Biological activity, effect reaction rates

Electrostatically-controlled pre-association interactions have an important effect on rates for [Pd(dien)Cl]+ reacting with thione-containing nucleosides, nucleotides and oligonucleotides, as is often the case for reactions between metal complexes and this type of biological ligand. Interaction between the charged complex and the polyanionic oligonucleotide surface leads to an increase in both enthalpy and entropy of activation in the DNA or model environment (252). 

Enzymes provide the catalytic entities in biological transformations. The unique characteristics of biocatalysts, namely, activation of substrates and acceleration of reaction rates at ambient temperatures, specificity towards substrates, and stereospecificity and chiroselectivity towards product formation, generate most sophisticated and effective catalysts [101]. Accordingly, extensive efforts of chemists and biochemists are directed towards harnessing chemical transformations by means of technological approaches utilizing enzymes [102, 103]. 

Metal Ion Effects. The metal ion effects on the acid-catalyzed hydrolysis of PPS also were examined by Benkovic and Hevey (5). However, they observed that in water near pH 3, the rate enhancement in the presence of an excess of metal ion was at most only threefold (Mg2+, Ca2+, Al3+) and in some cases (Zn2+, Co2+, Cu2+) the rate was actually retarded. We thought that the substrate PPS and Mg2+ ion should be hydrated heavily in water so that their complexa-tion for rate enhancement is weak. If, however, the hydrolysis is carried out in a solvent of low water content, such complexation would not occur, and therefore, the rate enhancement might be more pronounced. This possibility appears to be supported by the fact that the active sites of many enzymes are hydrophobic. Of course, there is a possibility that the S—O fission may not require metal ion activation. In this connection, it is interesting to note that in biological phosphoryl-transfer reactions the enzymes generally require divalent metal ions for activity (7, 8, 9), but such metal ion dependency appears to be less important for sulfate-transfer enzymes. For example, many phosphatases require metal ions, but no sulfatase is known to be metal... 

There exist a large number of phenomenological laws for example, Fick s law relates to the flow of a substance and its concentration gradient, and the mass action law explores the reaction rate and chemical concentrations or affinities. When two or more of these phenomena occur simultaneously in a system, they may couple and induce new effects, such as facilitated and active transport in biological systems. In active transport, a substrate can flow against the direction imposed by its thermodynamic force. Without the coupling, such uphill transport would be in violation of the second law of thermodynamics. Therefore, dissipation due to either diffusion or chemical reaction can be negative only if these two processes couple and produce a positive total entropy production. 

Because of the relative simplicity of carboxylic ester hydrolysis, in general, and that of base catalyzed ester hydrolysis, in particular, these reactions have served well as model systems in investigations of micellar effects on reaction rates and activation parameters. In addition, the prevalence in biological systems of carboxylic ester hydrolyses catalyzed by nucleophiles and by enzymes renders the investigation of micelle-catalyzed ester hydrolyses of obvious importance. 

In addition to chemicals, biological catalysts such as enzymes can be used to catalyze reactions in SC CO2. Since the first attempt to operate reactions in supercritical fluids published by Randolph et al. [34], various type of enzymes were studied lipase, oxidase, decarboxylase, dehydrogenase, proteinase, etc. [33,35-37]. The effect of different parameters was extensively reported by Ballesteros et al. [35]. Enzyme activity and stability in supercritical conditions as well as the benefits of using supercritical fluids for enzymatic reactions (improved reaction rates, control of selectivity, etc.) have been demonstrated [36]. 

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