Linear Relationships between the Ligand Binding Energy and the Activation Energy of Time-dependent Inhibition of Steroid 5α-Reductase by Δ1-4-Azasteroids

Abstract

The inhibition of steroid 5␣-reductase (5AR) by ⌬ 1 -4-azasteroids is characterized by a two-step time-dependent kinetic mechanism where inhibitor combines with enzyme in a fast equilibrium, defined by the inhibition constant K i , to form an initial reversible enzyme-inhibitor complex, which subsequently undergoes a time-dependent chemical rearrangement, defined by the rate constant k 3 , leading to the formation of an apparently irreversible, tight-binding enzyme-inhibitor complex (Tian, G., Mook, R. A., Jr., Moss, M. L., and Frye, S. V. (1995) Biochemistry 34, 13453-13459). A detailed kinetic analysis of this process with a series of ⌬ 1 -4-azasteroids having different C-17 substituents was performed to understand the relationships between the rate of time-dependent inhibition and the affinity of the time-dependent inhibitors for the enzyme. A linear correlation was observed between ln(1/K i ), which is proportional to the ligand binding energy for the formation of the enzymeinhibitor complex, and ln(1/(k 3 /K i )), which is proportional to the activation energy for the inhibition reaction under the second order reaction condition, which leads to the formation of the irreversible, tight-binding enzyme-inhibitor complex. The coefficient of the correlation was ؊0.88 ؎ 0.07 for type 1 5AR and ؊1.0 ؎ 0.2 for type 2 5AR. In comparison, there was no obvious correlation between ln(1/K i ) and ln(1/k 3 ), which is proportional to the activation energy of the second, time-dependent step of the inhibition reaction. These data are consistent with a model where ligand binding energies provided at C-17 of ⌬ 1 -4-azasteroids is fully expressed to lower the activation energy of k 3 /K i with little perturbation of the energy barrier of the second, time-dependent step. 5AR1 catalyzes the NADPH-dependent reductive conversion of testosterone to dihydrotestosterone. Two isozymes of 5AR, designated types 1 and 2, have been described (1, 2). Although 5AR1 is predominantly expressed in skin and liver, 5AR2 is mainly expressed in prostate, seminal vesicles, liver, and epididymis (3). Both 5AR1 and 5AR2 are implicated in benign prostatic hyperplasia (2), a condition affecting the majority of men over age of 60 (4). Intense efforts made over the past decade to develop drugs against the activity of this enzyme has led to the discovery of potent, time-dependent ⌬ 1 -4-azasteroidal inhibitors of 5AR, including finasteride and GG745 (see Fig. 1).Finasteride inhibits both 5AR1 and 5AR2 in a time-dependent manner (5-7). The kinetic mechanism of this time-dependent inhibition is characterized by a fast binding step for the formation of an initial enzyme-inhibitor complex (EI), followed by a time-dependent event leading to the formation of an apparently irreversible enzyme-inhibitor complex (EI*). This timedependent event involves a chemical transformation at the ⌬ 1 double bond (8). The finding of a NADP-dihydrofinasteride adduct as a product of this time-dependent inhibition reaction suggests that the chemical event is a combination of a n...

Quantitative analysis of the effect of tubulin isotype expression on sensitivity of cancer cell lines to a set of novel colchicine derivatives

“…The studies of Tian and Haffner show a linear relation between cytotoxicity-derived values of logIC 50 and the binding free energy [44],…”

“…Although it is difficult to directly measure the binding free energy for the entire cell line, the information regarding the interaction between tubulin isotypes and colchicine derivatives can still be described by the cytotoxicity measurement of the values of log IC 50 . The studies of Tian and Haffner show a linear relation between cytotoxicity-derived values of logIC 50 and the binding free energy [ 44 ], …”

Synthesis and Molecular Docking of 2,4,5-Trisubstituted-1,3-Thiazole Derivatives as Antibacterial Agents

“…Furthermore, based on molecular docking prediction in Table 2 below, T3 exhibited the best performance against the target enzyme as supported by its lowest binding energy (-7.09 kcal mol -1 ) as compared to other ligands and as reported by other studies that state low binding energy is required for inhibition of enzyme [14,35]. More negative binding energy is proportional to the activation energy for inhibition reaction which leads to tight binding of enzyme-inhibitor complex [16]. Meanwhile, T1 and T2 showed higher binding energy compared to T3 which coincide with the antibacterial screening result.…”

“…Therefore there are a large number of studies on molecular docking for antibacterial agents focusing on GlcN-6-P synthase as their target enzyme [14,15]. The potential inhibitor will form interaction with the active site of the enzyme and form a ligand-enzyme complex which is responsible in inhibition mechanism [16]. Thus, molecular docking studies are widely required in this field as the process will provide information on the interaction energy between ligand and target site of the enzyme.…”

Targeting the Epidermal Growth Factor Receptor: Exploring the Potential of Novel Inhibitor N-(3-Ethynylphenyl)-6, 7-bis (2-methoxyethoxy) Quinolin- 4-Amine Using Docking and Molecular Dynamics Simulation

“…Compound 943, a potent inhibitor of EGFR, has the absolute free energy of binding G -6.24756 kcal/mol and binding energy of -6.17 kcal/mol. An inverse relation exists between gibbs energy and binding energy for the formation of protein-inhibitor complex similar to the Ki (inhibitor constant) and G values [45,46]. The IC 50 value of the compound 943 is calculated to be 30nM with a high absorption and distribution rate ( Table 2).…”