You have discovered a new enzyme that is allosterically inhibited by a negative modulator.

You have discovered a new enzyme that is allosterically inhibited by a negative modulator. Upon solving the structure of the active state of the enzyme by X-ray crystallography, you note that it is tetrameric with each monomer consisting of two domains. You were unable to solve the crystal structure of the enzyme in the inactive state.

a) When determining the crystal structure, you used multiple isomorphous replacement (MIR) to solve the phase problem. Describe what is meant by the ‘phase problem’ in X-ray crystallography and the steps required to solve the phase problem by MIR.

b) Small angle X-ray scattering (SAXS) is a technique that can measure protein conformational change in the solution phase. Design and describe experiment/s using SAXS you could use to analyse the conformation of the new enzyme in the active state and in the inactive state. Include a plan for data reduction.

c) Your experiments reveal differences between the SAXS scattering profiles for the active state and inactive state. Describe one analysis that can be used to compare your SAXS data with your crystal structure of the new enzyme including how discrepancies between the SAXS data and crystal structure are quantified .