Question

If the two disulfide bonds in a protein containing only four Cys residues are broken, and then allowed to reform, how many possible combinations of disulfide bonds are possible?

• A. 3
• B. 2
• C. 4
• D. 6

Hint:

For 4 items, there are only 3 possible pair combinations: (1-2, 3-4), (1-3, 2-4), (1-4, 2-3).

Answer 1

The Correct Option is A

Step 1: Concept
This is a combinatorial problem where we need to find the number of ways to pair 4 distinct Cysteine (Cys) residues to form 2 disulfide bonds.

Step 2: Analysis
Let the residues be 1, 2, 3, and 4. If residue 1 pairs with 2, the remaining pair must be 3-4. If residue 1 pairs with 3, the remaining pair must be 2-4. If residue 1 pairs with 4, the remaining pair must be 2-3.

Step 3: Calculation
The formula for pairing $n$ items is $(n-1)!!$. For $n=4$, it is $(4-1)!! = 3 \times 1 = 3$.

Step 4: Conclusion
There are exactly 3 unique ways to form two disulfide bonds from four Cys residues. Final Answer: (A)