An interesting read. We discussed umami receptors when I was taking Human A&P in college, and I wasn't aware that there was any controversy concerning their existence.
Our book identified umami as a receptor for savory (read, meat) flavors by responding to the presence of amino acids and did not mention any specific amino acid. I would have to assume that this meant that the receptor responded to nitrogen, but that's just a guess.
I'm curious about this statement that umami responds specifically and solely to glutamate, especially considering that it is a non-essential amino acid. My book is in my office at the gym, I'll look tomorrow and see if it gives any reference.