Below is a protein sequence (in single letter abbreviations) and this protein ha

Question

Below is a protein sequence (in single letter abbreviations) and this protein has an optimal enzyme activity at pH 7.5. Label all the individual charges (+ or -) along this protein when it is at optimal pH. What is the overall charge at this pH?

N-terminus: M-A-R-K-E-D-F-W-P-E-H-R-H-L-S-D-V-G :C-terminus

Explanation / Answer

At physiological pH~7.0, then amino acids are zwitter ions (both C-terminus and N-terminus have a charge). Also remember that if ph>pka of a sidegroup for positive charges makes the charge 0, and for negatively charged side groups its -1. The only things with charges are H, R, K, D, E. So... you have to know that H R K D E pka= 6 12.5 10.8 3.9 4.1 So for pH=7 then we get the following charges: H R K D E 0 + + - - and for pH=7.5 nothing is changed. So in order we get the charges M-A-R-K-E-D-F-W-P-E-H-R-H-L-S-D-V-G + 0 + + - - 0 0 0 - 0 + 0 0 0 - 0 - = 4+ and 4- = net charge of zero.

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