Biological complexity: how we smell odors (Introduction)

by David Turell , Thursday, May 26, 2016, 15:53 (3103 days ago) @ David Turell

It is a highly complex arrangement where neurons are on the body's surface and are specialized:-http://inference-review.com/article/smells-spanners-and-switches-"Primary olfactory neurons are, under normal circumstances, the only part of our central nervous system that is exposed to the outside air. Each neuron carries on its surface one of a large number of different protein receptors. An even larger number of volatile drugs called odorants are capable of turning on these receptors. The result is the sensation of smell. Two things are peculiar about smell. First, the sensation unerringly discriminates among odorants. As far as we know, no two odorants in the pure state smell the same to us. Second, it has turned out to be impossible to predict the smell of a molecule from its molecular structure.-***-"The most commonly accepted theory is that odor is not written into the molecule, but arises because the odorant happens to fit the shape of its many receptors. If you could by some means change the specificity of each receptor, the perceived odor would be completely different. This makes sense. Lock and key molecular recognition is everywhere in biology; antibodies and enzymes would not work without it.-***
"If both assumptions are correct, then even if receptors have only two positions, N receptors will be able to distinguish 2N smells. For fruit flies, this would mean 2^63, or approximately 10^19 smells. (my bold)
***-"There are sixty-three receptors in fruit flies, approximately four hundred in human beings, and close to a thousand in mice.-***-"To most biochemists, the notion that a vast number of olfactory receptors results in an effectively infinite range of sensations therefore poses no conceptual problem at all. It is small wonder that we cannot predict odor from molecular structure. The task would be difficult enough if there were one receptor involved, as is the case when designing drugs. When there are dozens of receptors, the problem is simply intractable. Whatever the steps from molecular structure to odor, they cannot be followed in reverse. If this is correct, all we can do is retreat and declare victory, which is what mainstream thinking in the field appears to be doing.-***-"The Database of Odorant Responses compiled at the University of Konstanz lists identified receptors in the fruit fly and the molecules known to turn them on (or off) What is striking is the vast range of odorants to which each receptor responds. If a particular odorant generates a significant receptor response, it would be tempting to call the excited receptor the receptor for that odorant. But that would be foolish, since another as yet untried odorant might always give a still larger response.3 Conversely, some odorants inhibit the receptor. This pattern is true of all fruit fly receptors, and there are strong indications of something similar occurring in vertebrate receptors.***-***-"Most odorants activate most receptors to some extent, and if there is a pattern, it must lie in the relative degree of excitation. It is very hard to estimate the discriminatory ability of such a system. Exciting many receptors promiscuously, certain odors may reduce a system's discrimination, while a system's ability to make use of different degrees of excitation may increase it. The balance between the two determines how many odorants can be discriminated. Given our current state of knowledge, we cannot estimate, even approximately, this balance. -***-"No matter its structure, if a molecule contains an -SH group, a rotten egg character is superimposed upon the molecule's other smells. If the molecule smells of pine needles, the total will smell of eggy pine needles, such as in the case of pinanethiol, a grapefruit odorant.-***-"But the situation is more tangled than that. Our ability to detect the presence of distinctive groups of atoms—called functional groups because in large part they determine the chemistry of the molecule—extends far beyond -SH. -***-"No receptor could unerringly detect these functional groups, whatever the molecular context, by conventional molecular recognition. They are too small to have a distinctive shape, and they only interact through one, or perhaps two, hydrogen bonds. Are our noses somehow reading the atomic composition of the molecule?"-Comment: I've presented the beginning of the article, the problem. The author is looking for a solution. We can smell too much for lock and key. (Note my bolded sentence) He proposes looking at electron currents in his future research.