Biological complexity: how we smell odors; very well (Introduction)

by David Turell , Wednesday, August 04, 2021, 22:55 (996 days ago) @ David Turell

New research on how receptors work to sort out odors:

https://phys.org/news/2021-08-reveals-receptors.html

"'The olfactory system has to recognize a vast number of molecules with only a few hundred odor receptors or even less," says Rockefeller neuroscientist Vanessa Ruta. "It's clear that it had to evolve a different kind of logic than other sensory systems."

***

"The findings, published in Nature, reveal that olfactory receptors indeed follow a logic rarely seen in other receptors of the nervous system. While most receptors are precisely shaped to pair with only a few select molecules in a lock-and-key fashion, most olfactory receptors each bind to a large number of different molecules. Their promiscuity in pairing with a variety of odors allows each receptor to respond to many chemical components. From there, the brain can figure out the odor by considering the activation pattern of combinations of receptors.

***

"The team turned to the jumping bristletail, a ground-dwelling insect whose genome has been recently sequenced and has only five kinds of olfactory receptors. Although the jumping bristletail's olfactory system is simple, its receptors belong to a large family of receptors with tens of millions of variants thought to exist in the hundreds of thousands of different insect species. Despite their diversity, these receptors function the same way: They form an ion channel—a pore through which charged particles flow—that opens only when the receptor encounters its target odorant, ultimately activating the sensory cells that initiate the sense of smell.

"The researchers chose OR5, a receptor from the jumping bristletail with broad recognition ability, responding to 60 percent of small molecules they tested.

***

"It turned out that both DEET and eugenol bind at the same location and fit entirely inside a simple pocket within the receptor. And surprisingly, the amino acids lining the pocket didn't form strong, selective chemical bonds with the odorants, but only weak bonds. Whereas in most other systems, receptors and their target molecules are good chemical matches, here they seemed more like friendly acquaintances. "These kinds of nonspecific chemical interactions allow different odorants to be recognized," Ruta says. "In this way, the receptor is not selective to a specific chemical feature. Rather, it's recognizing the more general chemical nature of the odorant," Ruta says.

"And as computational modeling revealed, the same pocket could accommodate many other odor molecules in just the same way.

"But the receptor's promiscuity doesn't mean it has no specificity, Ruta says. Although each receptor responds to a large number of molecules, it is insensitive to others. Moreover, a simple mutation in the amino acids of the binding site would broadly reconfigure the receptor, changing the molecules with which it prefers to bind. This latter finding also helps to explain how insects have been able to evolve many millions of odor receptor varieties suited for the wide range of lifestyles and habitats they encounter."

Comment: this gives us a general idea of how the receptors attach to odorant chemicals, bu t not how the brain sorts out the complex messages it must be getting.