Jan 132009
 
ResearchBlogging.orgAlthough we still do not know the full breadth of our flavor-sensing capabilities, human beings are known to possess receptors for at least five basic tastes. Probably you have known about the sweet, sour, salty, and bitter flavors since you were in grade school, but the fifth, umami, was less widely accepted in the West until recently. Umami is a savory flavor element that is found in many foods, including tomatoes, parmesan cheese, truffles, and many kinds of meat and seafood. The umami taste primarily detects the amino acid glutamate (hence the popularity of the food additive monosodium glutamate, or MSG), but the effect is also intensified by the presence of the nucleotide inosine monophosphate (IMP). In a recent (open access) paper in PNAS, researchers from two corporations examined the umami taste receptor to understand how this happens.

The umami flavor is detected by a pair of G-protein coupled receptors (GPCRs) that have an external venus flytrap (VFT) domain in addition to their classic 7-helix trans-membrane domain (TMD). This complex is closely related to the sensor for the sweet flavor: in fact one of the receptors (called T1R3) is the same in both sensors. It is the second receptor (T1R1 for umami, T1R2 for sweet) that determines what taste is recognized. What we don’t know for sure is whether it is the TMD or the VFT of these receptors that identifies the flavor component.

In order to answer this question, the researchers performed an experiment known as a “domain swap”. Using recombinant DNA technology they assembled two chimeric proteins, one with the VFT of umami and the TMD of sweet, and one with the VFT of sweet and the TMD of umami. They then inserted these proteins into cultured cells that would fluoresce when the receptors were activated. The authors suspected that the VFT is primarily responsible for binding the ligand. As you can see from figures 1 & 2 (this is an open access paper, so go ahead and take a look), the experiment bears this out. The chimera with the VFT of sweet caused a fluorescent response in the presence of compounds such as sucrose and aspartame, while the umami-VFT chimera reacted to glutamate and aspartate. You can also see in figure 2C that the presence of IMP dramatically enhanced the activity of glutamate in this chimera. This indicates that the VFT is also responsible for IMP synergy in the umami receptor.

The hurdle in going further than this is that no structure of the umami VFT is available, which makes it difficult to figure out exactly how everything fits together. However, T1R1 has a close evolutionary relationship to the metabotropic glutamate receptors (mGluR), and a crystal structure of that VFT is available. Using conserved and homologous residues as a guide, the authors made a model of the T1R1 fold from the mGluR data. Based on this model they predicted certain amino acids that would be essential for glutamate binding in T1R1 and then mutated them in order to measure the effect. Residues that were predicted by the model to interact with the zwitterionic amino acid backbone proved to be essential for ligand recognition. Interestingly, the amino acids that contact the side-chain carboxylic acid of glutamate in mGluR are not conserved in T1R1, and mutations at the matching sites do not alter glutamate binding. However, these mutations eliminate the effect of IMP.

In order to understand this behavior, the authors modeled the binding cleft in the closed state, with IMP and glutamate in place. Glutamate binds at the bottom of the cleft, with its side chain pointed outwards. This conformation puts several positively-charged residues from the two lobes of the VFT close together higher up in the cleft. The authors propose, in keeping with previous models of VFT behavior, that the binding of the glutamate lowers the energy barrier between the open and closed states of the domain, but that glutamate alone is not sufficient to hold the domain closed. Their model places IMP higher up in the cleft, where its negatively-charged phosphate interacts with the positive residues. Thus, IMP stabilizes the closed conformation of the VFT domain.

Some more work here would be welcome, particularly in the form of experimental crystal structures of the T1R1 VFT that can confirm the homology model. The VFT is rather large, but using a perdeuterated sample in a high-field magnet it might be possible to confirm the population-shift mechanism using NMR experiments. Lower-resolution techniques such as FRET may also be able to catch this stabilization behavior. If the model proves to be accurate, it would serve as an interesting example of positive allostery from a population shift.

Although these experiments only concerned the umami taste receptor, this allosteric mechanism may be a more general feature of certain GPCRs. The authors indicate that they have unpublished data showing similar behavior in the sweet receptor, and it may be possible to design an allosteric stabilizer for any GPCR with a VFT domain. Because the related mGluR receptors are involved in many neurological and psychological diseases, successful design of such activators may have some therapeutic value.

F. Zhang, B. Klebansky, R. M. Fine, H. Xu, A. Pronin, H. Liu, C. Tachdjian, X. Li (2008). Molecular mechanism for the umami taste synergism Proceedings of the National Academy of Sciences, 105 (52), 20930-20934 DOI: 10.1073/pnas.0810174106 OPEN ACCESS

 Posted by at 11:30 PM

  2 Responses to “How we taste umami”

  1. I'm glad they're starting to figure out how these taste receptors work. I will be so happy when they figure out how to turn that particular receptor off.

    I hate the taste of glutamate (and it ticks me off that so many people think it's — as I read in one article — "the essence of deliciousness"). About the only fairly strongly umami thing I can tolerate eating is tomatoes, and even then, I prefer underripe ones of the "cardboard" variety. I've never quite been able to figure out why people like eating strongly umami things, since to me they tend to taste mouldy, sweaty, or dirty.

    So far, I haven't seen anything on the subject that even acknowledges the existence of people with a negative reaction to the taste of glutamate.

  2. I’ve been a vegetarian all my life, and I’m the only one in my family who is. I’ve only recently realized it’s because of my strong distaste for umami. (In addition to meat, when I was younger I also steered away from strong umami flavored non-meat products, like cheeses, tomatoes, and mushrooms.) I’d be very interested to know what causes my strong reaction against the umami flavor, and why.

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