||P.8 bottom paragraph to p.9 top paragraph: "The bacterium Escherichia coli illustrates this idea: It has five different chemoreceptor proteins with
different ligand spectra. In principle, E. coli could therefore analyze odor mixtures in 5-dimensional space and react differently to every possible mixture. But it doesn’t. The outputs of all five receptors readily converge on a single variable, namely the concentration of the CheY signaling molecule (Grebe and Stock, 1998 PMID 9501057). This single variable controls the bacterium’s locomotor decisions, so [investigators] may identify it with the odor percept in the present use of the term. E. coli projects the 5-dimensional receptor space onto a 1-dimensional perceptual space of attraction/repulsion. Why then does it need five receptors, including different receptor proteins for aspartate and for serine, both amino acids? Presumably it is difficult to make a generic amino acid receptor with high sensitivity. Both the amino and the carboxyl ends of the molecule vary in their charge distribution depending on pH, and a single binding pocket directed at these regions would not offer sufficient affinity under all conditions. Instead, the two receptor proteins focus on other more stable portions of the ligand, but those are
also unique between serine and aspartate."