Nonenzymatic chemical reactions examples

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... 

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). 

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-... 

One of the most outstanding features in enzyme-catalyzed reactions is stereochemical completeness. A high percentage of stereospecific reactions, such that only one chiral product is formed in an excellent enantiomer excess (e.e.) and that only one of two enantiomers is available as a substrate, readily take place. In addition, the spectacular feature that an enzyme can also differentiate enantiotopic groups or faces that are nonenzymatically (chemically) equivalent has also been found. For example, enzymes distinguish two hydrogens attached to a prochiral carbon, >CH2. Even the three... 

Bacteria are involved in many, but not all, types of food spoilage. The stale, putrid odor of spoiled meat, the foul odor of spoiled egg, and the sour odor of spoiled milk are examples of bacteria food spoilage. Both fresh and canned foods are subject to food spoilage. Some types of food spoilage are caused by enzymes, and others are caused by chemical reactions— for example, by nonenzymatic browning. The control of bacterial spoilage has received a tremendous amount of research attention since the days of Louis Fhsteur, who pioneered in the study of bacteria. 

The earliest examples of analytical methods based on chemical kinetics, which date from the late nineteenth century, took advantage of the catalytic activity of enzymes. Typically, the enzyme was added to a solution containing a suitable substrate, and the reaction between the two was monitored for a fixed time. The enzyme s activity was determined by measuring the amount of substrate that had reacted. Enzymes also were used in procedures for the quantitative analysis of hydrogen peroxide and carbohydrates. The application of catalytic reactions continued in the first half of the twentieth century, and developments included the use of nonenzymatic catalysts, noncatalytic reactions, and differences in reaction rates when analyzing samples with several analytes. 

It has been illustrated with several examples (Table 2.1) that enzymes are substantially better catalysts than are protons or other ionic species used in nonenzymatic reactions. Enzymes invariably surpass all chemical catalysts in relation to substrate and reaction specificities. 

---