And now we come to umami science at its most fundamental: the chemical process that plays out in our taste cells when we eat savory foods and experience umami.
Umami, we've learned, is caused by the glutamate present in abundance in ramen's ingredients, and amplified by nucleotides through the phenomenon known as "umami synergy." But how exactly do these molecules make us perceive umami? In the final installment of Ramen Chemistry's series on umami science, we're going to answer that question by visiting the surface of a taste cell, where a venus flytrap (of a molecular sort) lurks on the surface. This venus flytrap has an appetite for just one thing--glutamate, preferably with a side of nucleotide--and when it bites down on a meal, it sets off a cascade of cellular signals that causes the brain to say "delicious!"
Venus Flytraps. Well, not this kind of venus flytrap. Conservatory of Flowers, GG Park. Photo: Ramen Chemistry.
Background: Sensory Perception and Cellular Communication
We learned in Part II how the "basic tastes" all derive from the interactions of particular dietary molecules--i.e., glutamate (umami), sugars (sweetness), acids (sourness), sodium chloride (saltiness)--with chemical receptors on the surface of taste cells. There is a special receptor for each basic taste--one for sweetness, one for umami, and so on. It's important to understand that when I use the term "receptor" here, what I mean is a protein; a protein that sits on a taste cell and makes physical contact with a taste molecule like glutamate or sugar.
Taste Signaling. Those things marked "T1R2, T1R3" are the venus flytrap receptor proteins that recognize sweet chemicals, setting off a chain of chemical events inside the taste cell, ultimately causing the brain to perceive sweetness. Umami works the same way. Image www.qiagen.com (full link here).
This interaction between the taste molecule and its protein receptor is the first step in the perception of taste. It starts what scientists call a signal transduction pathway. Signal transduction is how we perceive and respond to external stimuli, and how our cells communicate with each other to get anything done in our bodies.
Now, don't let your eyes glaze over at the sight of a term like signal transduction! It's easy to understand. It works like this: a stimulus--a taste or smell molecule, light, a hormone, a neurotransmitter, etc.--starts the process by interacting with a receptor protein on a cell surface. The receptor protein responds to the stimulus by changing its shape and, in effect, turning "on." That shape change ripples through the cell surface, causing something to happen inside the cell. What follows is a cascade of events in which a succession of proteins is turned on, each causing the next event in the molecular sequence. The end result is a precise physiological response--the taste of umami, the scent of jasmine, or the perception of the color blue.
Umami and the Venus Flytrap
We just learned that the first step in any taste process occurs when a taste molecule causes a taste receptor protein to change its shape. In the umami receptor, that shape change happens in a part of the protein called a "venus flytrap domain" (VFT). The VFT is made of two lobes connected by a sort of atomic hinge. Those lobes can be open or they can snap shut, which is why its called a venus flytrap. Under normal circumstances, the VFT prefers to be open. It's more stable that way.
Tangential Relationship. Ramen from Ramen Kyouka, Hiroko's ramen school teacher's restaurant in Tokyo (left). Venus flytrap (right).
Things change when glutamate comes along. That's because right near the VFT's hinge is a tiny pocket that is specially adapted to fit a glutamate molecule. When glutamate enters this pocket, its atoms interact with the atoms in both lobes, causing the two lobes to close around it. Glutamate acts as a molecular glue, increasing the stability of the closed VFT. This is hugely important, because the closed form of the VFT is the active form: an umami taste signal is sent to the brain only when the VFT is closed.
Umami Signal Cascade. When the VFT closes around glutamate, it causes a shape distortion in another part of the umami receptor, triggering a series of molecular events resulting in umami taste.
It turns out that this opening and closing of the VFT also explains umami synergy. Inside the VFT there are actually two pockets. One for glutamate and another, further from the hinge, for the nucleotides IMP and GMP. Umami synergy occurs when glutamate is in its pocket, and IMP or GMP is simultaneously in the adjacent nucleotide pocket. The nucleotide, in essence, increases the strength of the glue, making the closed VFT even more stable. The more stable the closed VFT becomes, the more umami signal is sent to the brain.
Synergy Happens in the Flytrap. Glutamate (yellow) up against the VFT's hinge. IMP (green) sits next door, at the mouth of the flytrap. The contacting amino acids in the VFT surround. This 2008 paper in the Proceedings of the National Academy of Sciences, entitled "Molecular mechanism for the umami taste synergism" showed the molecular basis for umami synergy: http://www.pnas.org/content/105/52/20930.full
And with that, we've finished umami science. I have tried to write this series for scientists and non-scientists alike, and it's my hope that all readers have learned something new. Up next, Ramen Chemistry will get back to the business of starting a restaurant. Stay tuned.