Diffusible intermediates

FIGURE 18.5 Schematic representation of types of multienzyme systems carrying out a metabolic pathway (a) Physically separate, soluble enzymes with diffusing intermediates, (b) A multienzyme complex. Substrate enters the complex, becomes covalently bound and then sequentially modified by enzymes Ei to E5 before product is released. No intermediates are free to diffuse away, (c) A membrane-bound multienzyme system. 

The condition of unstable, and therefore non-diffusing, intermediates implies that... 

Depending on Ao, this half plane is divided vertically into three regions - diffusion, intermediate and hopping. For the first two regimes one can simplify equation (4.3.21) putting there G = 0, thus getting... 

In the last years supercritical fluid (SCF) technology has occupied a significant place in the high pressure chemical engineering. Due to their specific properties as liquid-like densities, gas-like viscosities and diffusivities intermediate between gas and liquid values, SCF have large potential in extraction and separation processes, polymer science and technology and in elaboration of new materials [1-4], In these last cases, to control the size of particles, we have to deal with the kinetics of their formation. ... 

The natural product bleomycin mediates strand scission in the presence of iron and oxygen by a related mechanism (6, 12, 14) however, no diffusible intermediate is generated. Electrochemical and related studies showed that the mechanism of iron bleomycin (Fe-BLM) involves first reduction of Fe(III)-BLM, reaction of Fe(II)-BLM with 02, and reduction of the Fe(II)-BLM-02 adduct to activated Fe-BLM (15). 

When Pt2(pop)44- is irradiated at 450 nm in the presence of DNA, strand scission occurs, and scavenger studies show that no diffusible intermediates are involved (22). Thus, scission must involve abstraction of hydrogen atoms from the sugar functionalities by the metal complex itself. Because Pt2(pop)44- is an anion, binding of the complex to DNA does not occur. This presents the possibility that Pt2(pop)44- may be a particularly sensitive probe of solvent accessibility and hence nucleic acid structure. 

This reaction is easily shown to follow the polystep reaction mechanism via diffusing intermediates. Actually the reaction path is already contained... 

The most appropriate model for mass transfer through air-filled pores can be determined by reference to the Knudsen number, Kn, which is the ratio of the mean free path of the water molecule and the pore diameter [Eq. (8)]. A Knudsen number of 10 or greater is indicative of Knudsen diffusion, whereas a value of 0.01 or less is indicative of Fickian diffusion. Intermediate... 

A very clever three-phase test for the detection of metaphosphate intermediates in phosphoryl transfer reactions has been described by Rebek and coworkers (44). The basis of this test is the use of two polymers suspended in solution. The donor polymer contains a potential precursor to metaphosphate anion, e.g., an acyl phosphate or a phosphoramidate, and the recipient polymer contains an acceptor nucleophile, e.g., an amine. After reaction and physical separation of the polymers, the recipient polymer is analyzed for covalently bound phosphate. Since very few of the phosphoryl groups to be transferred will be on the surface of the donor polymer, detection of significant transfer to the recipient polymer provides evidence for a diffusible intermediate, i.e., free metaphosphate anion. Significant transfer did occur in dioxane or acetonitrile suspensions of the polymers, thereby providing evidence for an intermediate. However, this test for diffusible and, therefore, relatively stable metaphosphate anion is compromised by the choice of solvent. Both dioxane and acetonitrile can provide unshared electron pairs for the highly electrophilic metaphosphate anion such that the actual species that migrates from the donor polymer to the recipient polymer may be a complex between metaphosphate anion and the solvent. Such a role for solvent has been investigated stereochemically, the results of which will be described later in this section. 

Cineol, fenchol (the alcohol corresponding to fenchone), y-terpinene and bornyl pyrophosphate, for instance, are built from geranyl pyrophosphate and/or neryl pyrophosphate by polyfunctional enzyme proteins, the so-called carbo-cyclases. No free diffusable intermediates occur. 

F. W. Brinkley and C. B. Mills, One-Dimensional Diffusion Intermediate Reactor Code (Fire Code), Trans. Am. Nuc. Soc. 2, 234 (June 1959). 

Manoj KM. Chlorinations catalyzed by chloroperoxidase occur via diffusible intermediate(s) and the reaction components play multiple roles in the overall process. Biochim. Biophys. Acta 2006 1764 1325-1339. 

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