Is there a biological environment that we can we assume glutamate exists as glutamic acid?
At typical physiological pHs glutamate does exist as glutamate. 

Broadly the acidity in the digestive track is enough to reduce glutamate to glutamic acid. The stomach for example has a healthy pH of between 1.5 to 3.5.

Canadianer points out in the comments that the active site during catalysis are protonated. There are obviously a lot of examples of this and sensitive evaluation by looking at the isocontours and surface potential of the protein structure are needed to draw conclusions if this might be important.