Question

The sulfhydryl group of Cys-575 in Taq polymerase has a pKa of 8.47. In a solution of Taq polymerase, what fraction of sulfhydryl groups will be deprotonated at pH 7.50?

1) 9.68%

2) 90.3%

3) 88.8%

4) 17.1%

Answer 1

we know that

pH = pKa + log [deprotonated / protonated]

given

pH = 7.5

pKa = 8.47

so

7.5 = 8.47 + log [deprotonated / protonated]

log [deprotonated / protonated] = 7.5 - 8.47

log [deprotonated / protonated] = -0.97

[deprotonated / protonated] = 10^(-0.97)

[deprotonated / protonated] = 0.107

[deprotonated] = 0.107 [protonated]

now

% deprotonated = [ deprotonated] x 100 / ( [ deprotonated] + [protonated] )

= 0.107 x [protonated] x 100 / ( 0.107 [protonated] + [protonated] )

= 10.7 [protonated] / ( 1.107 protonated] )

= 9.68

so

the answer is 1) 9.68 %