From all the studied compounds, compounds 1 and 3 were found to be the most effective ones against all the studied cell lines with a GI50 of 2 nM for the HeLa cell line. These compounds have identical scaffolds and only differ in the group that is attached to position 2 of the psoralen. Compound 3 has a carboxylic group in this position and it interacts very closely with iron through this group. In such case a strong chelation with the metal cannot be excluded. Compound 1 has an ester group in this position and it binds very similarly to what was observed for the potent xanthotoxin inhibitor that shows a close interaction between the oxygen of the furan ring with the iron from the heme (2.83Å).
Compounds 4 and 5 can also interact with the iron of the heme with one oxygen atom, but due to the steric effect of the bulkier groups attached to position 2 of the psoralen, it is the oxygen from the lactone ring that interacts directly with this center. This interaction is however weaker when compared to those observed in compounds 1 and 3 due to the proximity of the ketone group that steals electron density from this atom. This effect is perhaps the main cause for the lower biological activity of these compounds against all cell lines. From these results we can conclude that due to the small size of the active site, the presence of bulkier groups at position 2 of the coumarin ring tend to decrease the biological activity of the compounds, due to the improper alignment of the psoralen in the active site.
Compound 6 is the only one that does not contain any group attached to the position 2 of the psoralen ring and does not have an oxygen atom pointing towards the iron. Even so, this compound shows good results in two cell lines: HeLa and TCC-SUP.
Comparing the binding pose of this compound with natural substrate (coumarin) present in the bound X-ray structure 1Z10, we can conclude that they are very similar. This means that in such position compound 6 is in an optimal position to be hydroxylated at position 9 or 11. We believe therefore that this compound can react in the active site and the product of this reaction might not inhibit only CYP2A6, but rather inhibit another enzyme and this might be the reason of the good results that are observed in some cell lines.
The results obtained in this study provide good indicators about the reactivity of the psoralenes in the CYP2A6. It has been shown that the interaction between the oxygen from the furan ring and the iron from the heme gives rises to the best biological activity. The residue Asn297 is important to orient and align the substrate in the active site but is not vital to induce enzyme inhibition. The presence of bulky substituents in the coumarin ring should be avoided since they tend to change the orientation of the psoralen in the active site which has a negative impact on their biological activity.
From all the studied compounds, compounds 1 and 3 were found to be the most effective ones against all the studied cell lines with a GI50 of 2 nM for the HeLa cell line. These compounds have identical scaffolds and only differ in the group that is attached to position 2 of the psoralen. Compound 3 has a carboxylic group in this position and it interacts very closely with iron through this group. In such case a strong chelation with the metal cannot be excluded. Compound 1 has an ester group in this position and it binds very similarly to what was observed for the potent xanthotoxin inhibitor that shows a close interaction between the oxygen of the furan ring with the iron from the heme (2.83Å).
Compounds 4 and 5 can also interact with the iron of the heme with one oxygen atom, but due to the steric effect of the bulkier groups attached to position 2 of the psoralen, it is the oxygen from the lactone ring that interacts directly with this center. This interaction is however weaker when compared to those observed in compounds 1 and 3 due to the proximity of the ketone group that steals electron density from this atom. This effect is perhaps the main cause for the lower biological activity of these compounds against all cell lines. From these results we can conclude that due to the small size of the active site, the presence of bulkier groups at position 2 of the coumarin ring tend to decrease the biological activity of the compounds, due to the improper alignment of the psoralen in the active site.
Compound 6 is the only one that does not contain any group attached to the position 2 of the psoralen ring and does not have an oxygen atom pointing towards the iron. Even so, this compound shows good results in two cell lines: HeLa and TCC-SUP.
Comparing the binding pose of this compound with natural substrate (coumarin) present in the bound X-ray structure 1Z10, we can conclude that they are very similar. This means that in such position compound 6 is in an optimal position to be hydroxylated at position 9 or 11. We believe therefore that this compound can react in the active site and the product of this reaction might not inhibit only CYP2A6, but rather inhibit another enzyme and this might be the reason of the good results that are observed in some cell lines.
The results obtained in this study provide good indicators about the reactivity of the psoralenes in the CYP2A6. It has been shown that the interaction between the oxygen from the furan ring and the iron from the heme gives rises to the best biological activity. The residue Asn297 is important to orient and align the substrate in the active site but is not vital to induce enzyme inhibition. The presence of bulky substituents in the coumarin ring should be avoided since they tend to change the orientation of the psoralen in the active site which has a negative impact on their biological activity.
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