(Biochemistry) The activity of Lysozyme is greater than 80% of maximum between p

Question

(Biochemistry)

The activity of Lysozyme is greater than 80% of maximum between pH 3.8 and 6.1 with peak activity occurring around pH 5. Below 3.8 and above 6.1, the enzymatic activity drops dramatically. Which of the following provides the best explanation for these observation?

A) Two histidine residues in the active site. One must be protonated and the other deprotonated for enzyme activity (pKR His 6)

B) the active site contains an aspartic acid and glutamic acid, both must be deprotonated for activity (pKR Asp 3.8, pKR Glu 4.2)

C) The active site contains an aspartic acid that must be deprotonated and a cysteine (pKR Cys 8.2) that must be deprotonated

D) the active site contains a histidine that must be protonated and a glutamic acid that must be deprotonated for enzyme activity

E) Both A and B are reasonable conclusions

Phenylalanine hydroxylase (PAH) catalyzes the rate-limiting step in the conversion of phenylalanine to tyrosine. Normal PAH possesses a Leu at position 321. Mutations of residue 321 to either Val or Ile do not decrease the catalytic activity of PAH, but mutations that change residue 321 to Arg, Glu, or Ser result in significant decreases in PAH activity. Which of the following statements best accounts for these observations?

A) Mutating a Leu to either Val or Ile constitute nonpolar to nonpolar substitutions and will likely not change the structure and function of the enzyme.

B) Mutating Leu to Arg, Glu, or Ser significantly changes the chemical properties of position 321 and will likely alter the structure and function of the enzyme.

C) Leu is likely a residue buried near the interior of PAH’s tertiary structure. Replacing it with Arg, Glu, or Ser would have a net destabilizing effect on the folded state of the enzyme.

D) Mutating Leu to Val or Ile increases the nonpolar nature of the R-group at position 321 which ensures the residue will continue to interact with the hydrophobic core of the enzyme and maintain proper structure/function

E) All of the above

Explanation / Answer

1-

The active site of lysozyme has Aspartic acid at 52nd position and glutamic acid at the 35th position. Both this residue must be deprotonated for the activity of the enzyme.

So correct answer is an option - B) the active site contains an aspartic acid and glutamic acid, both must be deprotonated for activity (pKR Asp 3.8, pKR Glu 4.2)

2-

Mutation of Leu to Val or Ile doesn't affect the hydrophobic interaction of the amino acid, whereas when Leu is mutated to Glu or Ser will disrupt the hydrophobic interaction.

HYdrophobic residues are buried inside the protein. So when the hydrophobic residue is mutated to hydrophilic reside this will disrupt the overall tertiary structure of the protein. So protein will not show its function.

So all the options are correct.

SO correct answer is option E- All of the above

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