Quaternary Structure Example: Cholera Toxin B-subunit Use the structure below to
ID: 274479 • Letter: Q
Question
Quaternary Structure Example: Cholera Toxin B-subunit Use the structure below to answer the questions about quaternary structure in the structure activity on the course canvas site. 1. Cholera toxin (ctx) is responsible for most of the nasty symptoms exhibited by a cholera infection. Ctx is a multimeric protein (colored by chain in the structure), how would you classify it in the aß system? (Hint: assume structure similarity is equal to sequence similarity) 2. Do any of the individual chains have symmetry in and of themselves? 3. If you ignore the chain colored in light blue (A-subunit), does the rest of the structure show symmetry? If so, how would you classify it? Hint: use a word from geometry. 4. What types of weak intermolecular forces do you expect to find at the interface between the chains? Why? Smol spin on spin off A-subunit on A-subunit offExplanation / Answer
1] The light blue chain is the alpha sub-unit or A- chain.
The other five chains form the beta sub-unit or the B- chain.
2] Yes, the green and the red chains in the beta sub-unit show symmetry .
3] Yes, the rest of the structure shows helical symmetry. The protein molecule is a 3-D structure and has a screw axis or , in other words, rotary translation .
4] The chains are held together by weak disulphide bonds.
In biological systems, multivalent interactions play unique roles. They can even induce reorganisation of receptors on the cell surface which can enhance receptor binding selectivity. These proteins need to bind to the cell surface to interact.
Cholera toxins act by the binding of the B-subunit ring to the GM1 gangliosides on the surface of the target cell. Eventually , the entire toxin complex is endocytosed by the cell and the cholera toxin A1 [CTA1] chain is released by the reduction of the disulphide bridge.