Non-enzymic systems

Many mechanistic aspects of the hydrolysis of phosphate esters in protic media remain uncertain. In spite of predictions that racemization at phosphorus should be the final outcome if indeed the (hypothetical) metaphosphate intermediate is involved in the solvolysis of monoesters, the results of several studies on the methanolysis of appropriately O-isotopically labelled compounds are consistent with reactions proceeding with inversion of configuration, as observed for all enzymic and non-enzymic systems so far examined this has resulted in the suggestion that if metaphosphate is actually formed, then it must be in a masked form. 

This chapter is concerned with mechanistic observations on hydride-transfer processes in non-enzymic systems, but under conditions and with structures such that the observations are considered relevant to enzyme-catalyzed reactions. Even... 

The existence of a specific cinnabarinate synthase or synthetase in the nuclear fraction is still sub judice. It has been noted [197] that the nuclear fraction is capable of releasing small amounts of hydrogen peroxide. Therefore, H2O2 plus Mn2+ ions could be an efficient, non-enzymic system for HA conversion to CA [17]. Moreover, purification and characterisation of a true cinnabarinate synthase from that source have never been reported. 

The ease with which ascorbic acid may be oxidized in the presence of molecular oxygen in both enzymic and non-enzymic systems has been demonstrated by many workers over the last decade. Both copper and iron salts catalyze its oxidation, and the oxidative activity of many tissues may, in part, be attributed to them, despite the fact that the activity of these metals is reduced by other natural constituents such as glutathione, amino acids, or proteins. Many quinones which could be formed from a variety of compounds present in plant tissues are also capable of oxidizing ascorbic acid. 

Compared with tar, which has a relatively short lifetime in the marine environment, the residence times of plastic, glass and non-corrodible metallic debris are indefinite. Most plastic articles are fabricated from polyethylene, polystyrene or polyvinyl chloride. With molecular weights ranging to over 500,000, the only chemical reactivity of these polymers is derived from any residual unsaturation and, therefore, they are essentially inert chemically and photochemically. Further, since indigenous microflora lack the enzyme systems necessary to degrade most of these polymers, articles manufactured from them are highly resistant or virtually immune to biodegradation. That is, the properties that render plastics so durable... 

Enzyme labels are usually associated with solid-phase antibodies in the technique known as enzyme-linked immunosorbent assay (ELISA). There are several variants of this technique employing both competitive and non-competitive systems. However it is best used in combination with two monoclonal antibodies in the two-site format in which an excess of antibody is bound to a solid phase such as a test-tube or microtitre plate the test antigen is then added and is largely sequestered by the antibody (Figure 7.12). After washing... 

The xylanolytic enzyme system of Trichoderma reesei, a well-known producer of cellulolytic enzymes, is versatile and well suited for the total hydrolysis of different xylans. It consists of two major, specific and several non-specific xylanases, at least one / -xylosidase, a-arabinosidase and a-glucuronidase and at least two acetyl esterases. The hydrolysis of polymeric xylans starts by the action of endoxylanases. The side-groupcleaving enzymes have their highest activities towards soluble, short xylo-oligosaccharides, and make the substituted oligosaccharides again accessible for xylanases and / -xylosidase. 

Product inhibition and substrate inhibition are effects also known in enzyme catalysis that can reduce catalytic efficiency. Generally, catalytic systems (natural or artificial) based on covalent interactions are more sensitive towards inhibitions than non-covalent systems utilizing weak interactions Garcia-Junceda, E. (2008) Multi-Step Enzyme Catalysis, Wiley-VCH Verlag GmbH, Weinheim, Germany. 

---