Nonenzymatic chemical reactions

Amino acids play a role in food processing in the development of a cooked flavor as the result of a chemical reaction called the nonenzymatic browning reaction (228). 

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. 

Fig. 11.15. Mechanisms of ring opening of (R)-thiazolidine-4-carboxylic acids (11.113) as derivatives of and chemical delivery systems for l-cysteine (11.114). Activation was shown to be by nonenzymatic, hydrolytic reaction (Pathway a), or by mitochondrial oxidation (Pathway b) to the (R)-4,5-dihydrothiazole-4-carboxylic acid (11.115), followed by a (presumably nonenzymatic) hydrolysis to the IV-acylcysteine, and then by cytosolic hydrolysis to cysteine [138].

The crucial difference between nonenzymatic and enzymatic reactions is that the former generally take place in a homogeneous solution, whereas the latter occur on the surface of a protein that is asymmetric. Because of this, an asymmetric enzyme is able to confer asymmetry on the reactions of symmetric substrates. For example, although the two hydrogen atoms in CH2RR are equivalent in simple chemical reactions, the equivalence may be lost when the compound binds to the asymmetric active site of an enzyme.3 The attachment to the enzyme by R and R in structure 8.7 causes the two hydrogen atoms to be exposed to... 

A nonenzymatic. chemical method for specific cleavage of polypeptides involves the reaction of cyanogen bromide, CNBr, with methionine residues ... 

In the analysis of proteins in forensic applications, the chemical modifications that occur to proteins, posttranslationally and nonenzymatically, are of primary importance. These chemical changes are a result of chemical reactions between side chains of the protein and reactive groups of metabolites and/or exogenous toxicants, including drugs present in extracellular fluid such as serum. The analytical accessibility of these modified proteins depends on their rate of turnover. For example, those with a slow turnover rate will be long-lived, and such problems will be much more easy to identify than those with faster turnover rates. 

This section discusses two families of structural proteins, namely, extracellular keratins and collagens. Each example represents a family of closely related chemical entities, which means dealing with complex protein mixtures. We consider two types of chemical modifications modifications arising from increased levels of glucose in body fluids (obviously related to diabetes) and those derived from acetaldehyde, the first metabolic product of ethanol ingestion. As noted, a common feature of the latter nonenzymatic modification reactions is the presence of an aldehydic functionality in the nonprotein reactant (Jelfnkovd et al 1995 Deyl and Mikgik, 1995). 

In rare instances, the derivatization with a CDA was carried out enzymatically (see below). Clearly, the requirements for optimization of such reactions are different from those of nonenzymatic chemical transformations and should be carefully examined before an enzymatic derivatization reaction is routinely applied for quantification. 

Because of their remarkable catalytic activity, a given number of enzyme molecules convert an enormous number of substrate molecules to products within a short time. Therefore, the appearance of increased amounts of enzymes in the blood stream is easily detected, although the amount of enzyme protein released from damaged cells is small compared with the total level of nonenzymatic proteins in bloods Thus a particular enzyme is recognized by its characteristic effect on a given chemical reaction despite the presence of a vast excess of other proteins. 

There are various chemical reactions that proceed and may be accelerated at low values of water activity. Maillard reactions leading to lysine loss and brown color develop peaks at aw values around 0.5-0.8. Nonenzymatic lipid oxidation increases rapidly below aw = 0.4. Enzymic hydrolysis decreases with water activity to aw= 0.3, after which, it is negligible. 

Figure 2. Examples of the types of chemical reactions promoted by aquatic microbes, including reductant excretion (1), transplasma membrane electron transport (2), redox reactions involving extracellular solutes (3) and reductants excreted by the cell (3/>), hydrolysis reactions (4), ami nonenzymatically mediated redox reactions (5). The cell wall and membrane are represented by the curved parallel lines and extracellular enzymes by ovals.

Unlike DDH, 5,6-LAM, and MCM, for which chemical examples of radical isomerizations are available, there is no nonenzymatic model reaction for the isomerization of the 4-glutamyl radical identified as the intermediate observable by EPR spectroscopy at the active site of The accepted mechanism is the radical fragmenta-... 

It must be remembered that compounds formed by enzymatic processes can react subsequently in a nonenzymatic transformation. Additionally, by examining the chemical transformations involved in the production of compounds via nonenzymatic browning reactions, several routes can be involved in the production of the same compounds from different precursors (2337). 

The nonenzymatic glycosylation of certain proteins may start a cascade of chemical reactions, which finally lead to irreversible cross-linMng of neighboring proteins. Advanced glycosidation end products (AGE) are mostly yellowish-brown and they fluoresce. One AGE structure, namely 2-furanyl-4(5)-2-furanyl-... 

The presence or activity of water in foods may also enhance the rate at which deteriorative chemical reactions occur. Some products may become rancid through free radical oxidation even at low humidities and thus become unacceptable. Labile nutrients such as vitamins and natural color compounds are oxidized more rapidly when stored at low moisture levels. Enzyme-mediated hydrolytic reactions may reduce the quality of the food product. Other reactions such as the Maillard type of nonenzymatic browning may be enhanced by the presence of higher levels of water. On the other hand, water content is crucial for the textural characteristics and the sensory perception of foods. A food may be found unacceptable by consumers simply because it does not satisfy their textural (sensory) anticipation. 

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