Oxygen as substrate

Many enzymic reactions have high negative AG° values, for example glycosyl or peptide bond hydrolysis reactions in aqueous media, oxidations with oxygen as substrate etc. Some thermodynamic data of industrially applied enzymic reactions are described by Bmns and Schulze (1962), Tewari (1990) and Biselli, Kragl and Wandrey (1995). 

Stopped-flow kinetic studies of Amadoriase I using fructosylpropylamine and oxygen as substrates in 10 mM Tris hydrochloride buffer (pH 7.9) at 4 °C pointed to the pyranose form as being the active configuration. The redox potentials were found to be + 48 and -52 mV for the oxidised enzyme/anionic quinone and anionic semi-quinone/reduced enzyme reactions, respectively, at pH 7.0 and 25 °C.620... 

As discussed y)xevio xs, y,Saccharomyces cerevisiae does not need oxygen to obtain energy when fermenting grape juice. However, there are some essential biosynthetic pathways that use oxygen as substrate. This is the case for the biosynthesis of sterols and unsaturated fatty acids (Ratledge and Evans 1989). 

Figure 1.7 illustrates the synthesis of sterols in yeasts. Basically, sterols are synthesised by the mevalonate pathway. The key stage in this pathway is, without any doubt, the reaction catalysed by squalene monooxygenase. This reaction, which uses oxygen as substrate, transforms squalene into squalene 2,3, epoxide. Later, squalene epoxide lanosterol cyclase catalyses the synthesis of the first sterol of the pathway. 

The addition usually takes place from the sterically less hindered side of the alkene. The stereochemical course of the addition can be controlled by suitably positioned oxygen center that can coordinate to the organozinc reagent. For example the reaction with 4-hydroxycyclopentene 6 as substrate exclusively yields the c -3-hydroxybicyclo [3.1.0] hexane 7 ... 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Figure 4.10 Typical effects of yield coefficient on oxygen requirement when only biomass and CO2 are produced (methanol as substrate)...

Figure 4.11 Typical relationship between minimum oxygen-transfer rates and yield coefficients at various productivities (methanol as substrate).

In biochemical engineering processes, measurement of dissolved oxygen (DO) is essential. The production of SCP may reach a steady-state condition by keeping the DO level constant, while the viable protein is continuously harvested. The concentration of protein is proportional to oxygen uptake rate. Control of DO would lead us to achieve steady SCP production. Variation of DO may affect retention time and other process variables such as substrate and product concentrations, retention time, dilution rate and aeration rate. Microbial activities are monitored by the oxygen uptake rate from the supplied ah or oxygen. 

Oxidases catalyze the removal of hydrogen from a substrate using oxygen as a hydrogen acceptor. They form water or hydrogen peroxide as a reaction product (Figure 11-1). 

Enzymes are nature s catalysts. For the moment it is sufficient to consider an enzyme as a large protein, the structure of which results in a very shape-specific active site (Fig. 1.3). Flaving shapes that are optimally suited to guide reactant molecules (usually referred to as substrates) in the optimum configuration for reaction, enzymes are highly specific and efficient catalysts. For example, the enzyme catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen... 

Stereoselective oxygen transfer to the sulphur atom of alkyl aryl sulphides catalyzed by 2-flavoenzyme monooxygenases afforded optically active sulphoxides in high optical yields . For instance, with ethyl p-tolyl sulphide as substrate cyclohexanone monooxygenase from Actinetobacter produces predominantly (— )-(S)-sulphoxide with 64% e.e. In contrast, FAD-containing dimethylaniline monooxygenase purified from hog liver microsomes affords (+ )-(i )-enantiomer of this sulphoxide with 90% optical purity . ... 

This conclusion is in agreement with experiments in which a smootb quartz and cellulose were used as substrates. For above materials the transfer of excitation energy of the dye into the substrate is low which is confirmed by intensive luminescence of adsorbed tripaflavine. Note, that the activation energy of emission of singlet oxygen is close for zinc oxide oxidized by oxygen atoms, quartz and cellulose and amounts to 5-10 kcal/mol [83]. 

Such enzymes catalyse reactions involving electron transfer. Oxidases use molecular oxygen as an electron acceptor (Scheme 10.10). Dehydrogenases remove hydrogen atoms from the substrate and transfer them to an acceptor other than oxygen. 

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