Water, in enzyme catalysis

Review of the role of water in enzyme catalysis, with carbonic anhydrase as the featured example. 

Review of the roles of water in enzyme catalysis as revealed by studies in water-poor solutes. 

In an investigation of the role of water in enzymic catalysis. Brooks and Karplus (1989) chose lysozyme for their study. Stochastic boundary molecular dynamics methodology was applied, with which it was possible to focus on a small part of the overall system (i.e., the active site, substrate, and surrounding solvent). It was shown that both structure and dynamics are affected by solvent. These effects are mediated through solvation of polar residues, as well as stabilization of like-charged ion pairs. Conversely, the effects of the protein on solvent dynamics and... 

The above constitute only a few examples of the extensive role of water in enzyme catalysis. Water probably controls many aspects of enzyme catalysis that continuously occur within biological cells and the number of such instances is enormous. However, our understanding of these phenomena remains poor in most cases, although failure of any enzymatic action can lead to serious consequences. 

An attractive alternative is to study intramolecular reactions. These are generally faster than the corresponding intermolecular processes, and are frequently so much faster that it is possible to observe those types of reaction involved in enzyme catalysis. Thus groups like carboxyl and imidazole are involved at the active sites of many enzymes hydrolysing aliphatic esters and amides. Bimolecular reactions in water between acetic acid or imidazole and substrates such as ethyl acetate and simple amides are frequently too slow to... 

HsO ) in one region of an aqueous solution to produce a hydronium ion at a distant site. Note that the proton released locally from the initial HsO remains in its vicinity, and is not the same as the proton forming the hydronium ion at the distant site. For this reason, the ionic mobility appears to be much greater than would be expected on the basis of diffusion alone. Facilitated proton transfer along rigidly and accurately positioned hydrogen bonds could be of fundamental importance in enzyme catalysis. See Water... 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

Zacharis, E., Omar, I.C., Partridge, J. and Robb, D.A. (1997) Selection of salt hydrate pairs for use in water control in enzyme catalysis in organic solvents. Biotechnol. 

Eppler et al. [103] viewed these results as having a potential relationship to salt-activated enzyme preparations, particularly in relation to the mobility of enzyme-bound water. Specifically, the authors examined both water mobility [as measured by T2-derived correlation times, (tc)D20] and NaF-activated enzyme activity and observed a linear relationship. This suggests that the salt-activated enzymes contain a more mobile water population than salt-free enzymes, which facilitates a more aqueous-like local environment and dramatically increases enzyme activity through increased flexibility. Therefore, enzyme activation appears to correlate with the properties of enzyme-associated water. Once again, the physicochemical properties of water dictate enzyme structure, function, and dynamics. Hence, salt activation has proven to be a useful technique in activating enzymes for use in organic solvents and has provided a quantitative tool to better understand the role of water in enzymatic catalysis in dehydrated media. 

Using carbon dioxide as a feedstock in synthetic chemistry is an important area of green chemistry. It is significantly soluble in water, and water is therefore a good medium for its conversion. However, when it dissolves it forms carbonic acid (Figure 3.13). Considerable efforts have been made to understand this process and control the pH of aqueous-carbon dioxide systems.This is also highly relevant to studies involving supercritical carbon dioxide and water in biphasic catalysis, especially for pH-sensitive enzymes. 

Hailing, P. Enzymic conversions in organic and other low-water media. In Enzyme Catalysis in Organic Synthesis 2nd Ed. Drauz, K., Waldmann, H., Eds. Wiley-VCH Weinheim, Germany, 2002 259-285. 

W. A, Waters Oxford University) The concentration of ions on a surface is not often appreciated, and may be very important in enzyme catalysis. 

This chapter first explains enzyme nomenclature, describes enzymatic, supercritical reactor configurations, and gives a compilation of published experimental results. The- most important topics concerning enzymatic reactions in SCFs are then covered. These are factors affecting enzyme stability, the role of water in enzymatic catalysis, and the effect of pressure on reaction rates. Studies on mass transfer effects are also reviewed as are factors that have an effect on reaction selectivities. Finally, a rough cost calculation for a hypothetical industrial process is given. 

As already mentioned, control of vrater content is of great importance in enzyme catalysis. Studies on Pseudomonas sp. lipase have also revealed a strong influence of the vrater content of the reaction medium [70]. In order to compare the enzyme activity and selectivity as a fonction of the vrater present in solvents of different polarities, it is necessary to use the vrater activity a in these solvents. We used the method of vrater activity equilibration over saturated salt solutions [71] and could demonstrate that, in contrast to MTBE, which is commonly used for this type of reaction, the enantiosdectivity of the lipase is less influenced either by the water content or the temperature when the reaction is performed in [BMIM][(CFjS02)2N]. 

Water was used as solvent for the first time in the lipase-catalyzed ROP of five lactone monomers, e-CL, OL, UDL, DDL, and PDL (Scheme 5) [69, 70]. Macrolides of UDL, DDL, and PDL are less reactive than lactones of smaller ring size due to lower ring strain when using a usual chemical catalyst [71]. However, they showed higher reactivity in enzyme catalysis and were polymerized by lipase in water to produce the corresponding polyesters typically, UDL gave polyUDL with 1,300 (Mw/M = 2.1) in 79% yields at 60°C for 72 h. DDL is... 

Several dozens of aldolases have been identified so far in nature [23,24], and many of these enzymes are commercially available at a scale sufficient for preparative applications. Enzyme catalysis is more attractive for the synthesis and modification of biologically relevant classes of organic compounds that are typically complex, multifunctional, and water soluble. Typical examples are those structurally related to amino acids [5-10] or carbohydrates [25-28], which are difficult to prepare and to handle by conventional methods of chemical synthesis and mandate the laborious manipulation of protective groups. 

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

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