to 7,8-dihydrofohc acid (2) is shown. 

Conversion. Conversion describes the enzymatic starch hydrolysis processes, Hquification, and saccharification. 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

The primary steps in the conversion of starch are Uquefaction, saccharification, and isomerization. By controlling the enzymatic reactions, sugars of different sweetness can be produced to suit the various needs of manufacturers of food and nonalcohoUc beverages. 

Many products made by fermentation are also based on the conversion of starch. Some examples of the use of enzymatically hydrolyzed starches are the production of alcohol, ascorbic acid, enzymes, lysine, and penicillin. 

A number of examples of monoacylated diols produced by enzymatic hydrolysis of prochiral carboxylates are presented in Table 3. PLE-catalyzed conversions of acycHc diesters strongly depend on the stmcture of the substituent and are usually poor for alkyl derivatives. Lipases are much less sensitive to the stmcture of the side chain the yields and selectivity of the hydrolysis of both alkyl (26) and aryl (24) derivatives are similar. The enzyme selectivity depends not only on the stmcture of the alcohol, but also on the nature of the acyl moiety (48). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

A non-linear regression analysis is employed using die Solver in Microsoft Excel spreadsheet to determine die values of and in die following examples. Example 1-5 (Chapter 1) involves the enzymatic reaction in the conversion of urea to ammonia and carbon dioxide and Example 11-1 deals with the interconversion of D-glyceraldehyde 3-Phosphate and dihydroxyacetone phosphate. The Solver (EXAMPLEll-l.xls and EXAMPLEll-3.xls) uses the Michaehs-Menten (MM) formula to compute v i- The residual sums of squares between Vg(,j, and v j is then calculated. Using guessed values of and the Solver uses a search optimization technique to determine MM parameters. The values of and in Example 11-1 are ... 

The enzymatic conversion of a-(aminomethyl)pyrroles is also used by nature to produce porphyrinogens like uroporphyrinogen III (see introduction, compound 8), which is the key building block in the biosynthesis of all known porphinoid natural products. This biomimetic method is a powerful tool for the synthesis of different porphyrins, e.g. for the preparation of JV,Af, V ,Ar"-tetramethylporphyrin-2,3,7,8,12,13,17,18-octaacetic acid dibromide 12.36... 

The strategy we hope you identified is to first produce 6-aminopenidllanic acid, then attempt to add different moieties to the 6-amino group. This can be achieved either chemically or enzymatically. In the following section we will consider the conversion of penicillin C into 6-aminopenicillanic acid and follow this by examining how 6-aminopenidllanic acid may be converted into ampirillin and amoxicillin. 

Also illustrated in Figure 6.17 there is another important antibiotic, amoxicillin. Both amoxicillin and ampiciilin can be made enzymatically or chemically. Although enzymes are available that can be applied very well for the conversion of 6-APA into a variety of semi-synthetic penicillins, economic reasons are still impeding large scale applications. 

Figure A8.12 Enzymatic conversion of frans-cinnamic acid to L-phenylalanine.

A new development is the industrial production of L-phenylalanine by converting phenylpyruvic add with pyridoxalphosphate-dependent phenylalanine transaminase (see Figure A8.16). The biotransformation step is complicated by an unfavourable equilibrium and the need for an amino-donor (aspartic add). For a complete conversion of phenylpyruvic add, oxaloacetic add (deamination product of aspartic add) is decarboxylated enzymatically or chemically to pyruvic add. The use of immobilised . coli (covalent attachment and entrapment of whole cells with polyazetidine) is preferred in this process (Figure A8.17). 

A solution of 2.25 g (25 mmol) of D-glyccraldehyde in 300 mL of water is combined with a solution of 20 mmol of dihydroxyacetonc phosphate (DIIAP) in 200 mL of water freshly adjusted to pH 6.8. The mixture is incubated with 100 U of L-rhamnulose 1-phosphate aldolase at r.t. for 24 h with monitoring of conversion by TLC (2-propanol/sat. ammonia/water 6 4 2) and by enzymatic assay for DHAP55. 

Almost all types of cell can be used to convert an added compound into another compound, involving many forms of enzymatic reaction including dehydration, oxidation, hydroxyla-tion, animation, isomerisation, etc. These types of conversion have advantages over chemical processes in that the reaction can be very specific, and produced at moderate temperatures. Examples of transformations using enzymes include the production of steroids, conversion of antibiotics and prostaglandins. Industrial transformation requires the production of large quantities of enzyme, but the half-life of enzymes can be improved by immobilisation and extraction simplified by the use of whole cells. 

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