question about amino acids

[morrisol]

i was doing a practice question that asked what would happen if you change isoleucine to lysine in a protein where the isoleucine was located close to Glu. According to the answer, it says that this change will make it more stable (which i get) but it then goes on to say that because it is more stable is is more likely to give up its proton (making it more acidic). if something is more stable, then wouldnt it be less likely to give up its proton? basically why would it be more acidic? isnt lysine a basic amino acid?

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[BobbyKoch]

Maybe by "give up its proton" it means in donating it to glutamic acid in the form of hydrogen bonding. This is what is stabilizing the interaction, in any case.

 

[morrisol]

Maybe by "give up its proton" it means in donating it to glutamic acid in the form of hydrogen bonding. This is what is stabilizing the interaction, in any case.

so the lysine's H from its NH3+ r-group, will h-bond with the acidic side chain of glut? cuz i was thinking that the reason it would stabilize is because the lysine has a +1 charge and the glut has a -1 charge and simply by convention net charge will be neutral?

 

[BobbyKoch]

Yes, at physiological pH the glutamate is deprotonated (the carboxylic R group). The lysine is protonated. The lysine donates a proton and the lone pairs on the glutamate’s R group oxygen(s) accept. There is also the +\- charge interaction.

If the question was which interaction contributes most to stability I would probably say the H bond but that doesn’t seem to be the question.

 

[morrisol]

Yes, at physiological pH the glutamate is deprotonated (the carboxylic R group). The lysine is protonated. The lysine donates a proton and the lone pairs on the glutamate’s R group oxygen(s) accept. There is also the +\- charge interaction.

If the question was which interaction contributes most to stability I would probably say the H bond but that doesn’t seem to be the question.

so then if its neutral how is it acidic? thats what i dont get.

 

[Wahkoon]

so then if its neutral how is it acidic? thats what i dont get.

Glutamate is deprotonated (O-), Lysine is protonated (NH3+). There charges interact to stabilize one another... see below

Glutamate is labelled as an acidic amino acid because it donates protons at physiologic pH (deprotonates, gives H+, decreases pH). The overall charge is neutral since there is +1 and -1.