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Allosteric regulation in enzymes is where a molecule binds at a site other than the active site and thus changes the tertiary structure of the active site, hence altering the binding of substrate and formation of enzyme-substrate complexes.

As I understand it, if this is extended to receptors, for example G protein-coupled receptors, which effectively causes a conformation change which is transmitted across the membrane and 'activates' a G protein which is associated with the receptor. The Gα subunit swaps a GDP for a GTP which causes the dissociation of the alpha from the beta-gamma coupled subunits and from the receptor. The dissociated Gα and Gβγ subunits then take part in a cascade process interacting with other molecules.

However, photoreceptors such as rods/cones in the eye respond to light. These photons tend to cause changes in the structure of the rhodopsin/iodopsin pigments and not the actual 'receptors' themselves. G proteins do seem to be involved in this process too.

Therefore, are photoreceptors allosteric? And if not, are they an anomaly in mammalian receptors, or are there many other examples of receptors that do not rely on allostery?

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  • $\begingroup$ Signaling 'receptors' are not necessarily enzymes; 'receptor-proteins' is a different functional-class of protein. But yes all they work in lock and key mechanism. I can assume fro where this confusion arise. Older biochemistry texts classifying protein by work (such as motor protein, carrier protein and such and such) that lists lacked receptor-proteins. Me too once had this confusion so I can understand what you're trying to say. $\endgroup$ – Always Confused Nov 22 '16 at 17:56
  • $\begingroup$ Not only for mammals; for all sort of signaling-receptors of universe; you could compare it with allosteric enzyme. Because just like that enzyme's 2 sites : allosteric site and active site; the signal-receptor proteins have main 2 types of sites: the 'ligand' binding site (compare it with enzyme's "allosteric" site) and the next-messenger's binding site (compare with the enzyme's "active site"). $\endgroup$ – Always Confused Nov 22 '16 at 18:06
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"Allosteric" refers to an alternate site of action, but it is not limited to a discussion of enzymes. For a protein that is a receptor, allosteric modulation refers to modulation of that receptor away from the binding site or "active site" of the endogenous ligand.

Although it is true that the receptor may have enzymatic activity when a ligand is bound, it would be non-standard to use the term "allosteric" to refer to the normal ligand binding site, even if this terminology is technically correct with respect to the enzymatic activity.

There are many receptors which are not enzymes at all: a great example would be ligand-gated ion channels - all of the activity of these proteins is due to their conformational changes, and not due to any catalyzed reaction.

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  • $\begingroup$ So would ligand-gated ion channels not be described as allosteric since they are not enzymes? Or they would because they rely upon conformational changes? $\endgroup$ – D.J. Lawson Nov 22 '16 at 18:01
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    $\begingroup$ I think you have to be careful of how you attribute the word "allosteric" - in the canonical case, an enzyme itself is not allosteric - the modulation of the enzyme by some other factor that binds outside where the substrate binds is allosteric. "Allosteric" literally means "other site". I suppose you could consider ligand binding on an ion channel to be "allosterically" changing the protein structure at the pore, therefore allowing ions to pass. $\endgroup$ – Bryan Krause Nov 22 '16 at 19:26
  • $\begingroup$ I think you recognize a very important concept in biology: changes in protein function result from conformational changes. You are wanting to call this "allosteric" - you can probably twist the concepts to say that (most) proteins' conformation changes somewhere when something binds elsewhere on the protein, therefore an allosteric effect. Does that really get you any further than the initial statement that changes in protein function result from conformational changes? $\endgroup$ – Bryan Krause Nov 22 '16 at 19:29
  • $\begingroup$ To me, it makes more sense to reserve the term allosteric regulation for a) modulation of enzyme function at a site that is not the normal substrate binding site, or b) modulation of receptor function by changing efficacy, promoting or preventing ligand binding, etc, at a site that is neither the endogenous ligand binding site nor the site of enzymatic or other functional activity (e.g. an ion pore). $\endgroup$ – Bryan Krause Nov 22 '16 at 19:31

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