Proline is a kinky amino acid. Kinky in the sense that it is only one of the twenty with a fixed configuration of its alpha carbon because of the ring (which may be why there is more of it in organisms living at high temperature) and kinky in the sense that when present in alpha helices it produces a kink. The previous post shows how it is used to schlep the body weight’s worth of ATP we make each day out of our mitochondria — https://luysii.wordpress.com/2019/01/30/3939/.
Well here it is in one of the marijuana receptors (CB1). Binding of delta9 THC in the 7 transmembrane alpha helix bundles of the G Protein Coupled Receptor (GPCR) causes an alteration in the kink allowing transmembrane helix 6 (TM6) to move outward toward the cytoplasm, creating a cavity on the intracellular side, where the G protein trimer can bind.
You can read much more about this in an exquisite paper [ Cell vol. 176 pp. 448 – 458 `19 ] describing the CB1 receptor bound to a synthetic ligand 20 times more potent that delta-9 tetrahydrocannabinol (delta9 THC). It is a cryoEM study which used 177,000 projections to come up with a 3 Angstrom resolution structure of CB1 bound to MBDB-FUBINACA in complex with its G protein trimer. They had to use a single chain variable fragment (scFv6) along with a positive allosteric modulator (PAM) called ZCZ-011 to stabilize the complex.
MBDB-FUBINACA is a story in itself. It is presently the fentanyl of synthetic cannabinoids, which “has been linked to thousands of hospitalizations and numerous fatalities” [ New England Journal of Medicine vol. 376 pp. 235 – 242 ’17 ]. I’m surprised I’ve never heard of it — have you? But then I’ve been retired from clinical practice for some time. Perhaps the mainstream press, pushing marihuana legalization as it has been, kept it quiet, or more likely there have been no further episodes of mass intoxication from the AMB-FUBINACA (aka the zombie drug) since 2017.
I’ve never knowingly used marihuana. Frankly it scares me — for why please see — https://luysii.wordpress.com/2014/05/13/why-marihuana-scares-me/.
There are 4 molecular switches buried in GPCRs [ Current Med. Chem. vol. 19 pp. 1090 – 1109 ’12 ]
1. The ionic lock switch between the D/E R Y sequence at the cytoplasmic end of TM3 and E286 at the cytoplasmic end of TM6 (single letter amino acid code used) –http://130.88.97.239/bioactivity/aacodefrm.html
2. TM3 – TM7 lock switch. In rhodopsin it is between the protonated Schiff base of lysine and a glutamic acid and it broken on light activation,.=
3. Toggle switch linked with the n P x x Y motif in TM7 (x stands for any amino acid) — much more about this later in the post.
4. Transmission switch — produced by agonist binding, the outward movement of TM6 to to ligand binding creating a hole fo the G protein to bind to the receptor on the cytoplasmic side.
So why did I call the Cell paper exquisite? Because of the molecular detail it provides about just how MDMB FUBINACA activates CB1. Here’s the structure of AB-FUBINACA — https://en.wikipedia.org/wiki/AB-FUBINACA. Both look like drugs designed by a committee. They both have a para-iodophenyl group, an amide, and a fused indole ring with an extra nitrogen (imidazole ring — I never could keep heterocyclic nomenclature straight). MDMB has a methyl ester (in place of the amide) and a tertiary butyl group (in place of the isoPropyl group).
I don’t have time to look up how Pfizer came up with it. The FUBINACAs do not resemble delta9 THC at all — https://en.wikipedia.org/wiki/Tetrahydrocannabinol.
The pictures in the paper show how the hydrophobic aromatic side chains of FIVE phenylalanines and 2 tryptophans create a nice oily space for delta9 THC and MBDB-FUBINACA to bind.
F200 (phenylAlanine 200) and W356 are the toggle twin switch which stabilize the inactive conformation of CB1. The rotation of F200 to interact with the imidazole of FUBINACA, allows W356 to rotate outward, changing the kink produced the the proline #358 in TM6 allowing the helix to straighten and rotate outward toward the cytoplasm, creating a cavity for the G protein to bind to.
Definitely a tour de force for the blind watchman.
Chemiotics II 