Suppose that additional experiments with Protein Z demonstrate that the residues

Question

Suppose that additional experiments with Protein Z demonstrate that the residues mutated in this study do not, in fact, contact DNA. Given these observations, suggest an alternate explanation for the binding data with the mutant proteins. (3)

Protein Z is a transcription factor that binds DNA. The interaction of protein Z with DNA was analyzed via alanine scanning. The following table reports binding data for the wild- type protein (wt) and for various mutant proteins. (Nomenclature: 115A has a substitution of alanine for isoleucine at position 15.) Protein wt I15A D16A R17A K18A F24A S25A M26A Kd (nM) 1.4 nM 1.3 nM 1.4 nM 12 nM 31 nM 1.6 nM 1.4 nM 1.5 nM

Explanation / Answer

Kd is inversely proportional to binding affinity.

Greater the Kd value, lesser the affinity.

Smaller the Kd value, greater the affinity.

From the given data,

I15A: Increased the affinity

D16A and S25A: No effect on affinity

All the other mutations reduced the affinity.

It is given that none of the mutated residues contact the DNA.

Explanation:

It is highly possible for an amino acid to exert differential binding property even if it is not located at the contacting surface. These internal amino acids may alter the structure of the protein so that the contacting surface will change ultimately resulting in the differential binding property.

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