Use phosphate-buffered solutions in experiments

1. Biologists frequently use phosphate-buffered solutions in experiments.  You need to prepare a sodium phosphate solution at pH 6.7 by mixing NaH2PO4 and Na2HPO4 (pKa =7.2).  What are the respective concentration of each of these substances if you want the final concentration of phosphate ([HPO4-2]+[H2PO4-] to be 150 mM?

2. Activity of the bacterial enzyme acetoacetate decarboxylase sharply declines below pH 6, suggesting there is an ionizable catalytic residue with pKa ~ 6. Using mutagenesis, this critical catalytic residue was determined to be Lys-115.

MCB354FA15_PS01

a) For catalytic Lys-115, what is the ratio of deprotonated to protonated side chain at pH 7.5?

b) The neighboring residue at position 116 is also lysine. Explain how this perturbs the pKa of the catalytic lysine residue.

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