Many enzymes are inhibited irreversibly by heavy metal ions such as Hg2, Cu, or

Question

Many enzymes are inhibited irreversibly by heavy metal ions such as Hg2, Cu, or Ag', which can react with essential sulfhydryl groups to form mercaptides: Enz-SH + Ag+ Enz-S-Ag H + The affinity of Agt for sulfhydryl groups is so great that Ag can be used to titrate -SH groups quantitatively. To 10.0 mL of a solution containing 3.0 mg/mL of a pure enzyme, an investigator added just enough AgNO3 to completely inactivate the enzyme. A total of 0.385 ol of AgNO3 was required. Calculate the minimum molecular weight (M) of the enzyme. Number minimurn M,= The M you determined above was not exact, but was a minimum value for the M of the enzyme. Why does the M value obtained this way give only the minimum molecular weight? O O O O The Ag' would also react with the terminal carboxyl group. There are no disulfide bonds in the protein. There is only one titratable - SH group per protein molecule There is only one polypeptide chain. 0 Previous Give Up & View Solution e Check AnswerNext Exit- Hint

Explanation / Answer

Ans. Part 1: Moles of Ag+ (= AgNO3) consumed = 0.385 umol = 3.85 x 10-7 mol

# Following stoichiometry, 1 mol Ag+ binds to 1 mol –SH group.

So,

            Moles of –SH group in protein = 3.85 x 10-7 mol

# Given, amount of protein = 10 mL of 3.0 mg/ mL

Now,

            Mass of protein = [Protein] x Volume of solution

                                                = (3.0 mg/ mL) x 10.0 mL

                                                = 30.0 mg

                                                = 0.030 g

# Assuming there is only one –SH group per molecule of protein, the number of –SH groups titrated must be equal to that of number of protein molecules.

So, we get-

            Mass of protein in sample = 0.030 g

            Number of protein molecules (= -SH group) = 3.85 x 10-7 mol

Now,

            Minimum molar mass (Mr) of protein = Mass of protein / Moles of protein

                                                            = 0.030 g / (3.85 x 10-7 mol)

                                                            = 7.7922 x 104 g/ mol

# Part 2: The minimum molar mass value indicates that maximum moles of Ag+ have reacted with the protein sample – that is Ag+ has reacted to –SH as well as other groups. If Ag+ would react only with –SH group, the actual moles of Ag+ reacting with –SH would be less than 0.385 mol. Since moles of Ag+ is inversely proportional to Mr, decrease in actual moles of Ag+ would give higher value of Mr.

# Therefore, the minimum molar mass value suggests that Ag+ has reacted to some groups other than –SH.

So, correct option is – A. The Ag+ would react with the terminal carboxyl group.

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