Substrate binding constants

Receptor—substrate-binding constants are typically between 10 and 10, at equimolar ratios, implying that when a receptor—substrate... 

More recently, Kaiser and coworkers reported enantiomeric specificity in the reaction of cyclohexaamylose with 3-carboxy-2,2,5,5-tetramethyl-pyrrolidin-l-oxy m-nitrophenyl ester (1), a spin label useful for identifying enzyme-substrate interactions (Flohr et al., 1971). In this case, the catalytic mechanism is identical to the scheme derived for the reactions of the cycloamyloses with phenyl acetates. In fact, the covalent intermediate, an acyl-cyclohexaamylose, was isolated. Maximal rate constants for appearance of m-nitrophenol at pH 8.62 (fc2), rate constants for hydrolysis of the covalent intermediate (fc3), and substrate binding constants (Kd) for the two enantiomers are presented in Table VIII. Significantly, specificity appears in the rates of acylation (fc2) rather than in either the strength of binding or the rate of deacylation. 

Vesicle size was found to affect reaction kinetics for the alkaline hydrolysis and thiolysis of p-nitrophenyl octanoate, with small vesicles being more effective as catalysts, and it was concluded that this size dependence itself was brought about by differences in ion dissociation, substrate binding constants, and intrinsic reactiv-... 

It was found that the catalytic reactivity and the substrate binding profile depend upon the size and the generation of the dendrons. Since the reactions obey Michaelis-Menten kinetics, this family of catalysts was given the name dendrizyme , alluding to enzymes. The dendrizyme/substrate binding constant... 

Comparison of the bis(4-nitrophenyl) phosphate hydrolysis reactivity of 1 and 2 (Fig. 27) in DMSO buffered water (1 1) at 50 °C revealed that the complex having the greater Zn-Zn separation is more reactive (1 k = (1.6 + 0.1) x 10-4M-1 s-1 2 (4.6 + 0.1) x 10 4 M 1 s 1).154 pH-rate studies are consistent with the involvement of a Zn-OH moiety in these hydrolytic reactions. Saturation-type behavior is observed in terms of substrate concentration for both reactions. Substrate binding constants were determined (1 k= 19.6 + 2.1 M-1 2 17.9+ 1.3M-1) and found to be nearly identical. Notably, kCSLt values for 1 ((4.9 + 0.4) x 10 6s 1) and 2 ((2.3 + 0.1) x 10-5s-1) differ by a factor of approximately 5. Thus, the enhanced hydrolytic efficiency of 2 for the hydrolysis of bis(4-nitrophenyl) phosphate is not due to differing substrate affinities for the complexes, but instead to intrinsically higher reactivity for the system having the larger Zn-Zn separation. 

The Michaelis-Menten equation developed in 1913 ushered in the era of enzyme kinetics and mechanism (chapter 2). Experimentally, its application involves graphing rates (velocities) of reaction (v) against trial concentrations of substrate ([S]). A "saturation" curve is usually observed in which there is a leveling off of v, so as to approach the maximum rate (V a ) as [S] reaches saturation concentration. In practice, it is difficult to accurately determine the onset of saturation and this led to considerable uncertainty in the values of Vmax as well as the enzyme-substrate binding constants (K in chapter 2). 

Let us carry over the binding sequence in a dimeric protein in Fig. 16 to a dimeric enzyme operating under the rapid equilibrium conditions, in order to express the binding sequences in terms of all three types of constants the substrate binding constant, Ks, the transformation constant, Kt> and the interaction constants, ICaa, and iCgB (Eq. (13.79)). 

Catalysis of 6-nitrobenzisoxazole-3-carboxylate (NBOC) decarboxylation by N,N dimethyl dialkylam-monium bromide vesicles is modulated by the bilayer fluidity [40], Catalytic efficiency increases with temperature, but at the phase transition there is a sharp increase in the catalysis. Several parameters, such as substrate binding constants, extent of ion dissociation and reactivity of NBOC, may change simultaneously at the Tc, com-... 

Table 1. Substrate Binding Constants for IPP in the Presence of Different APP-lnitiating Substrates and the Different APPs ...

Inhibition of alkaline hydrolysis of acetylcholine (60 C, pH = 10) caused by complexation of the positively charged fragment of acetylchoUne with caUx[4] resorcinol tetraanion was shown [87]. The tetradentate complexes of this ca-lix[4]iesorcinarene with copper (H) and zinc (H) ions exerted similar reactivity as original ligand [88]. The substrate binding constant depended on the structure and UpophiUcity of caIix[4]resorcinarene and was equal to (4-36) X 10 mol/L. 

The group of Hansch performed a complete QSAR-study of various kinetic constants, enzyme-substrate binding constants and inhibition constants for practically all R>j NHCHR2C0R compounds for which such data are available. No QSAR directly related to the bimolecular, low substrate concentration rate constants was published by this group. They correlated large series of compounds, N=33-71 and even... 

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