Question

a. Draw the reaction that takes place when a disulfide bond is formed. What conditions are required for this to occur? What is the major role of disulfide bonds? What kinds of proteins normally have disulfide bonds? Where do you expect to find ribonuclease, the enzyme used in Anfinsen’s famous experiment?

b. Ohm loops are also considered secondary structure. How does their structure differ from other secondary elements? What function do these loops serve? Where are loops typically found on proteins?

c. The fatty acid binding protein and bacterial porin exist in completely different environments, one hydrophobic and one hydrophilic, and also transport molecules with the opposite properties. Describe the structural features these proteins have in common and how this structure can serve these two opposing roles.

Answer 1

a. Disulphide bond formation

In chemistry, a disulfide is the functional group with the structure R−S−S−R′. The linkage is also called an S-S-bond (Disulfide Bridge) and is formed by the coupling of two thiol groups. In biological contexts disulfide bonds (-S-S-) are formed by the oxidation of sulfhydryl (−SH) groups. For example in proteins, disulfide bonds formed between the thiol groups in the cysteine residues by oxidative folding. The reaction mechanism of disulphide bond formation is shown below.

The protein disulfide bond formation is influenced by the spatial accessibility of cysteine residues forming the disulfide bond and the difference between the pK_a of the thiol groups and the pH. The disulfide bond stabilizes the folded form of a protein. The disulfide bond holds two portions of the protein together.

Extracellular proteins contain several disulfide bonds. In proteins, the disulphide bonds are formed between the thiol groups of two cysteine amino acids.

In the 1950s, Christian Anfinsen used urea and β-mercaptoethanol (denaturing agents,), to break down the secondary and tertiary structure of ribonuclease. Ribonuclease is a polypeptide consists of 124 amino acids and contains four disulfide bonds within its tertiary structure.