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Optically active drugs help please

"Many drugs work by a molecule of the active ingredient binding with a receptor or the active site of an enzyme. Because receptors have a 3D structure, only one of a pair of optical isomers will fit. In some cases, one optical isomer is an effective drug and the other is inactive"

If only one enantiomer can bind with the receptor, how do some drugs have both enantiomers taking effect? E.g. in thalidomide both the S-form which caused birth defects and the R from which was a safe sedative took effect? Or was it only the S-from that was active? I'm confused.
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Pigster
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There may be more than one type of receptor in the body? One may only accept the R form, another the S form. Some receptors are more forgiving of shape than others?
Original post by anactualmess
"Many drugs work by a molecule of the active ingredient binding with a receptor or the active site of an enzyme. Because receptors have a 3D structure, only one of a pair of optical isomers will fit. In some cases, one optical isomer is an effective drug and the other is inactive"

If only one enantiomer can bind with the receptor, how do some drugs have both enantiomers taking effect? E.g. in thalidomide both the S-form which caused birth defects and the R from which was a safe sedative took effect? Or was it only the S-from that was active? I'm confused.


In the case of thalidomide, enantiomers reacted in vivo producing a racemic mixture, so it did not matter if you were only taking the "safe" enantiomer, the teratogenic form was always produced.

https://en.wikipedia.org/wiki/Racemization
Original post by charco
In the case of thalidomide, enantiomers reacted in vivo producing a racemic mixture, so it did not matter if you were only taking the "safe" enantiomer, the teratogenic form was always produced.

https://en.wikipedia.org/wiki/Racemization

so both enantiomers were active? "Because receptors have a 3D structure, only one of a pair of optical isomers will fit" can you please help me clarify this statement, of course there is more than one type of receptor in the body so I understand the two different enantiomers could bind to two different receptors, but the way the textbook phrases it it makes it seem that there is only one target receptor and so only one enantiomer, the one which is complementary to that receptor, will be active? I know I am probably just interpreting this wrong, please help :smile:
Original post by anactualmess
so both enantiomers were active? "Because receptors have a 3D structure, only one of a pair of optical isomers will fit" can you please help me clarify this statement, of course there is more than one type of receptor in the body so I understand the two different enantiomers could bind to two different receptors, but the way the textbook phrases it it makes it seem that there is only one target receptor and so only one enantiomer, the one which is complementary to that receptor, will be active? I know I am probably just interpreting this wrong, please help :smile:

No.
You didn't read it carefully.

ONLY ONE of the enantiomers is teratogenic (caused deformity the unborn foetus), HOWEVER the other enantionmer undergoes racemization once inside the body, FORMING both enantiomers.

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