The essential enzyme ILoveBiochemase is a tetrameric protein containing 4 identi

Question

The essential enzyme ILoveBiochemase is a tetrameric protein containing 4 identical binding sites. It it up-regulated after college exams when students become saturated with the ligand IWantAnA. This protein-ligand interaction was recently reported to have a Kd of 5nM. Unlike hemoglobin, a protein similar in structure, the identical binding sites of ILoveBiochemase are considered to be non-cooperative.

a) Briefly describe what is meant by a non-cooperative multi-binding site protein and relate how it is different than hemoglobin.

b) Assume that the monomeric protein BeatGeorgiaStatease also binds IWantAnA with the same affinity as ILoveBiochemase, but is found 4x greater in concentration in tissue. Describe the difference in specific activity.

c) What would the binding curve of ILoveBiochemase and BeatGeorgiaStatease look like? Draw a potential binding curves and indicate the ligand concentration that could correspond to theta = 0.5

Explanation / Answer

a) Non-cooperative binding means the binding of one IWantAnA ligand molecule with one subunit of ILoveBiochemase tetramer is not affect (increase or decrease) the another IWantAnA ligand binding with the second subunit of ILoveBiochemase. It might be independent binding to each subunit. Contrastingly, in the Hb (heme) the binding of one O2 molecule will permit the binding another O2 molecule with the different subunit of Hb (heme). Binding of last (4th) O2 molecule with the Hb (heme) has 100 times more affinity than the first O2 molecule. During this process, Hb will convert from T-form to R-form that might not happen in your enzyme ILoveBiochemase, so that it is non-cooperative and affinity for all the ligand molecules would be same to interact with your enzyme.

b) Specific activity: It is the activity of an enzyme per milligram of total protein (mol min1mg1). It gives detail of enzyme purity in the mixture. It represents that the amount of product formed by an enzyme in a given time period under given conditions per milligram of total proteins. It also represents that, it equals to the rate of reaction multiplied by the volume of reaction that divided by the mass of total protein. In your case, both the enzymes specific activities would be same under standard conditions (if 4 ligands bind to the one tetrameric non-cooperative enzyme and 4 ligands bind to the monomeric 4X enzyme at the same time) since, they have similar specificity for the substrate and their individual concentrations would be same in the total proteins and both can produce the same amount of product in the given time point.

c) You could see a hyperbolic curve in both cases, unlike Hb sigmoidal curve. I don't that problem

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