Enzyme Transformations

Malic add has a limited use in the food industry as an addifying agent where it is an alternative to dtric add. In nature, only L(-) malic add is found whereas the relatively cheap, chemical synthetic methods yield D/L mixtures. The favoured industrial way to produce the L(-) add is by enzymic transformation from fumaric add. Either whole cells or isolated and immobilised enzymes can be used, with high conversion effidendes. 

The study of enzyme kinetics—the factors that affect the rates of enzyme-catalyzed reactions—reveals the individual steps by which enzymes transform substrates into products. 

Nevertheless, considerable research effort has been expended on studies of mechanisms of GTN-NO bioconversion. Heat treatment of smooth muscle or endothelial cells is known to abrogate their ability to release NO from GTN [27, 28, 36], and the present consensus is firmly in favour of enzymic transformation of GTN to NO in vivo. 

Fio. 1. Some of the possible effects of enzyme transformations on the activities of alkaloids in mammalian systems. 

The preparation of the sample prior to its analysis will depend upon the nature of both the sample and the analytical method chosen and may involve the disruption of cells, homogenization and extraction procedures as well as the removal of protein or other interfering substances. It may be necessary to prevent the decomposition and degradation of the carbohydrate content during such treatments or during storage by the addition of antibacterial agents such as thymol or merthiolate, or substances such as fluoride ions, which will inhibit the enzymic transformation of the carbohydrates. 

Those nucleosides found in the nucleic acids DNA and RNA involve the joining of ribose of deoxyribose to a purine or a pyrimidine base. One such nucleoside is adenosine, in which a nitrogen of adenine is linked to carbon 1 of the pentose, ribose. In this form it is a component of RNA but as a phosphory-lated derivative of adenosine (e.g. ATP), which is a high energy compound, it fulfils an important role in metabolism. The dinucleotides NAD and NADP are two cofactors necessary for many enzymic transformations and these also contain /V-glycosides of ribose phosphate. Other important nucleosides are found... 

The mechanism of toxification of isoniazid was investigated in rats pretreated with inducers or inhibitors of microsomal enzymes or an inhibitor of acylamidases. In animals pretreated with the acylamidase inhibitor bis(4-nitrophenyl) phosphate, isoniazid and acetylisoniazid produced less liver necrosis than in control animals. The treatment had no effect on the necrosis due to acetylhydrazine [173], In animals pretreated with inducers of microsomal cytochrome P450 such as phenobarbital, acetylisoniazid, and acetylhydrazine caused markedly increased necrosis, while pretreatment with cytochrome P450 inhibitors decreased necrosis. In contrast, the toxicity of isoniazid and hydrazine was not modified by phenobarbital pretreatment. From these observations, Trimbell et al. [173] concluded that the hydrolysis of acetylisoniazid is a prerequisite for hepatotoxicity, and that microsomal enzymes transform acetylhydrazine, the product of hydrolysis, to a toxic species. 

Berkessel, A. and Thauer, R. K. (1995) On the mechanism of catalysis by a metal-free hydro-genase from methanogenic archaea Enzymic transformation of H2 without a metal and its analogy to the chemistry of alkanes in superacidic solution. Angew. Cbem., Int. Ed. Engl., 34, 2247-50. 

The two established Hnls, those from L. usitatissimum and P. amygdalus, have found biocatalytic applications for the production of (i )-cyanohydrins. The former of these Hnls is the least widely applied, the natural substrates being acetone cyanohydrin or (i )-2-butanone cyanohydrin (Table 1) [28]. Although an improved procedure for the purification of this enzyme has been reported [27] it is still only available in limited quantities (from 100 g of seedlings approximately 350 U of enzyme are obtained). It was found that this enzyme transforms a range of aliphatic aldehyde and ketone substrates [27], the latter of which included five-membered cyclic (e.g. 2-methylcyclopentanone) and chlorinated ketone substrates. In contrast, attempts to transform substituted cyclohexanones and 3-methylcyclopentanone failed and it was even found that benzaldehyde deactivated the enzyme. 

Holland HL (1981) Microbial and in vitro enzymic transformation of alkaloids (chap. 5). In Manske RHF, Rodrigo RGA (eds) The alkaloids, vol 18. Academic Press, New York p 323... 

An exactly analogous enzymic transformation is encountered during the formation of oestrogen and androgen sex hormones, e.g. estradiol and testosterone respectively, where dehydroepiandrosterone is oxidized to androstenedione. 

In recent years, C-NMR spectroscopy has found extensive use in studies on carbohydrate polymers, in some series overshadowing the importance of H-NMR spectroscopy. Applications range through determination of primary structure, analysis of mixtures, monitoring of chemical and enzymic transformations or of chelation reactions, and studies on conformational change. Measurements of coupling are utilized in determining the... 

Microbial and enzymic transformations of steroids have been reviewed. 5a-Pregnane-7,20-dione gave a reasonable yield of the la, 12/3-dihydroxy-derivative on incubation with Calonectria decora High yields of 10 8-hydroxy-19-nortes-tosterone were obtained by incubation of 19-nortestosterone with Rhizopus arr-hizus (Fischer 11). Oxidation of canrenone with a Penicillium species provided the 15a-hydroxy-derivative (348) which is a product of human metabolism of spironolactone.16j8-Hydroxylation of androstenolone, androst-4-ene-3,17-... 

In the case of most enzymic transformations the reaction rate can be described as a hyperbolic function of the concentration of substrate the characteristic parameters of these hyperboles are the and the KM values, which can be determined easily by different linearized plots. Different factors such as temperature, pH, chemical modification of the functional groups in the side chains of the protein, reversible inhibitors, activators, allosteric effectors, influence the catalytic activity of the enzymes. 

Two characteristics, the Michaelis constant KM and the maximal velocity V are the most important numeric data. The well-known Michaelis-Menten equation describes the relationship between the initial reaction rate and the substrate concentration with these two constants. The actual form of the rate equation of an enzymic process depends on the chemical mechanism of the enzymic transformation of the substrate to product (Table 8.1). 

The conversion of D-mannose 6-phosphate into D-mannosyl phosphate is catalyzed by phosphomannomutase, an enzyme distinct from phosphoglucomutase. Both enzymes have been detected by Mathe-son705 in cassia seeds, mung beans, orchid tubers, and pea seedlings. These enzymes from cassia seeds have been separated from one another by chromatography on DEAE- and O-phosphono-cellulose columns, and further characterized. Phosphomannomutase from animal sources requires the presence of either D-galactose 1,6-bisphosphate or D-mannose 1,6-bisphosphate for activity.708 D-Mannosyl phosphate may then be enzymically transformed into GDP-D-mannose in the... 

M.-R. Kula and C. Wandrey, Continuous enzymic transformation in an enzyme-membrane-reactor with simultaneous NADH regeneration, Meth. Enzymol. 1987, 136, 9-21. 

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