hich of the following statements about protein-ligand binding is correct? A) The

Question

hich of the following statements about protein-ligand binding is correct? A) The K, is qual to the concentration of ligand when all of the binding sites are occupied The K, is independent of such conditions as salt concentration and pH. ) The larger the K, (association constant), the weaker the affinity. D) The larger the K, the faster is the binding. ) The larger the K,, the smaller the Ka (dissociation constant). 2. An allosteric interaction between a ligand and a protein is one in which: A) to the same site. to a binding site affects binding of additional molener site binding of a molecule binding of a molecule to a binding site affects binding properties of anot B) C) D) binding of the ligand to the protein is covalent. multiple molecules of the same ligand can bind to the same binding site. E) two different ligands can bind to the same binding site. 3. In hemoglobin, the transition from T state to R state (low to high affinity) A) Fe?* binding. B) heme binding. C) oxygen binding. D) subunit association. E) subunit dissociation. 4. Which of the following is not correct concerning cooperative binding of a ligan A) It is usually a form of allosteric interaction. B) It is usually associated with proteins with multiple subunits C) It rarely occurs in enzymes D) It results in a nonlinear Hill Plot. E) It results in a sigmoidal binding curve. 5. Carbon monoxide (CO) is toxic to humans because: A) it binds to myoglobin and causes it to denature. B) it is rapidly converted to toxic CO2 C) it binds to the globin portion of hemoglobin and prevents the binding of O2 D) it binds to the Fe in hemoglobin and prevents the binding of O2. E) it binds to the heme portion of hemoglobin and causes heme to unbind from hemoglobin The fundamental cause of sickle-cell disease is a change in the structure of: A) blood. B) capillaries C) hemoglobin. D) red cells. E) the heart. 6.

Explanation / Answer

For protein -ligand binding ,

P+L<--->PL

ka=[PL]/[P][L]

Kd=1/ka

[P]=protein concentration

[L]=ligand conc

[PL]=proten ligand complex

ka=association constant

kd=dissociation constant

A) [Pt]=total protein =[Pb](protein bound)+[P](unbound)

r=fraction of bound sites=[Pb]/[Pt]=[Pb]/[Pb]+[P]

[pb]=ka[p][L]

So r=ka[L]/Ka[L]+1

for n number of sites per protein

r=n[ka[L]/Ka[L]+1]

If all the sites are bound,r=1

Ka[L]+1=nKa[L]

Ka[L](n-1)=1

Ka=(1/n-1)*1/[L]

Ka is not equal to ligand concentration

false statement

B)Incorrect ,Protein concentration depends on pH as they have different pka values for their proton dissociation /association.Proteins react at a specific pH values only

C)ka=[PL]/[P][L]

larger ka ,larger [PL] or complex formed

Incorrect statement

D) Correct statement

ka=[PL]/[P][L]

P+L<--->PL

ka=k1/k-1

k-1=rate constant for forward reaction,

k1=rate constant for reverse reaction

if higher ka then higher k1 or forward reaction rate or [PL] formation is faster

E) correct statement

ka=1/kd

kd=[P][L]/[PL]

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