Figure 1. Two-dimensional sketches of nicotine and its three important metabolites. Heteroatoms oxygen (O) and nitrogen (N) with valanced hydrogens are colored red and blue, respectively.

Figure 1. Two-dimensional sketches of nicotine and its three important metabolites. Heteroatoms oxygen (O) and nitrogen (N) with valanced hydrogens are colored red and blue, respectively.

Figure 2. Self-docking analyses of the native ligands, i.e., dihydrotestosterone (DHT), cortisol, and thyroxine, respectively, for sex-hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), and thyroxin-binding globulin (TBG). The ligand binding sites of the proteins are shown as a surface with light orange color. The docked pose of the native ligand is with a backbone colored in blue, whereas the original bound pose is with a backbone in white.

Figure 2. Self-docking analyses of the native ligands, i.e., dihydrotestosterone (DHT), cortisol, and thyroxine, respectively, for sex-hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), and thyroxin-binding globulin (TBG). The ligand binding sites of the proteins are shown as a surface with light orange color. The docked pose of the native ligand is with a backbone colored in blue, whereas the original bound pose is with a backbone in white.

Figure 3. Molecular docking of nicotine and its three important metabolites to the ligand-binding site of sex-hormone-binding globulin (SHBG). SHBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (dihydrotestosterone) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 3. Molecular docking of nicotine and its three important metabolites to the ligand-binding site of sex-hormone-binding globulin (SHBG). SHBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (dihydrotestosterone) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 4. Ligand–protein interaction plots of the native ligand (dihydrotestosterone; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with sex-hormone-binding globulin (SHBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Figure 4. Ligand–protein interaction plots of the native ligand (dihydrotestosterone; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with sex-hormone-binding globulin (SHBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Figure 5. Molecular docking of nicotine and its three important metabolites with the ligand-binding site of corticosteroid-binding globulin (CBG). CBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (cortisol) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 5. Molecular docking of nicotine and its three important metabolites with the ligand-binding site of corticosteroid-binding globulin (CBG). CBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (cortisol) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 6. Ligand–protein interaction plots of the native ligand (cortisol; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with corticosteroid-binding globulin (CBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Figure 6. Ligand–protein interaction plots of the native ligand (cortisol; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with corticosteroid-binding globulin (CBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Figure 7. Molecular docking of nicotine and its three important metabolites with the ligand-binding site of thyroxine-binding globulin (TBG). TBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (cortisol) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 7. Molecular docking of nicotine and its three important metabolites with the ligand-binding site of thyroxine-binding globulin (TBG). TBG is in surface representation colored light orange. The ligands are shown in stick representation together in the binding site for easy comparison of the binding poses. The backbones of the compounds are colored differently: native ligand (cortisol) in white, nicotine in magenta, cotinine in cyan, trans-3′-hydroxycotinine in green, and 5′-hydroxycotinine in yellow. The heteroatoms of the compounds oxygen (O) and nitrogen (N) are in blue and red colors, respectively.

Figure 8. Ligand–protein interaction plots of the native ligand (thyroxine; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with thyroxine-binding globulin (TBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Figure 8. Ligand–protein interaction plots of the native ligand (thyroxine; Panel A), nicotine (Panel B), cotinine (Panel C), trans-3′-hydroxycotinine (Panel D), and 5′-hydroxycotinine (Panel E) in complex with thyroxine-binding globulin (TBG). The ligands are placed at the center of each plot surrounded by the interacting residues. The ligands and the residues forming hydrogen bonds are shown in the ball and stick representation. The balls represent the atoms, and the sticks represent the bond between them. The color of the balls differentiates the atom types, i.e., the black balls represent carbon atoms, red balls oxygen atoms, and blue balls nitrogen atoms. The hydrogen bonds are shown as green dashed lines labeled with bond length (in Å). The residues forming nonbonded contacts are shown as arcs with bristles. The interacting residues for ligands which are common with the interacting residues for the native ligand are encircled.

Table 1. Self-docking analyses of the native ligands (dihydrotestosterone, cortisol, and thyroxine) for sex-hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), and thyroxin-binding globulin (TBG). For the root-mean-square deviation (RMSD) calculation, the heavy atoms of ligands (excluding hydrogen atoms) were considered.

Table 1. Self-docking analyses of the native ligands (dihydrotestosterone, cortisol, and thyroxine) for sex-hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), and thyroxin-binding globulin (TBG). For the root-mean-square deviation (RMSD) calculation, the heavy atoms of ligands (excluding hydrogen atoms) were considered.

Self-Docking of Native LigandRMSDTotalRMSDNetDihydrotestosterone with SHBG 0.000 (for all 21 atoms)0.000 (for all 21 atoms)Cortisol with CBG0.358 (for all 26 atoms)0.000 (for 23 atoms, excluding 3 atoms)Thyroxine with TBG1.482 (for all 24 atoms)0.095 (for 18 atoms, excluding 6 atoms)

Table 2. The binding strength scores for nicotine and its metabolites in complex with sex-hormone-binding globulin (SHBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Table 2. The binding strength scores for nicotine and its metabolites in complex with sex-hormone-binding globulin (SHBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Ligands Interacting with SHBGBinding Energy (Kcal/mol)pKdDock/Grid ScoreNicotine−6.945.09−29.70Cotinine−6.915.07−30.77Trans-3′-hydroxycotinine−6.925.08−33.145′-Hydroxycotinine−7.045.16−31.59

Table 3. The amino acid residues of sex-hormone-binding globulin (SHBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts and loss in accessible surface area (ΔASA) due to ligand binding. Amino acid residues with an asterisk are the most important residues showing the maximum number of nonbonded contacts and/or the maximum loss in ASA.

Table 3. The amino acid residues of sex-hormone-binding globulin (SHBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts and loss in accessible surface area (ΔASA) due to ligand binding. Amino acid residues with an asterisk are the most important residues showing the maximum number of nonbonded contacts and/or the maximum loss in ASA.

Ligands Interacting with SHBGInteracting ResiduesNonbonded ContactsΔASA (Å2)NicotineAsp-65410.88Trp-6665.36Phe-67 *1024.83Leu-8016.02Asn-82111.02Val-112111.82CotininePhe-5626.06Gly-5816.54Asp-65410.81Trp-6655.36Phe-67 *724.83Asn-82111.85Val-10519.58Trans-3′-hydroxycotinine Phe-5626.06Gly-5816.54Asp-65410.35Trp-6665.36Phe-67 *1124.83Leu-8026.1Asn-82111.33Val-105210.51Met-107121.4Val-112311.855′-HydroxycotininePhe-5626.06Asp-65110.29Trp-6655.36Phe-67 *1124.83Leu-8026.1Asn-82111.1Val-10519.5Met-107121.52Val-112111.86Met-139 *229.38

Table 4. The binding strength scores for nicotine and its metabolites in complex with corticosteroid-binding globulin (CBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Table 4. The binding strength scores for nicotine and its metabolites in complex with corticosteroid-binding globulin (CBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Ligands Interacting with CBGBinding Energy (Kcal/mol)pKdDock/Grid ScoreNicotine−6.945.09−25.54Cotinine−6.865.03−27.22Trans 3′-hydroxycotinine−6.885.05−28.265′-Hydroxycotinine−6.995.12−27.63

Table 5. The amino acid residues of corticosteroid-binding globulin (CBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts, loss in accessible surface area (ΔASA) due to ligand binding, and hydrogen bonding (H-bond) interaction (when applicable). Amino acid residues with an asterisk are the most important residues showing the maximum number of nonbonded contacts and/or the maximum loss in ASA or H-bond.

Table 5. The amino acid residues of corticosteroid-binding globulin (CBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts, loss in accessible surface area (ΔASA) due to ligand binding, and hydrogen bonding (H-bond) interaction (when applicable). Amino acid residues with an asterisk are the most important residues showing the maximum number of nonbonded contacts and/or the maximum loss in ASA or H-bond.

Ligands Interacting with CBGInteracting ResiduesNonbonded ContactsΔASA (Å2)NicotineVal-22110.09Thr-24014.97Phe-24217.83Ile-263225.78Asn-264321.43Phe-366316.65Trp-371 *343.56CotinineThr-24014.97Phe-24227.89Arg-26029.57Ile-263425.78Asn-264322.39Ser-267111.74Phe-366316.65Trp-371 *945.77Trans 3′-hydroxycotininePhe-24237.89Arg-260312.12Ile-263425.78Asn-264 * (H-bond)227.32Ser-267113.63Phe-366416.65Trp-371 *548.855′-HydroxycotinineIle-263125.78Asn-264 * (H-bond)231.07Phe-366516.65Trp-371 *1866.17

Table 6. The binding strength scores for nicotine and its metabolites in complex with thyroxine-binding globulin (TBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Table 6. The binding strength scores for nicotine and its metabolites in complex with thyroxine-binding globulin (TBG). The ‘Kd’ denotes the dissociation constant. The binding energy and pKd or −log (Kd) values were calculated using X-Score. Lower (more negative) binding energy or Dock/Grid score and higher dissociation constant denote better docking.

Ligands Interacting with TBGBinding Energy (Kcal/mol)pKdDock ScoreNicotine−6.945.09−26.72Cotinine−6.955.09−29.44Trans 3′-hydroxycotinine−6.855.02−30.345′-Hydroxycotinine−6.965.10−30.30

Table 7. The amino acid residues of thyroxine-binding globulin (TBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts, loss in accessible surface area (ΔASA) due to ligand binding, and hydrogen bonding (H-bond) interaction (when applicable). Amino acid residues with an asterisk are the most important residues as they showed the maximum number of nonbonded contacts and/or the maximum loss in ASA or H-bond.

Table 7. The amino acid residues of thyroxine-binding globulin (TBG) interacting with nicotine and its metabolites. Each residue is listed with the number of nonbonded contacts, loss in accessible surface area (ΔASA) due to ligand binding, and hydrogen bonding (H-bond) interaction (when applicable). Amino acid residues with an asterisk are the most important residues as they showed the maximum number of nonbonded contacts and/or the maximum loss in ASA or H-bond.

Ligands Interacting with TBGInteracting ResiduesNon-Bonded ContactsΔASA (Å2)NicotineLeu-269 *729.65Lys-270 * (H-bond)119.59Asn-273 *235.92Leu-376326.83CotinineSer-23 * (H-bond)27.47Leu-269324.78Lys-270431.27Leu-376124.01Arg-381 *1464.05Trans 3′-hydroxycotinineLeu-246117.83Leu-269129.57Asn-273 *440.32Leu-27619.53Leu-376227.17Glu-37710.62Arg-381 *544.895′-HydroxycotinineAsn-273 * (H-bond)541.78Leu-27619.37Leu-376227.17Arg-381 *646.26