Glu vs Asp

[letmein1992]

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I'm doing TBR passages, and I have a quick questions. The questions asks, "At a pH of about 5, which of the amino acid residues in te active site of the lysozyme enzyme provided the general base catalyst and which provides the general catalysis?"

The answer is, "Glu provides the general acid catalysis, because it's side chain gamma-carbonyl group is about 9% deprotonated at a pH of 5. Asp provides the general base catalysis, because its side gain beta-carboxyl group is about 9% protonated at a pH of 5.

Can someone explain why this is true? I just didn't know how to answer Thai problem because the pKa of both AA are below 5, and I thought both of acidic AA, so idk how to identify which would act as a base..

Thanks!​

 

[Quinoline]

The pka of the R of asp is 3.9, and glu is 4.07
Using the henderson-hasselbalch equation pH=pka+log(A-/HA), you can find the ratios of A- to HA for each AA. Also if you know the pkas for each, you can see that asp is more acidic so it is more deprotonated at pH 5 than glu so it is more likely to act as a base.