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In the body we almost always assume that glutamate exists as glutamate rather than glutamic acid. It is so commonly glutamate yet glutamic acid and glutamate share the abbreviations of Glu and E. From the wikipedia page:

The side chain carboxylic acid functional group has a pKa of 4.1 and therefore exists almost entirely in its negatively charged deprotonated carboxylate form at pH values greater than 4.1; therefore, it is negatively charged at physiological pH ranging from 7.35 to 7.45.

Glutamic acid:

glutamic acid (amino acid contains hydrogen)

Glutamate:

enter image description here

Given that the pKa of glutamate is 4.1, is their any biological situation (a cell type, organel etc.) that glutamic acid is preferred?

Additionally, does this have any large biochemical implications, or is that outweighed by the (relatively speaking) "extreme" acidic environment?

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    $\begingroup$ I'm confused by your question. Glutamate predominates at physiological pH. $\endgroup$
    – canadianer
    Commented Feb 6, 2015 at 17:13
  • $\begingroup$ Completely got them mixed up as I was writing, whoops! The underlying point is "where does the protonated form occur biologically, and does it matter?" $\endgroup$
    – James
    Commented Feb 6, 2015 at 17:55
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    $\begingroup$ I figured as much. I might search for some examples later, but here's some speculation on where you might find it: proteins naturally found in the digestive track, acidophiles, active sites during catalysis. 4.1 is the pKa of glutamate free in solution. In a protein, this can change depending on interactions with other residues. $\endgroup$
    – canadianer
    Commented Feb 6, 2015 at 18:02
  • $\begingroup$ onlinelibrary.wiley.com/store/10.1111/j.1432-1033.1995.0839a.x/… $\endgroup$
    – canadianer
    Commented Feb 6, 2015 at 18:03
  • $\begingroup$ lysosomes, stomach acid. $\endgroup$
    – user137
    Commented Feb 7, 2015 at 5:50

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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.

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