1. (10 points) In a recently published article it was shown that the two active

Question

1. (10 points) In a recently published article it was shown that the two active site cysteine
residues (Cys 12 and Cys 15) of the small thioredoxin protein BC3987 have pKa values of
7.2 and 5.1 respectively. The typical pKa value for a cysteine side chain is 8.
a. What does the pKa value of an amino acid side chain indicate?
b. At physiological pH (7.4) calculate the fraction of each side chain that is charged.
c. Provide a general explanation as to how the measured pKa values for the two
cysteine residues in the active site of BC3987 could be lower than the typical value.

Explanation / Answer

(a) pka value is the value at which the side chain like amino and carboxyl gain H(+) ions from the surrounding and loses H(+) ions to the surrounding. It's helps in finding the pI value of the amino acids. As

pl=( pka1+pka 2)/2..

(b) at ph=7.4, as lysine is basic amino acids hence as pH is greater than pI, as pI is equal to 6.15, hence the one carboxyl group will lose its H(+) ions hence becoming negatively charge..

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