Allosteric Effects

Q1: The specificity of a ligand binding site on a protein is based on

A the absence of competing ligands

B the amino acid residues lining the binding site

C the presence of hydrating water molecules

D the opposite chirality of the binding ligand

ANS:B - the amino acid residues lining the binding site

No answer description is available. 

Q2: An allosteric activator

A increases the binding affinity

B decreases the binding affinity

C stabilizes the R state of the protein

D both (a) and (c)

ANS:D - both (a) and (c)

The T state of the protein is the Tensed state, when no substrate is bound. T state has low energy and is more stable than are state.

The are state of the protein is the Relaxed state, when substrates are bound. R state has higher energy, thus it is more unstable.

Allosteric activators stabilize the are state, by moving the equilibrium towards the R state, thus increasing the affinity of the enzyme for the substrate.

Q3: A protein that shows infinite cooperative for binding of n ligands will

A show a Hill coefficient (nH) of 0.0

B only be found in either the unliganded form or the fully liganded form

C show a Hill coefficient (nH) of n

D both (b) and (c)

ANS:D - both (b) and (c)

No answer description is available. 

Q4: In hemoglobin, allosteric effects occur

A only in humans

B for maintaining Fe in the Fe2+ state

C to minimize oxygen delivery to the tissues

D to maximize oxygen delivery to the tissues

ANS:C - to minimize oxygen delivery to the tissues

No answer description is available. 

Q5: Small molecules affect hemoglobin (Hb) by

A decreasing Hb affinity for O2

B increasing [H+]

C increasing Hb affinity for O2

D increasing [H+] and decreasing Hb affinity for O2

ANS:D - increasing [H+] and decreasing Hb affinity for O2

No answer description is available. 

Q6: The conformational changes from the T to the R state is initiated by

A binding of oxygen to the heme

B movement of the proximal histidine towards the heme

C movement of the F-helix, which contains the proximal His

D reorganization of protein-protein contacts between the individual subunits

ANS:A - binding of oxygen to the heme

T stands for tensed state and R stands for relaxed state.

Q7: Bisphosphoglycerate (BPG) cannot bind to the oxygenated R state of hemoglobin because

A it is displaced from the heme by oxygen

B it is displaced from the heme by movement of the proximal histidine

C its binding pocket becomes too small to accommodate BPG

D BPG binds to the R state with the same affinity as the T state

ANS:C - its binding pocket becomes too small to accommodate BPG

The hole in the centre of the Hemoglobin consists of 6-positive charged amino residues. 2.3 BPG has a negative charge and can bind on the hemoglobin's hole solely on the T-state, because the hole that exists in the T-state essentially collapses on the R-state.

Q8: When protein binds two ligands in a non-cooperative manner, then the x-intercept of the Scatchard Plot is

A 1

B 2

C not defined

D none of the above

ANS:B - 2

No answer description is available. 

Q9: A protein that binds two ligands in a non-cooperative manner will show

A a sigmodial binding curve

B a hyperbolic binding curve

C a linear Scatchard Plot

D both (b) and (c)

ANS:D - both (b) and (c)

No answer description is available. 

Q10: The Hill coefficient (nH) for myoglobin and hemoglobin are respectively

A 2.8 and 1.0

B 1.0 and 2.8

C 1.2 and 4.5

D 4.5 and 1.2

ANS:B - 1.0 and 2.8

No answer description is available. 

Q11: O2 binding to hemoglobin results in

A 100-fold higher affinity for the last O2 bound than for the first

B extensive protein conformational change

C both (a) and (b)

D 100-fold lower affinity for the last O2 bound than for the first

ANS:C - both (a) and (b)

No answer description is available. 



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