Chemical secondary, enzyme reactions

In these processes light is used to produce a chemical change which acts as a trigger for some complex enzyme-catalysed reaction. The primary photochemical process may be relatively simple, but the following dark reactions are often quite complex. These secondary (dark) reactions are not considered in this book, since they are part of biochemistry rather than photochemistry. 

Even in systems where chemical stabilization is used, radicals detected in solution are usually transient. This makes quantitation more difficult in these systems than in ones where the paramagnetic species are kinetically stable. Of course, quantitation is extremely important in all radical systems. It will distinguish between situations in which a radical is an obligate intermediate in an enzyme reaction and one in which the radical is formed in a secondary reaction or side reaction of low efficiency. However, in most biological ESR to date few attempts have been made to distinguish between such possibilities. 

The majority of the biosensors that have been reported are based on the deposition of biologically active species such as enzymes and antibodies at the surface of an electrochemical or optical transducer. The most common principle is to identify the analyte by use of a chemically selective enzyme. The enzyme-substrate reaction produces a secondary chemical signal by means of catalysis, e.g. H" or H2O2. This signal is then recognized and quantitatively converted to an electrical signal, e.g. a potential, a current, or a change of absorption or fluorescence, by a suitable transducer. 

IlEible 14-2. Examples for enzyme reactions producing secondary chemical signals and corresponding transducers. Trivial names of the enzymes and systematic numbers (EC-numbers) are given. 

Solute transport across the cytoplasmic membrane of bacteria occurs by two major mechanisms (i) Secondary transport systems transport by these systems is driven by electrochemical gradients and will lead to the translocation of solute in unmodified form (ii) Group translocation solute is substrate for a specific enzyme system in the membrane the enzyme reaction results in a chemical modification of the solute and release of the products at the cytoplasmic side. The only well-established group translocation system is the phosphoenolpyruvate phospho-transferase system (PTS) (see below). 

Tapia, O. and Andres, J. On a quantum theory of chemical reactions and the role of in vacuum transition stmetures. Primary and secondary sources of enzyme catalysis, J.Mol.Str (THEOCHEM), 335 (1995), 267-286... 

Cook, P.F., Oppenheimer, N.J. and Cleland, W.W. (1981). Secondary deuterium and nitrogen-15 isotope effects in enzyme-catalyzed reactions. Chemical mechanism of liver alcohol dehydrogenase. Biochemistry 20, 1817-1825... 

The reaction is exactly analogous to the chemical aldol reaction (also shown), but it utilizes an enamine as the nucleophile, and it can thus be achieved under typical enzymic conditions, i.e. around neutrality and at room temperature. There is one subtle difference though, in that the enzyme produces an enamine from a primary amine. We have indicated that enamine formation is a property of secondary amines, whereas primary amines react with aldehydes and ketones to form imines (see Section 7.7.1). Thus, a further property of the enzyme is to help stabilize the enamine tautomer relative to the imine. 

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