Substrate channeling

Substrate channeling is a process by which two or more sequential enzymes in a pathway interact to transfer a metabolite (or intermediate) from one enzyme to another without allowing free diffusion of the metabolite into bulk solvent. (Ovadi, 1991 Srere, 1987 Anderson, 1999). The substrate tunneling is one of fundamental process of regulating enzymatic processes in cells. Glycolysis, biosynthesis of nucleic acids, aminoacids, and fatty acids are found to be among these processes. 

Techniques for demonstrating channeling behavior include enzyme buffering, ligand exchange kinetics, isotope dilution, and estimation of the transition time. One of the most 

More direct approach to the problem is based on measuring rapid presteady state kinetics with the use rapid chemical quench and stop-flow techniques (Johnson, 1995 Fierke and Hammes, 1995). These techniques allow monitoring individual rates of binding, conversion and dissociation of substrate. The most effective variant of such an approach is based on using a single turn over kinetics in which enzyme is taken in excess over radiolabeled substrate. 

Relationships between the energy and entropy activation of enzymatic 

Materials on the activation parameters of enzymatic processes have been analyzed in review articles (Likhtenshtein, 1966 1976a, 1979a,1988a Lumry and Rajender, 1970 Lumry ans Biltonen, 1969 Lumry and Gregory, 1995). Cases were indicated, where for the same enzymes the change of the activation energy and entropy of the process caused by variation of chemical structure of substrates and other conditions, mentioned above, take place in parallel. The following linear dependences are approximately satisfied  

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Amaro, R. Luthey-Schulten, Z., Molecular dynamics simulations of substrate channeling through an alpha-beta barrel protein, Chem. Phys. 2004, 307, 147-155... 

It should be noted that rather than exploiting the proactive aspects of a surface, it is equally valid to mask or negate wall effects by fluidically isolating the sample from the substrate channel wall to eliminate surface effects. This is carried out by using either a co-axial flow to keep the sample in the centre, i.e. away from the walls (Takagi et al. 2004) or in a similar way using multiple flow streams to surround the sample stream to form a sheath (Munson et al. 2004,2005). 

Those believing they have obtained evidence for direct nondissociative substrate channeling are strongly encouraged to consult the illuminating report of Wu et aV,... 

Johnson and Fierke Hammes have presented detailed accounts of how rapid reaction techniques allow one to analyze enzymic catalysis in terms of pre-steady-state events, single-turnover kinetics, substrate channeling, internal equilibria, and kinetic partitioning. See Chemical Kinetics Stopped-Flow Techniques... 

SUBSTRATE-ASSISTED CATALYSIS SUBSTRATE CHANNELING Substrate contaminated with the enzyme, BASAL RATE Substrate cycling,... 

S Additional information <5> (<5> ATP undergoes substrate channelling between enzyme and myosin ATPase [65]) [65]... 

Gregor, M. Janovska, A. Stefl, B. Zurmanova, J. Mejsnar, J. Substrate channelling in a creatine kinase system of rat skeletal muscle under various pH conditions. Exp. Physiol., 88, 1-6 (2003)... 

In Substrate Channeling, Intermediates Never Leave the Enzyme Surface... 

The long lipoyllysine arm swings from the active site of E, to E2 to E3, tethering the intermediates to the enzyme complex to allow substrate channeling. 

Substrate Channeling through Multienzyme Complexes May Occur in the Citric Acid Cycle... 

Review of the roles of swinging arms containing lipoate, biotin, and pantothenate in substrate channeling through multienzyme complexes. 

The last three steps of this four-step sequence are catalyzed by either of two sets of enzymes, with the enzymes employed depending on the length of the fatty acyl chain. For fatty acyl chains of 12 or more carbons, the reactions are catalyzed by a multienzyme complex associated with the inner mitochondrial membrane, the trifunctional protein (TFP). TFP is a heterooctamer of 4/34 subunits. Each a subunit contains two activities, the enoyl-CoA hydratase and the /3-hydroxyacyl-CoA dehydrogenase the /3 subunits contain the thiolase activity. This tight association of three enzymes may allow efficient substrate channeling from one active site to the... 

Substrate channeling The control over which biosynthetic route an intermediate should follow, by directing it to a specified enzyme rather than allowing competition from other enzymes in solution. 

Sulfation is expensive in energy terms for the cell, since two molecules of ATP are necessary for the synthesis of one molecule of 3 -phosphoadenosine 5 -phosphosulfate (PAPS). Both enzymes involved in the synthesis of PAPS, ATP sulfurylase, and APS kinase, reside within a single bifunctional cytosolic protein of approximately 56 kDa, where substrate channeling of APS from ATP sulfurylase to APS kinase occurs. Several group VI anions other than sulfate can also serve as substrates, although the resultant anhydrides are unstable. Because this instability would lead to the overall consumption of ATP, these other anions can exert a toxic effect by depleting the cell of ATP. 

Multidomain proteins tend to occur more frequently in eukaryotes than in prokaryotes. Often the eukaryotic counterpart to a set of individual prokaryotic enzymes that catalyze successive reactions is a single, multidomain protein. The theoretical advantages proposed for such an arrangement include (1) a geometry for the direct transfer of substrates from one active site to another, in a process known as substrate channeling, in order to increase the overall flux of the pathway, (2) the protection of intermediates that may be unstable in aqueous environments or may be acted on inappropriately by other enzymes, (3) the facilitation of interactions between domains for purposes of allosteric regulatory functions, and (4) the establishment of a fixed stoichiometric ratio of the... 

Anderson, K.S. (1999) Fundamental mechanisms of substrate channeling, in Schramm, V. L. and Purich, D. L. (eds.), Methods in Enzymology 308, Enzyme kinetics and Mechanism, Part E, Academic Press, San Diego, pp. 111-145. 

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