Substrate partition

The improvement of its activity and stability has been approach by the use of GE tools (see Refs. [398] and [399], respectively). A process drawback is the fact that the oxidation of hydrophobic compounds in an organic solvent becomes limited by substrate partition between the active site of the enzyme and the bulk solvent [398], To provide the biocatalyst soluble with a hydrophobic active site access, keeping its solubility in organic solvents, a double chemical modification on horse heart cytochrome c has been performed [400,401], First, to increase the active-site hydrophobicity, a methyl esterification on the heme propionates was performed. Then, polyethylene glycol (PEG) was used for a surface modification of the protein, yielding a protein-polymer conjugates that are soluble in organic solvents. 

Measurements of gas chromatographic retention time are often used as a fast and easy method of estimating vapor pressure. These estimated pressures are related to the gas/substrate partition coefficient, which can be regarded as a ratio of solubility of the substance in the gas to that in the substrate, both solubilities being of the substance in the liquid state. As a result the estimated vapor pressures are of the liquid state. To obtain the solid vapor pressure requires multiplication by the fugacity ratio. It is important to establish if the estimated and reported property is of the vapor or liquid. 

Vary the substrate partition coefficient in the range 1 to 1500 and see how this also influences the bioremediation time. 

Simplified schema of the substrate partition between metabolic pathways catabolism (used for ATP and ADP synthesis) and anabolism (for biomass synthesis). The quantities SPi indicate the fraction of S destined to formation of the product "i". 

Control of substrate partitioning by chemical cross-linking... 

The immediate vicinity of a solid phase may strongly affect the activity of the enzyme. The substrate partitions between the fluid phase and the charged polymer solid phase. This can be attributed to the ionic charges and to the limitation of diffusion of the solute to the solid phase due to an unstirred layer of about 1 xm (i.e., corresponding to a double-helix length of about 3000 bp or to more than 100 times the diameter of an average protein) (Trevan, 1980). 

Available evidence indicates that malic enzyme operates in two steps oxidation of malate to form enzyme-bound oxaloacetate, and decarboxylation of this material (27, 28). Malic enzyme will also decarboxylate oxaloacetate (28). Isotope effects indicate that oxidation and decarboxylation are separate steps and that both steps are partially rate determining (29). When the enzyme is supplied with oxaloacetate in the presence of NADPH, the substrate partitions in part toward malate formation and in part toward pyruvate formation (30). 

Here, the charged substrate partitioning in bulk solution (S ) and in the porous support (S ) is given by... 

There are a number of factors which may influence the activity or selectivity of a polymer-immobilized catalyst. Substrate diffusion is but one. This article has reviewed the mathematical formalism for interpreting reaction rate data. The same approach that has been employed extensively in heterogeneous systems is applicable to polymer-immobilized systems. The formalism requires an understanding of the extent of substrate partitioning, the appropriate intrinsic kinetic expression and a value for the substrate s diffusion coefficient. A simple method for estimating diffusion coefficients was discussed as were general criteria for establishing the presence of substrate transport limitations. Application of these principles should permit one to identify experimental conditions which will result in the intrinsic reaction rate data needed to probe the catalytic properties of immobilized catalysts. 

FIGURE 10.2. A model for the mediated reduction of species Y by the 0/R redox couple immobilized in a polymeric film at the electrode surface. The governing process may be either substrate diffusion to the film substrate partition into and diffusion within the layer charge transport within the PME film (governed by relative rates of electron hopping, redox site diffusion, counterion transport, or polymer motions) and electrode or mediation kinetics. 

FIGURE 2.15. (a) Variation of the plateau current l for mediated electrocatalysis with layer thickness L. (b) Analysis of substrate partitioning (ip) and diffusion (is) in electroactive polymer films via analysis of the rotation speed and thickness dependence of the RDE current response. 

If the rate of substrate partitioning into the film at the layer/solution interface is slow and not at equilibrium, then the Andrieux-Saveant equations must be modified to take this into account. This is quite readily... 

Jacobs, K.A., Paul, D.R., Geor, R.J., Hinchcliff, K.W., and Sherman, W.M., Dietary composition influences short-term endurance training-induced adaptations of substrate partitioning during exercise, Inl J. Sport Nutr. Exerc. Metab., 14, 38-61, 2004. 

Concerning the reaction S5rstems, hydrophobic aldehydes need cosolvents to enhance their solubility and up to 20% (v/v) dimethylformamide (DMF) or dimethylsulfoxide (DMSO) is usually tolerated by the enzyme without compromising its activity. Emulsions are alternative systems for hydrophobic acceptors, but substrate partitioning between phases may cause rate limitations, particularly for substrates with high values [96]. Examples of aldol additions catalyzed... 

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