Affinity constants

Between 5,000 and 100,000 GR molecules are found within almost every cell of the organism. The affinity constant for cortisol comes to around 30 nM, which is in the range of the concentration of free hormone in the plasma under normal conditions. Consequently, receptor occupancy can be expected to be 10-70%. This suggests that changes in cortisol secretion such as under stress conditions, directly translate into alterations in GR occupancy, leading to transcriptional responses. 

Since the binding capacity (Bmax) is saturable and depending on the affinity constant (Kaff), the free plasma fraction (jp) increases with increasing total and thus also bound concentrations. 

The two substrate kinetics of the overall reaction catalyzed by the complex in permeabilized membranes showed classical ping-pong kinetics in accordance with a phosphorylated enzyme intermediate. The affinity constants for fructose and P-enolpyruvate were 8 and 25 /iM, respectively. 

One of the earliest approximations studied is to assume that the solute concentration is small or rapidly becomes small in comparison to the affinity constant for uptake. This then allows the nonlinear Michaelis-Menten equation to be approximated by ... 

The affinity constant of the Fab fragments we produced for these studies (discussed below) was approximately 3.0xl09 M"1. This means that the Fab fragments would have about 138 to 750 times greater affinity for PCP than for the receptor. 

The affinity and cross-reactivity of the whole serum and Fab fragments were determined using equilibrium dialysis for the affinity determination and RIA for the cross-reactivity studies. The average intrinsic affinity constant (Ko) of the antibody (Nisonoff and Pressman 1958) changed very little throughout the... 

Confusingly, all of these terms are in current use to express the position of the equilibrium between a ligand and its receptors. The choice arises because the ratio of the rate constants k and k can be expressed either way up. In this chapter, we take KA to be k, kA and it is then strictly a dissociation equilibrium constant, often abbreviated to either dissociation constant or equilibrium constant. The inverse ratio, k+l k x, gives the association equilibrium constant, which is usually referred to as the affinity constant. 

Here, y is the fractional binding, x is the partial pressure of 02, K is an affinity constant, and n is a number which in Hill s work varied from 1.5 to 3.2. 

Kl, K. Dissociation equilibrium constants for binding of L and I (reciprocals of affinity constants)... 

Carrier-mediated transport is linear with mucosal solute concentration until this concentration exceeds the number of available carriers. At this point the maximal solute flux (7max) is independent of further increases in mucosal solute concentration. In the linear range of solute flux versus mucosal concentration (C), the proportionality constant is the ratio of / to the solute-carrier affinity constant (Km). This description of Michaelis-Menten kinetics is directly analogous to time changes in mass per unit volume (velocity of concentration change) found in enzyme kinetics, while here the appropriate description is the time change in solute mass per unit surface area of membrane supporting the carrier. 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

Figure 17. Structure affinity relationship measurement using reflectometric interference spectroscopy of atrazine versus an antibody in water. A variety of derivatives of triazines are measured at a number of concentrations to obtain affinity constants.

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