Naloxone
(Converted Wikipedia SMILES)
Naloxone is a full antagonist. It binds to the mu opiod receptor with these common binding spots (gold) to morphinan agonists. However, with this longer alkyl chain off of the in morphinan agonists nitrogen (as shown in orange), the same conformation change in the mu opioid receptor presumably does not take place, making this molecule an antagonist. It should be noted that naloxone has a very high affinity for the mu opioid receptor, making it a suitable heroin overdose antidote.
Naloxone in wireframe with the receptor binding similarities to morphinan agonists in gold, with the region preventing the full conformational change of the mu opioid receptor in orange.
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Some Additional References 1. Susanth. (2015). Opiod Structure Activity Relationship. Slideshare.net. Retrieved 17 March 2019, from https://www.slideshare.net/sujisusanth/opiod-sar-48199676
2. Buprenorphine Education: Technical explanation of Buprenorphine mu receptor, affinity of agonist and antagonist. (2019). Naabt.org. Retrieved 17 March 2019, from https://www.naabt.org/education/techntechnical_explanation_buprenorphine.cfm

Buprenorphine (based on converted Wikipedia SMILES File)
Buprenorphine is a partial agonist, meaning at low concentrations it does cause conformational change, but reaches its maximum effect at much lower concentrations. This partial activation of the mu opioid receptor is what makes buprenorphine a valuable drug in the treatment of opioid addiction. The part of buprenorphine that is similar to the morphinan opiates is shown in gold. The part that probably inhibits full activationis shown orange.
This wireframe model shows the same coding as the spacefilled model does above; gold similar structure to morphinan agonists; orange, an antagonist-like fulcrum.
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Methadone (based on Converted Wikipedia SMILES File)
Methadone is an agonist meaning it causes conformational change in the mu opioid receptor thus initiating various opiate signaling effects. Presumably, these regions (gold) are the locations that interact directly with the mu opioid receptor. Due to the symmetry of the methadone molecule, a large proportion of the molecule presumably can interact.
A wire-framed version of the above, showing in gold, the presumed regions of the methadone molecule that interact with the mu opioid receptor.
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