Enzymatic reaction modification

Process Va.ria.tlons. The conventional techniques for tea manufacture have been replaced in part by newer processing methods adopted for a greater degree of automation and control. These newer methods include withering modification (78), different types of maceration equipment (79), closed systems for fermentation (80), and fluid-bed dryers (81). A thermal process has been described which utilizes decreased time periods for enzymatic reactions but depends on heat treatment at 50—65°C to develop black tea character (82). It is claimed that tannin—protein complex formation is decreased and, therefore, greater tannin extractabiUty is achieved. Tea value is beheved to be increased through use of this process. 

A variety of chemical and enzymatic reactions produce derivatives of the simple sugars. These modifications produce a diverse array of saccharide derivatives. Some of the most common derivations are discussed here. 

Enzymatic Covalent Modification of Antibiotic—The Range of Reactions... 

One advantage of whole-cell biotransformation that has not been addressed adequately in this chapter is the ability to modify compounds with complex structure, such as natural products. Natural products are ideal substrates for biotransformation reactions since they are synthesized in a series of enzymatic reactions by the whole cells. The modification of natural products by biotransformation has been reviewed recently by Azerad [ 13] and a majority of the modifications were carried out by whole-cell biotransformations. Additional examples of modification of natural products by whole-cell biotransformations can also be found in the review article by Patel [2]. Natural products are an important source of new drugs and new drug leads [53]. The use of biotransformation, especially whole-cell biotransformation, in modification of natural products for lead optimization and generating libraries of derivatives for S AR and screening studies is important for the pharmaceutical industry. 

A promising approach to this topic is the development of biocompatible solid phase attachment systems for macrocycles that allow on-bead enzymatic and chemical modification [4]. While making use of recent developments in polymeric support for resins, we endeavored in constructing a new linker system, which allows easy attachment of macrocycles to the solid phase, simple organic or enzymatic reactions, and cleavage from solid support under mild conditions [98]. 

Variations of this method are possible in several ways. First of all, cyclodextrin which is available on a large scale by enzymatically catalyzed modification of starch can be tailored by chemical reactions. Furthermore, copolymerizations between different host-guest complexes are possible whereby in some cases the reactivity ratios differ from those reported in literature. 

GTPCH (EC 3.5.4.16) converts the substrate GTP to 7,8-dihydroneopterin triphosphate (H2NTP) and formate. GTPCH activity is determined by measuring neopterin, the completely oxidized and dephosphorylated H TP-product of the enzyme reaction. Conversion of H2NTP to neopterin is carried out after the enzymatic reaction in presence of iodine at pH 1.0, followed by dephosphorylation with alkaline phosphatase at pH 8.5-9.0. Neopterin is detected fluorimetrically at 350/440 nm upon HPLC separation. The assay is based with some modifications on the methods published by Viveros et al. and Hatakeyama and Yoneyama [15,16]. 

In most European countries, flavors that occur naturally or are generated during healing or processing by enzymatic reactions or modification generally arc considered natural flavors. Flavors that arc often referred to in the United Stales as synthetic are usually termed "artificial" in Europe. These would include such compounds as ethyl vanillin, allyl-of-iononc, and ethyl maltol. However, substances that are synthesized but chemically identical to the naturally occurring substances arc classified as "natural-identical." This class would include diacetyl, benzaldehyde, antsyl acetate, and benzophenone. 

Despite the fact that only 20 amino acids (plus selenocys-teine and formylmethionine in prokaryotic systems) are known to be directly specified by the genetic code, chemical analysis of mature proteins has revealed hundreds of different amino acids, all of them structural variants on the original 20. This structural diversity, which greatly expands the chemical lexicon of proteins, results from posttranslational modification of the primary products of translation. Our knowledge of the nature and significance of enzymatic reactions that bring about these important alterations is still very incomplete. 

Since none of the liposomal immunoassay approaches described in the scientific literature thus far took advantage of surface immobilization techniques, one could envision a double-amplification biosensor in which surface modification plays an important role [35]. For example, consider a dehydrogenase enzyme marker system which requires an electroactive cofactor such as NAD+. In the enzymatic reaction scheme ... 

It is evident that all the described methods are based on reduction reactions. Substrates react either with long-living radicals (TEAC, TRAP), AOC (ORAC) or with the iron complex compound (FRAP), AOM concentration are obtained as a result of enzymatic reactions [51] or their existing level is registered with electron-spin resonance [52], Various modifications of peroxidation lipid reaction can also be applied [53],... 

Dinitriles and diamides also can sometimes be partially hydrolyzed by use of enzymes. As with other enzymatic reactions, small structural modifications can substantially modify the suitability of a given substrate. Scheme 10.3 gives illustrative examples of the highly substrate-dependent hydrolysis of dinitriles and diamides by use of a microorganism. 

Thus this technique is especially suited to studying enzymatic reactions with considerable advantage. Until now the procedure for determining the active centrum of an enzyme was rather complex because enzymatic reactions are performed only in aqueous solutions and at very low concentrations. Thus, IR-spectroscopy and normal Raman techniques failed. Also, experiments to study enzymatic reactions in the crystalline phase will not succeed, for the native conformation of the enzymes in solution is a prerequisite for optimal reactivity. The usual technique, therefore, is a stepwise modification of the functional groups of either the enzyme or its substratum and the measurement of their kinetic behaviour. This very laborious procedure then provides some information concerning the active centrum. 

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