Biocatalysis enzymes, catalytic activity

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

The notion that RNA existed prior to enzymes and that RNA molecules can be catalytically active is by now well accepted. Indeed, RNA molecules have been observed to catalyze phophodiester bond breaking and synthesis (as occurs during replication and splicing) but also reactions more distant to its stmcture, such as amide bond formation (Wiegand, 1997). RNA thus provided the means to assemble peptides which may have led to the protein world of today (Zhang and Cech, 1998). Reactions catalyzed by RNA molecules have thus far not been employed in biocatalysis and it is unlikely that... 

One of the greatest hurdles for the application of biocatalysis is the need to operate processes under conditions that can differ dramatically from those in which the enzymes evolved. Many techniques are used in order to preserve catalytic activity and minimize the costs associated with the biocatalyst. In cases where the cost of the biocatalyst is a concern, an enzyme might be immobilized and used in a packed column or a fluidized bed reactor so as to enable reuse. Here also the enzyme must be stable for extended periods and may even be used under nonaqueous conditions and elevated temperatures. Recombinant technology has revolutionized the applications of biocat-... 

Thus, the results shown here demonstrate that thermosensitive microgel particles can serve as superior carriers for the adsorption of enzymes in which the activity of adsorbed enzymes are preserved. The catalytic activity of adsorbed P-D-glucosidase from almonds is increased by a factor of more than three. Moreover, the catalytic properties of immobilized enzymes can be manipulated by the volume transition of the microgel. Hence, such microgels present a novel class of active nanoreactors for biocatalysis. 

In the case of biocatalysis, enzymes [3] and catalytic antibodies [4] have attracted most attention. Since enzymes are inherently the more active catalysts, they have been used most often. Indeed, many industrial processes for the enantioselective production of certain chiral intermediates are based on the application of enzymes, as in the lipase-catalyzed kinetic resolution of an epoxy-ester used in the production of the anti-hypertensive therapeutic Diltiazem [5]. Recently, it has been noted that there seems to be a trend in industry to use enzymes more often than in the past... 

This Highlight is part of an extraordinary story (also a cautionary tale) in the area of biocatalysis. The point of particular interest was the incredible catalytic activity claimed for so-called pepzymes - small synthetic peptides modelled to mimic the active site structures of trypsin and chymotrypsin. One was claimed to hydrolyse a simple peptide (a trypsin substrate) with efficiency comparable to that of the native enzyme. This extraordinary result provoked at least as much scepticism as excitement, and in the following months several groups tried to reproduce the results. They failed, comprehensively. [1,2] Some reasons why this failure came as no surprise were subsequently summarised by Matthews, Craik and Neurath, [3] and by Corey and Corey [4]. The background has been discussed in an Angewandte Review on Enzyme Mechanisms, Models and Mimics. [5]... 

The application of aqueous / supercritical biphasic media is not restricted to metal complex catalysis but has proven effective also for enzymatic and whole-cell biocatalysis [36]. In general, water plays an important role in coimection with biocatalysis. If water is completely absent, enzymes are often not catalytically active under supercritical conditions [37]. In the literature many examples of biocatalysis with supercritical fluids containing various amounts of water are known and a detailed account of this field is outside the scope of the present discussion. One example to highlight the use of a true biphasic system is the carboxylation of pyrrole... 

Further advancement of biocatalysis will require the use of directed evolution to bridge the functional gap between wild-type and desired biocatalyst properties. These studies underscore the power of directed evolution to create artificial enzymes derived from wild-type enzymes with the desired catalytic activity. Directed evolution techniques will continue to fulfill the promise of biocatalysis for industrial applications. 

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

It was reported that PEGylated lipase entrapped in PVA cryogel could be conveniently used in organic solvent biocatalysis [279], This method for enzyme immobilization is more convenient in comparison to other types of immobilization that take advantage of enzyme covalent linkage to insoluble matrix, since the chemical step which is time consuming and harmful to enzyme activity is avoided. The application of this catalytic system to the hydrolysis of acetoxycoumarins demonstrated the feasibility of proposed method in the hydrolysis products of pharmaceutical interest and to obtain regioselective enrichment of one of the two monodeacetylated derivatives. 

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