Non-enzymatic reactions

A few biochemical reactions are not enzyme-catalysed. One example is the spontaneous but slow cyclisation of creatine phosphate to form creatinine (11.45). This relatively stable product is found in muscle and is expelled from the animal body in urine (Table 11.18). 

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Chorismate Mutase catalyzed Claisen Rearrangement- 10 rate enhancement over non-enzymatic reaction... 

Enzymes are exceptionally efficient catalytic proteins which increase the speed of a chemical reaction without themselves undergoing a permanent change. Under optimal conditions, most enzymatic reactions proceed from 10 to 10 times more rapidly than the corresponding non-enzymatic reactions. For example, one molecule of catalase, the enzyme which converts hydrogen peroxide into water and atomic oxygen, is able to deal with approximately 5 million molecules of H2O0 per minute. 

According to Kruger s original hypothesis to explain methyl-ation of rat liver DNA following administration of NDPA, NMPA should be formed from NOPPA. NOPPA is converted into NMPA in a base-catalyzed, non-enzymatic reaction, but this takes place only at high pH (16). We observed no detectable reaction at physio-... 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Mills and Alexander [4] have discussed the factors affecting the formation of dimethylnitrosamine in samples of soil. Dimethylnitrosamine was formed as readily in sterilized samples as in non-sterile samples, indicating that, although micro organisms can carry out an enzymatic nitrosation in some soils, dimethylnitrosamine can be formed by a non-enzymatic reaction, even at near neutral conditions. The presence of organic matter appears to be important in promoting nitrosation in the presence of the requisite precursors. 

The precursor, 7-dehydrocholesterol is converted by a non-enzymatic reaction to cholecalciferol (calciol). This reaction occurs in skin exposed to sunlight due to irradiation by UV-B light at a wavelength of about 300 nm. Cholecalciferol is transported via carrier proteins to the liver where hydroxylation at carbon-25 occurs in a reaction catalysed by a microsomal cytochrome P450 hydroxylase to form calcidiol. This compound travels to the kidney attached to specific binding proteins, where another cytochrome P450 enzyme, mitochondrial 1-a-hydroxylase, introduces a second hydroxyl group in to the molecule to form the active calcitriol. 

Figure 9.28 Non-enzymatic reactions leading to membranogenic polyprenyl or dipolyprenyl phosphates. (Modified from Ourisson and Nakatani, 1999.)...

Willeman et al. [26] modeled the enzyme-catalyzed cyanohydrin synthesis in a stirred batch tank reactor. Assumption of a mass transfer limitation (Figure 9.3b) is made, which results in a low concentration of substrate in the aqueous phase, thus suppressing the non-enzymatic reaction. In a well-stirred biphasic system the enzyme concentration was varied, keeping the phase ratio constant A maximum rate of conversion is reached at the concentration where mass transfer of the substrate becomes limiting. Further increase of enzyme concentration does not enhance the reaction rate [27]. The different results achieved by the two groups are explained by the different process strategies. No mass transfer limitation could be detected by Hickel et al. because the stirring rate in the aqueous phase was not varied [26]. 

The studies on the methylation of dihydroxybenzaldehyde and the earlier studies on the decarboxylation of oxaloacetic acid illustrate a hypothesis about metal-catalyzed enzymes that is not proved but has been substantiated in a number of instances in which it has been tried. The hypothesis is that, if a metal constitutes the active site of an enzyme, it should be possible to carry out the reaction with metal ions alone in the absence of the enzyme. The rates of non-enzymatic reactions may be much lower, and the metal ions may be more active metal ions than those that activate the enzyme, for the reasons already discussed. This hypothesis is the basis for much of the work on metal catalytic reactions that are models for enzyme systems. 

Although the acute vasodilator effects, as shown in in vitro studies (see above), may participate in the antihypertensive effects, the reduced blood pressure persisted even 42-48 h after the last administration of quercetin, when the plasma quercetin concentration and its metabolites fell bellow 25% of the peak post-administration levels [43]. Furthermore, the antihypertensive effects of quercetin did not appear to be related to its antioxidant properties since quercetin did not lower the urinary isoprostane F20 excretion, a prostaglandin-like compound produced in a non enzymatic reaction of arachidonic acid in membrane lipids and superoxide, which is currently used as a reliable marker of oxidative stress. The mechanisms involved in the antihypertensive effects and protection from organ damage... 

This is directed particularly towards Fe11,976 the substrate with the highest Vm and the lowest Km values. Ceruloplasmin-catalyzed oxidation of Fe is 10-100 times faster than the non-enzymatic reaction, and ceruloplasmin appears to be the only effective ferrooxidase in human serum. This is linked to the control of iron mobilization by ceruloplasmin. Iron is released as Fe from ferritin, and the rate at which it is converted to circulating Fem2-transferrin is dependent upon ceruloplasmin. This explains why Cu-deficient animals develop anaemia. 

Enzymatic ester hydrolysis is a common and widespread biochemical reaction. Since simple procedures are available to follow the kinetics of hydrolytic reactions, great efforts have been made during the last years to explain this form of catalysis in chemical terms, i.e., in analogy to known non-enzymatic reactions, and to define the components of the active sites. The ultimate aim of this research is the synthesis of an artificial enzyme with the same substrate specificity and comparable speeds of reaction as the natural catalyst. 

By simulating evolution in vitro it has become possible to isolate artificial ribozymes from synthetic combinatorial RNA libraries [1, 2]. This approach has great potential for many reasons. First, this strategy enables generation of catalysts that accelerate a variety of chemical reactions, e.g. amide bond formation, N-glycosidic bond formation, or Michael reactions. This combinatorial approach is a powerful tool for catalysis research, because neither prior knowledge of structural prerequisites or reaction mechanisms nor laborious trial-and-error syntheses are necessary (also for non-enzymatic reactions, as discussed in Chapter 5.4). The iterative procedure of in-vitro selection enables handling of up to 1016 different compounds... 

A further example of the importance of non-enzymatic reactions is the metabolism of cyclophosphamides in humans (Joqueviel, 1998). Ring-opening reactions with spontaneous elimination of acrolein and a range of hydrolysis reactions at the phosphorus center occur. 

One group of NADH oxidants, which does not fit the proposed reaction scheme in Fig. 2.4 are the metal complexes. Examples of this type include nickel hexacyanoferrate deposited on porous nickel electrodes [29], gold electrodes modified with cobalt hexacyanoferrate films [30] and adsorbed l,10-phenanthroline-5,6-dione complexes of ruthenium and osmium [31]. It is unclear how these systems work and no mechanism has been proposed to date. It may be worth noting that dihydronicotinamide groups have been shown to reduce aldehydes in a non-enzymatic reaction when the reaction is catalysed by zinc, a metal ion [15]. In a reaction between 1,10-phenanthroline-2-carboxaldehyde and N-propyl-l,4-dihydronicotinamide, no reaction was seen in the absence of zinc but when added to the system, the aldehyde was reduced and the nicotinamide was oxidised. This implies that either coordination to, or close proximity of, the metal ion activates... 

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