Question

Proinsulin is an 84 residue polypeptide with six cysteines. How many different disulfide combinations are possible?

Hint:

When calculating combinations of pairs from a set, use the combination formula \( C(n, 2) = \frac{n(n-1)}{2} \).

Answer 1

Correct Answer: 15

Step 1: Understanding disulfide bond formation.
Disulfide bonds are formed between pairs of cysteine residues. Given that there are six cysteines in the polypeptide, the number of possible disulfide pairings can be calculated using combinations. The number of ways to choose 2 cysteines from 6 is given by the combination formula: \[ C(n, 2) = \frac{n(n-1)}{2} \] where \( n = 6 \).
Step 2: Calculating the number of combinations.
\[ C(6, 2) = \frac{6(6-1)}{2} = \frac{6 \times 5}{2} = 15 \]
Step 3: Conclusion.
The number of different disulfide combinations possible is 15.