Access to substrate

After establishing the easy access to substrates 13 by the combination of the convenient synthetic method of propargylic alcohols and effective enzymatic resolution, the [2,3]-Wittig rearrangements were carried out. 

Biochemical studies have also suggested an asymmetric orientation of constituents in lipid globule membrane. By comparison of specific activities of enzymes in washed lipid globules and released membrane, Patton and Trams (1971) suggested that the active site of Mg2+-adeno-sine triphosphatase was accessible to substrates on both faces of the membrane and that of 5 -nucleotidase on the outer membrane face. Recent evidence from studies of Concanavalin A inhibition of globule membrane and plasma membrane 5 -nucleotidase support an outer surface localization for the active site of this enzyme (Carraway and Carra-way 1976 Snow et al. 1980). Kobylka and Carraway (1973) observed that exposure of lipid globules to proteolytic enzymes resulted in cleavage of all major membrane-associated proteins. They concluded that... 

Mordenite is a naturally occurring zeolite with a Si/Al ratio of ca. 10 and a structure composed of 12-ring and 8-ring tunnels with diameters of 0.39 nm and ca. 0.7 nm, respectively, extending through the entire framework (Fig. 2.7). Every framework atom forms a part of the walls of these tunnels and is accessible to substrate molecules diffusing through them. 

The specific activity of an enzyme almost always decreases on immobilization. The active sites are less accessible to substrate, and the diffusion of substrates and products across the stagnant layer of solution at the particle surface, and within polymer networks, lowers apparent values of Vmax and raises apparent Km values. The activity of an immobilized enzyme should be expressed as specific activity... 

The nature of the surface of the solid catalyst is ultimately vital to its performance and value. The number of catalytically active sites, their dispersion over the surface of the solid, their accessibility to substrate molecules and their activity or strength are fundamental properties that can be influenced by the nature of and any pretreatment of the support, the method of preparation of the... 

Binding of NTP allows the thiyl radical access to substrate C3 (H), which it abstracts to initiate reduction. Elimination of water by the 3 -radical, followed by or concurrent with reduction by an enzymatic dithiol, leads to the 3 -radical of the product dNTP. Abstraction of the thiol hydrogen atom from Cys408 by the product radical regenerates the thiyl radical and produces dNTP. The radical cationic species following dehydration shown in Figure 31 is a hypothetical intermediate. 

In certain applications, the size of the biocatalyst is not suitable for entrapment into the supporting sponge layer of asymmetric membranes. In these cases, the membrane acts mainly as a selective barrier it defines a reactive zone in the vessel, usually the shell, accessible to substrates and products mainly by diffusive mass transfer preventing the catalyst from pollution or inhibition possibly caused by other species in solution.54-69... 

When the solubility limit of water in the fluid is approached or exceeded, the reaction rates drop drastically [13,21]. The liquid water in the enzyme particles apparently forms a mass-transfer barrier. It has also been noted that enzyme particles tend to stick together when the optimum water concentration is exceeded [45]. Obviously this reduces the enzyme area that is accessible to substrates. 

Conformational and sterlc effects the enzyme may be conformatlonally different when fixed on a support alternatively It may be attached to the solid carrier in a way that would render certain parts of the enzyme molecule less accessible to substrate or effector. These effects are (in 1976) well understood."... 

To find out NP-5 effect on digestibility, solid residues recovered after pretreatment were analyzed and listed in Table 4. From this table, it shows ftiat hydrogen peroxide or NP-5 in ammonia solution did not affect that much on the composition of solid residue. Such a high digestibility caused by NP-5 must not be due to residual solid components because the compositions of both samples are almost same. Pretreated sample in our study is washed with water to remove chemicals and detached ink particles. In this stage detached ink particles are dispersed in the aqueous phase and some of ftiem redeposit on the fiber surftice. This redeposition may interfere enzyme access to substrate. After washing the pretreated sample with water, we found much less ink particles in the NP-5-contained sample than those in the control sample ftirou microscope. But we cannot explain this digestibility enhancement by NP-5 clearly yet. 

This latter was employed in the enantioselective addition of diethylzinc to aromatic and aliphatic aldehydes. For example, (S)-l-phenylpropanol was isolated in a yield of 92% and an ee of 96% when the reaction was performed in a 1/1 mixture of toluene and dichloromethane. The presence of dichloromethane proved to be essential for swelling of the polymer and making the catalytic sites accessibles to substrates. With other aromatic aldehydes and cyclohexane carboxaldehyde, the same range of chemical yield and ee was obtained. But with acyclic aliphatic aldehydes the chemical yield and ee dropped significantly. The recyclability of the chiral polymer 160 has been investigated and it was noticed a slightly decrease of the enantioselectivity of 1% per run. 

The Se-dependent GSH peroxidase is a tetramer of 84 kDa with very high activity towards both HjOj and organic hydroperoxides (kRooH 10 M" s ). It contains one residue of selenocysteine per mole at each of the active sites which, according to the crystal structure, is a depression near the molecular surface and readily accessible to substrate. This is consistent with the high reaction rate observed for this enzyme (Ladenstein and Epp 1984). 

As previously explained, formation of dinuclear species upon oxidation of Cu(I) complexes is a thermodynamic sink. Increasing the bulkiness of the ligand can hinder the coordination sphere sufficiently to prevent dimerization. But in that case, the access to the metal is also locked for the substrate. Embedding the metal site at the bottom of a molecular host can be a way to confine copper in a mononuclear environment, while keeping it accessible to substrates via the cavity of the host. Calix[6]arenes have been intensely used for this purpose. " Despite being highly flexible, they can be constrained into the cone conformation by coordination to a single metal center when three alternate phenolic units out of six are... 

Several successful chiral catalysts have been reported so far for catalytic asymmetric 1,3-dipolar cycloaddition reactions of nitrones with oxazolidinone derivatives (Figure 16.3) [27a,b,c] Recently, Kiindig et al. [27] had reported that the single coordination site Fe and Ru transition-metal Lewis acids can efficiently promote enantioselective 1,3-dipolar cycloadditions of nitrones with a,p-unsaturated aldehydes and represent a rapid access to substrates of high synthetic potential. The Fe catalyst (81) was found to be an excellent catalyst for 1,3-dipolar cycloaddition reactions between methacrolein and nitrone (83) (Scheme 16.24, Table 16.5). Isox-azolidine (84a) was formed as a single region- and diastereoisomer in excellent... 

An enzyme can be entrapped or enclosed in the cavities of a polymer network by polymerization of a monomer such as acrylamide or N,N -methylene-bis-acrylamide in the presence of enzyme, and still remain accessible to substrate through the network of pores. Furthermore,... 

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