Take a moment to breathe, and notice all the things you can smell. Right now, in my office, I can smell many things. A yeasty smell tells me that a lab down the hall is autoclaving bacterial growth media. The unique smell of old books reveals that I haven't entirely moved to digital media yet, but a faint whiff of dust reveals how long it's been since I've read them. The mug of tea on my desk provides a comforting scent throughout the day, and on other days, is replaced by the stronger scent of coffee. The sense of smell keeps us continually informed about our surroundings, giving us clues about what is happening close by and at longer distances. These many scents are recognized by a collection of olfactory odorant receptors that monitor the air we breathe and tell us when interesting molecules are nearby.
Odorants and Receptors
Odorants are typically small molecules that waft through the air and reach our nose, and have chemical properties that allow them to enter our mucous membranes and reach odorant receptors. The number of possible odorants has been tricky to pin down, with estimates ranging from 10,000 to 40 billion different molecules. Looking in our genome, we find that our noses contain about 400 different odorant receptors. These receptors are monitored by specialized nerve cells in the nose, typically with each individual nerve cell containing only one type of odorant receptor. The richness of our sense of smell, however, is achieved by combinatorics. Each odorant receptor typically binds to several similar types of odorant molecules, and each type of odorant will bind to multiple types of odorant receptors. The brain then sorts out all the combinations to identify specific scents.
Stinky Is Not Always Bad
The odorant receptor pictured here (PDB ID 8f76
) recognizes the odorant molecule propionate. Pure propionate has a pungent, unpleasant smell. However, the makers of swiss cheese carefully cultivate special bacteria that make propionate, and in small amounts, it adds an appealing tang to the cheese. This receptor also recognizes acetate, a slightly smaller molecule that gives vinegar its sharp odor. It is a G-protein-coupled receptor similar to light-sensing rhodopsins
and neurotransmitter receptors such as the adrenergic
receptors. As with other GPCRs, an odorant molecule binds to a specific extracellular site on the receptor, causing a change of shape in the receptor that is sensed by G-proteins
inside the cell, ultimately stimulating the nerve cell.
Odorant-binding proteins from pig and mosquito. Molecules bound in the odorant site are in yellow and orange, and the mosquito-repellent DEET is in green.Download high quality TIFF image
Many animals also build small proteins that capture odorant molecules and deliver them to odorant receptors. We only build a few of these, which are primarily involved in capture of pheromones. A similar protein from pigs is shown here from PDB ID 1e00
. Insects, however, often build many types of odorant-binding proteins, which help them capture odorants in their antenna and other sensory organs. Looking in the PDB archive, you can find many structures of odorant-binding proteins that moths use to find mates and mosquitoes use to find people to bite. The one pictured here, from PDB ID 3n7h
, has the mosquito repellent DEET bound to it, blocking a tunnel where odorants normally enter. Notice that the mammalian and insect proteins have entirely different protein folds, giving evidence that they evolved separately to perform a similar function.