Carbonyl enzymatic formation

The latter material has been used in the synthesis of asymmetrically labelled L-homoserine in a study on the mechanism of the enzymatic formation of L-threonine (9). However, we soon realized that under the experimental conditions used, the reduction of the carbonyl carbon and the saturation of the double bond are only two of the synthetic manifestations that are possible using an a-B-unsaturated aldehyde (10). Since then, we have been exploring this area and it now appears that a -unsaturated aldehydes can be reduced by baker s yeast to yield the synthetically useful chiral products indicated in Scheme 1 in a manner that depends upon the fermentation conditions and the nature of the a and Ysubstituents. 

The main components of tea are essential oil (0.5%) which is formed during fermentation, caffeine (1.8%-5.0%), and tannins (13%-18%). Enzymatic formation of black tea aroma follows biosynthetic pathways. The main precursors are amino acids and carotenoids (P-carotene, lutein, neoxanthin, and violaxanthin). Fermentation significantly reduces carotenoids and forms ionone and terpenoid carbonyls as a result of primary oxidations. During firing, secondary epoxida-tion takes place and forms epoxyionone, dihydroactinidiole, and trimethyl substituted cyclohexanones [35]. 

Grosch, W. Linoleic and Unolenic acid as substrate for enzymatic formation of volatile carbonyl compounds in pea. Z Lebensm. Unters.-Forsch. 137, 216- (1968)... 

The decarboxylation reaction usually proceeds from the dissociated form of a carboxyl group. As a result, the primary reaction intermediate is more or less a carbanion-like species. In one case, the carbanion is stabilized by the adjacent carbonyl group to form an enolate intermediate as seen in the case of decarboxylation of malonic acid and tropic acid derivatives. In the other case, the anion is stabilized by the aid of the thiazolium ring of TPP. This is the case of transketolases. The formation of carbanion equivalents is essentially important in the synthetic chemistry no matter what methods one takes, i.e., enzymatic or ordinary chemical. They undergo C—C bond-forming reactions with carbonyl compounds as well as a number of reactions with electrophiles, such as protonation, Michael-type addition, substitution with pyrophosphate and halides and so on. In this context,... 

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

Acyl nitroso compounds (3, Scheme 7.2) contain a nitroso group (-N=0) directly attached to a carbonyl carbon. Oxidation of an N-acyl hydroxylamine derivative provides the most direct method for the preparation of acyl C-nitroso compounds [10]. Treatment of hydroxamic acids, N-hydroxy carbamates or N-hydroxyureas with sodium periodate or tetra-alkyl ammonium periodate salts results in the formation of the corresponding acyl nitroso species (Scheme 7.2) [11-14]. Other oxidants including the Dess-Martin periodinane and both ruthenium (II) and iridium (I) based species efficiently convert N-acyl hydroxylamines to the corresponding acyl nitroso compounds [15-18]. The Swern oxidation also provides a useful alternative procedure for the oxidative preparation of acyl nitroso species [19]. Horseradish peroxidase (HRP) catalyzed oxidation of N-hydroxyurea with hydrogen peroxide forms an acyl nitroso species, which can be trapped with 1, 3-cyclohexanone, giving evidence of the formation of these species with enzymatic oxidants [20]. 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

As we have seen already, many enzymatic reactions depend upon formation of imines, which are commonly called Schiff bases. The two-step formation of Schiff bases consists of addition of an amino group to a carbonyl group to form a carbinolamine followed by elimination of water (Eq. 13-4).26 One group of aldolases (Section D) have, at their active centers,... 

Cleavage of the carbon backbone is reported to be mediated by hydrolases [63]. However, there are no indications that this side-reaction proceeds enzymatically. Instead formation of retro-Claisen products 12 and 13 proceeds via GSH addition to the carbonyl center in 2. The active role of GSH was demonstrated with GSH-depleted cells. After treatment with N-ethyl maleimide (NEM) [64] no retro-Claisen product was detectable. The same applied for NEM-treated cell liquor. 

Whilst PUFAs can be oxidised enzymatically within cells by the above mentioned reactions involving free radicals to yield prostaglandins and leukotrienes, it is important to stress that they can also be oxidised non-enzymatically to yield a variety of carbonyls. This latter mechanism involves the formation of acyclic fatty-acyl hydroperoxides through a radical-mediated peroxidative pathway. 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

Volatile aldehydes, and (E)-2-nonenal in particular, had already been identified as the cause of "rancid odors" in beer (7,8). These substances result firom the oxidation of unsaturated fatty acids. The direct precursor of (E)-2-nonenal and others carbonyl components is linoleic acid (Ci8 2 A 9,12) (9). Volatile aldehydes may be derived firom fatty acids in various manners. Chemical auto-oxidative factors would seem to provide the most likely explanation for the presence of these components in oak stave wood after seasoning in the open air. On the other hand, enzymatic factors may explain the presence of ese components while the tree is still standing or immediately after it has been cut. Additional research is necessary to pinpoint the exact formation and accumulation mechanisms of these molecules in the wood. 

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