J IRAN CHEM SOC
Table 2 Interaction results of molecular docking
Ligand
Contact residue
Hydrogen bond
Ser510
9a
Leu565, Ile572, Leu773, His518, Ile770, Val769, His513, Trp519, Gln514, Asp766, Phe576, Ser510,
Arg726, Gln716
9b
9c
Arg726, Gln716, his518, Gln514, Gly720, His513, Leu565, Ile572, Leu773, Asp766, Val769, Phe576,
Ile770
–
Leu565, His518, Gln716, His513, Ser510, Arg726, Ile770, Gln514, Phe576, Val769, Asp766, Leu773,
Ile572
His513
9d
9e
Leu565, Ile572, Leu773, Gln716, Ile770, Arg726, Ser510, Phe576, His518, Gln514, Trp519, His513
Gln514
Arg726, Gly720, His513, Phe576, Gln514, His518, Ile557, Leu565, leu773, Ile572, Gln716, Asp766,
Ile770, Thr728
His513, Asp766
9f
Ile572, Leu773, Leu565, Gln716, Ile770, Arg726, Ser510, Phe576, His518, His513, Gln514, Trp519
Gln514
4-MMPB Ile572, His518, Gln514, His513, Ser510, Arg726, Phe576, Val769, Gln716, Ile770, Leu773, Leu565
His518, His513, Ser510
Among the ligands 9a–f, compound 9e having the
4-phenylpiperazine substituent was the most potent inhibi-
tor of 15-LO at a concentration of 38 µM. The in vitro
acquired data for this class of heterocycles showed that
15-LO inhibitory activity can be manipulated by varying
the substitution of 5H-benzo[5,6][1,4]thiazino[3,2-e][1,2,4]
triazine at C-3. However, the standard ligand had the best
value of IC50 (18 µM).
two hydrogen bonds to the active site residues Asp766 and
His513. In contrast, the ligand 9b is unable to form hydro-
gen bond to the active site residues. Other designed ligands,
i.e. 9a, 9c, 9d and 9f, can produce just one hydrogen bond
to the active site residues. The standard ligand is capable
to constitute three hydrogen bonds to the active site resi-
dues His518, His513 and Ser510. As it is seen, the number
of hydrogen bonds has an important effect on the 15-LO
inhibitory activity of these ligands.
Molecular modelling and docking
Another point which could be considered is hydrophobic
interaction between 4-phenylpiperazine substitute in 9e and
hydrophobic amino acid residues Ile557, Leu565, Leu773
and Ile572. These interactions are absent in compound 9f
with 4-methylpiperazine substituent.
Generally from the docking outcomes and 15-LO inhibi-
tory activity assessment, this point could be understood that
with increasing the number of hydrogen bonds between
these ligands and active site residues, the connection of
ligand to the enzyme becomes stronger. It causes to rise of
Gibbs free energy of ligand binding to the 15-LO, and as a
result, the 15-LO inhibitory activity of ligand grows.
Docking studies were carried out to investigate the opti-
mal binding conformations of the compounds 9a–f and
4‑MMPB within the 15-LO active site. Docking was per-
formed by using of MOE software. For example, 3D repre-
sentation of docked ligand 9e into binding site of 15-LO is
The molecular docking process data are given in Table 1.
As it has been given in Table 1, all ∆Gb values for complex
formation between the ligand and the 15-LO were negative,
so a favourable complex may be formed between enzyme
and designed ligands. Compound 9e shows the best value
of ∆Gb (−37.08 kJ/mol), and compound 9b demonstrates
the smallest value of ∆Gb (−30.19 kcal/mol). These
results indicate that the ligand 9e can connect to the 15-LO
stronger than the other designed ligands. However, standard
ligand 4‑MMPB has the greatest value of ∆Gb (−43.61 kJ/
mol) when compared with the other six ligands. The cor-
relation between IC50 and obtained pKi (−log inhibition
constant) from docking was checked, and the correspond-
ing diagram is presented in Fig. 5. It was found that there is
a correlation between log IC50 and pKi although it was low
(R2 = 0.8263).
Conclusion
In the present study, a new group of 3-substituted-5H-benzo
[5,6][1,4]thiazino[3,2-e][1,2,4]triazines was synthesized.
The inhibitory activity of these compounds against the
15-lipoxygenase was estimated and compared with the
standard ligand 4‑MMPB. Our results show that com-
pound 9e, i.e. 3-(4-phenylpiperazino)-5H-benzo[5,6][1,4]
thiazino[3,2-e][1,2,4]triazine, has the best 15-LO inhibi-
tory activity (IC50 = 38 µM) between six designed ligands.
However, the 15-LO inhibitory activity of designed ligands
is smaller than the standard ligand (IC50 = 18 µM). The
docking studies indicated there are hydrogen bond inter-
actions between these ligands and active site residues of
15-LO. With increasing the number of hydrogen bonds, the
Display of residual interactions and hydrogen bonding
contacts between the ligands and the enzyme is shown in
The data of the interaction of residue contacts and
hydrogen bonding are given in Table 2. Ligand 9e can form
1 3