Enzyme association equilibrium constants

Table X. Comparison of Association Equilibrium Constants for Inactive and Active Enzymes (46)...

This association/dissociation is assumed to be a rapid equilibrium, and is the enzyme substrate dissociation constant. At equilibrium,... 

All enzymatic reactions are initiated by formation of a binary encounter complex between the enzyme and its substrate molecule (or one of its substrate molecules in the case of multiple substrate reactions see Section 2.6 below). Formation of this encounter complex is almost always driven by noncovalent interactions between the enzyme active site and the substrate. Hence the reaction represents a reversible equilibrium that can be described by a pseudo-first-order association rate constant (kon) and a first-order dissociation rate constant (kM) (see Appendix 1 for a refresher on biochemical reaction kinetics) ... 

For a binding reaction we can pick whether we show the reaction as favorable or unfavorable by picking the substrate concentration we use. Association constants have concentration units (M-1)- The equilibrium position of the reaction (how much ES is present) depends on what concentration we pick for the substrate. At a concentration of the substrate that is much less than the dissociation constant for the interaction, most of the enzyme will not have substrate bound, the ratio[ES]/[E] will be small, and the apparent equilibrium constant will also be small. This all means that at a substrate concentration much less than the dissociation constant, the binding of substrate is unfavorable. At substrate concentrations higher than the dissociation constant, most of the enzyme will have substrate bound and the reaction will be shown as favorable (downhill). (See also the discussion of saturation behavior in Chap. 8.)... 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

Kx is the equilibrium constant quantifying enzyme substrate association... 

A number of recent studies have shown that under certain conditions, FABMS indeed can very accurately measure the balance of ionic species in ongoing chemical reactions in solutions. These studies include the determination of acid dissociation constants (2), equilibrium constants for enzyme catalyzed reactions (1), metal-ligand association constants 03), and measurements of... 

An antenna remains in a plume 1 s and an antenna is not an isolated system, as is required to reach equilibrium. The kinetic properties of the PBP-ligand complexes may be more important to the function of PBPs as potential filters than the equilibrium dissociation constants. Thus, ligands with very fast association rate constants and very slow dissociation rate constants are more likely to be bound at the pore surfaces and to traverse the sensillar lymph unharmed by the powerful pheromone-degrading enzymes in the lymph (see below). Thus, in order to understand the function of PBPs, it is essential to obtain more data on binding kinetics. 

What is the equilibrium constant for the association of reactant A to the enzyme for the kinetic parameters used in Figure 3.4 How close is the reaction A + E C to equilibrium during the simulation that is illustrated How does the quasi-steady approximation depend on the equilibrium constant for enzyme binding ... 

Significant differences in the equilibrium constants for carbon monoxide binding to cytochromes P450 from bacterial, liver microsomal, and adrenal cortex microsomal sources, different isozymes of the liver microsomal proteins, and for substrate-free and substrate-bound enzymes, have been observed and have been related to similar factors that affect O2 and CO binding in oxygen transport and storage heme proteins. The importance of the cis and tmns effects, that is electronic effects associated with the porphyrin... 

Quantitative structure-activity relationships (QSAR). These involve any type qf property associated with biological activities. The data range from rate and equilibrium constants for pure enzymes to toxicities in large multicellular organisms. 

The reservoirs affect the concentration of the cycle species in two ways. The first is through the direct influx represented by the first term in each of eqs. (10.2). The second and more interesting way is through control of the enzyme activities, where the reservoir species F and T are allowed to become effectors of the enzymes. The type of control modeled is noncompetitive allosteric binding of the effectors, where each effector binds to the enzyme independently, as shown in fig. 10.2. In this scheme, the enzyme with effector bound is assumed to have altered catalytic activity toward its substrate compared to that of the enzyme without effector bound. The scheme as shown also relies on the simplifying assumptions that (1) the association and dissociation between enzyme and substrate are unaffected by the binding of the effector, and (2) the binding of substrate to enzyme is much faster than the conversion of bound substrate to product. Under these assumptions, the Michaelis constant Km represents the equilibrium constant for... 

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