A protein, shown schematically in Figure 6.30, may contain several cysteines (re

Question

A protein, shown schematically in Figure 6.30, may contain several cysteines (represented as light gray balls), which may pair together to form disulfide bonds. For six cysteines, three disulfide bonds can form; the figure shows the pairing arrangement 1-6, 2-5, 3-4. How many different disulfide pairing arrangements are possible?
Derive the general formula for the number of different pairing arrangements when there are n cysteines (and n is even).

Explanation / Answer

There will be 15 different disulfide pairing is possible with 6 cysteine amino acid residue.

The formula for different disulfide pairing between cysteine amino acids in given polypeptide chain is given by nCr formula of combination where n is number of total cysteine residues and r is number of cysteine amino acid required for disulfide bond formation.

So nCr = n!/r!(n-r)! = n*(n-1)*(n-2)!/2!*(n-2)! = 1/2 {n(n-1)} number of disulfide bonds in given polypeptide chain.

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