How exactly do molecules 'fit' into receptors?

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Offline Super Hans

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I regularly listen to AskTNS and I get slightly confused whenever the scientists talk about molecules fitting into a receptor.
So for example there was an episode that said something along the lines of some molecule in Propranolol 'fits' 
inside a receptor in the heart that otherwise adrenaline (or something) would usually fill except the Propranolol
doesn't allow salt through which as a result lowers the heart rate.

OK so, what do we mean when we say these chemicals 'fit' into the receptor, or are these terms just like an analogy for
a chemical attraction between molecules, or is it literally, the molecule fits perfectly into a receptor.

If the latter is the case, I just imagine the following: A washing machine is the body,  you put a bunch of padlocks and
a load of keys in the washing machine, if you turn it on with all the locks and the keys swishing around,  even though the
keys (molecules) fit in all the locks (receptor) and they're going around the washing machine (the body), they wouldn't
just fall into the right shape lock, they still need to be pushed in by intervention.

So how do these molecules just happen to 'fall'  into the right receptor, even if they do the keys and locks in the washer,
unless they're forced in, how do they actually get in there.

Apologise for the simple language, analogies and metaphors there but if I can visualise something it helps me understand.


PS. I'm on mobile so I'm sorry if the layout of this looks odd, the box isn't rendering all that we'll for me.

« Last Edit: 13/11/2014 02:48:55 by Super Hans »


Offline CliffordK

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Re: How exactly do molecules 'fit' into receptors?
« Reply #1 on: 13/11/2014 08:18:38 »
With a padlock, you have to force the key in past several tumblers, and then it must close around the exact tumbler combination.  Get the wrong key in the padlock, and it wouldn't necessarily fall out or be spit out. 

I would think biological receptors are more like a game with circles, squares, hexagons, and etc.  If a circle falls into a circle receptor, it gets stuck there for a while.  If the circle falls into a hexagon receptor, it just bounces out, and with some luck, it may encounter a circle receptor. 

Most of the receptors are HUGE protein molecules, relatively speaking.

Of course, there are a lot of cells in the human body.  But, consider ingesting a mole of some drug.  A mole of anything is A LOT, 6 x 1023