Enzymes continued oxidizing

This chapter begins, thus, with a short introduction to the chemical reactivity of epoxides. We continue with a description of the epoxides hydrolases and their biochemistry, and devote most of its length to a systematic discussion of the substrates hydrated by these enzymes. Arene oxides and diol epoxides will be presented first, followed by a large variety of alkene and cy-cloalkene oxides. 

When ethanol is metabolized in the liver, it is oxidized to ethanal (acetaldehyde). If too much ethanol is present in the body, an overabimdance of ethanal is formed, which causes many of the adverse effects of the "morning-after hangover." Continued oxidation of ethanal produces ethanoic acid (acetic acid), which is used as an energy source by the cell and eventually oxidized to CO2 and H2O. These reactions, summarized as follows, are catalyzed by liver enzymes. 

Given the tendency of peroxidases to undergo irreversible deactivation in the presence of substantial concentrations of H2O2 (which is common to many other enzymes) the oxidant has to be added continuously to the reaction in order to keep its concentration at a low level. This also suppresses its decomposition via the catalase activity of peroxidases (path 4, Scheme 2.174). In practice, this is conveniently achieved by using a H202-sensitive electrode coupled to an auto-titrator which adds the oxidant in a continuous fashion (peroxy-stat) [1352]. Furthermore, this minimizes the spontaneous (background) oxidation, which leads to the formation of racemic product. 

Improvement of the stability of coimmobilized xanthine oxidase Used as part of a coupled system of immobilized enzymes for the rapid and continuous estimation of a-amylase Operational studies on the batch-wise and continuous oxidation of D-glucose investigation of the associated macrokinetics... 

Spoilage of food is caused by growth of microorganisms, enzyme action, oxidation (chemical reactions), extremes in physical surroundings, anckor pests. Insofar as possible, the aim of preservation is to reduce, eliminate, and/or control the causes of spoilage. With the modern methods of preservation, seasonal harvests can be enjoyed year round, and food specifically grown in one area can be shipped across the country or even worldwide. Still, scientists continue to search for better methods of preservation that yield a nutritious, flavorful, and desirable product, with minimal long-term requirements. 

As previously stated, in an enzyme-catalyzed reaction involving the NAD" / NADH cofactor, NAD is reduced to NADH whereas the substrate is concurrently oxidized. In a BFC, any NAD that is reduced to NADH must be reoxidized in order to perpetuate the reaction cycle and provide a continuous oxidation of fuel. Direct oxidation of NADH at the electrodes, such as gold or glassy carbon, however, requires... 

Fermentation. The term fermentation arose from the misconception that black tea production is a microbial process (73). The conversion of green leaf to black tea was recognized as an oxidative process initiated by tea—enzyme catalysis circa 1901 (74). The process, which starts at the onset of maceration, is allowed to continue under ambient conditions. Leaf temperature is maintained at less than 25—30°C as lower (15—25°C) temperatures improve flavor (75). Temperature control and air diffusion are faciUtated by distributing macerated leaf in layers 5—8 cm deep on the factory floor, but more often on racked trays in a fermentation room maintained at a high rh and at the lowest feasible temperature. Depending on the nature of the leaf, the maceration techniques, the ambient temperature, and the style of tea desired, the fermentation time can vary from 45 min to 3 h. More highly controlled systems depend on the timed conveyance of macerated leaf on mesh belts for forced-air circulation. If the system is enclosed, humidity and temperature control are improved (76). 

The oxidation of heteroatoms and, in particular, the conversion of sulfides to asymmetric sulfoxides has continued to be a highly active field in biocatalysis. In particular, the diverse biotransformations at sulfur have received the majority of attention in the area of enzyme-mediated heteroatom oxidation. This is particularly due to the versatile applicability of sulfoxides as chiral auxiliaries in a variety of transformations coupled with facile protocols for the ultimate removal [187]. 

CK catalyzes the reversible phosphorylation of creatine in the presence of ATP and magnesium. When creatine phosphate is the substrate, the resulting creatine can be measured as the ninhydrin fluorescent compound, as in the continuous flow Auto Analyzer method. Kinetic methods based on coupled enzymatic reactions are also popular. Tanzer and Gilvarg (40) developed a kinetic method using the two exogenous enzymes pyruvate kinase and lactate dehydrogenase to measure the CK rate by following the oxidation of NADH. In this procedure the main reaction is run in a less favorable direction. 

In some cases it is more attractive to use whole microbial cells, rather than isolated enzymes, as biocatalysts. This is the case in many oxidative biotransformations where cofactor regeneration is required and/or the enzyme has low stability outside the cell. By performing the reaction as a fermentation, i.e. with growing microbial cells, the cofactor is continuously regenerated from the energy source, e.g. glucose. 

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