Quarternary Structure (Proteins)

Question 1

What defines a protein’s quaternary structure?

Answer 1

The 3D organization of a protein composed of two or more polypeptide chains

Question 2

What term refers to structurally similar units within a multimeric protein?

Answer 2

Protomers

Question 3

What types of forces stabilize quaternary structure?

Answer 3

Van der Waals forces, hydrogen bonds, ionic bonds, hydrophobic interactions, and occasionally covalent bonds

Question 4

How are subunits labeled in quaternary structures?

Answer 4

With Greek letters (𝛼, 𝛽, 𝛾,…) in order of increasing mass

Question 5

What is a homo-oligomeric protein?

Answer 5

A multimeric protein composed of identical subunits

Question 6

What is a hetero-oligomeric protein?

Answer 6

A protein composed of different subunits

Question 7

Which globular protein has a monomeric structure and stores oxygen in muscle?

Answer 7

Myoglobin

Question 8

What is the quaternary structure of hemoglobin?

Answer 8

A tetramer composed of two 𝛼 and two 𝛽 subunits (𝛼₂ 𝛽₂)

Question 9

What kind of oxygen saturation curve does myoglobin display?

Answer 9

A hyperbolic curve

Question 10

Why does hemoglobin display a sigmoidal oxygen saturation curve?

Answer 10

Due to cooperative binding of oxygen

Question 11

What is cooperative binding?

Answer 11

When binding of a ligand to one site increases affinity at other binding sites

Question 12

What are the T and R states of hemoglobin?

Answer 12

T (tense) state is low-affinity, R (relaxed) state is high-affinity

Question 13

What triggers the conformational shift from T to R state in hemoglobin?

Answer 13

The binding of oxygen to heme

Question 14

What structural change in heme facilitates the T to R transition?

Answer 14

The iron atom moves into the plane of the porphyrin ring upon oxygenation

Question 15

What is the Bohr effect?

Answer 15

A decrease in hemoglobin’s oxygen affinity at low pH or high CO₂

Question 16

What ion is involved in stabilizing the T state during the Bohr effect?

Answer 16

Protons (H⁺) binding to histidine residues

Question 17

How does CO₂ contribute to the Bohr effect?

Answer 17

By generating H⁺ ions through conversion to bicarbonate

Question 18

What is 2,3-Bisphosphoglycerate (2,3-BPG)?

Answer 18

An allosteric effector that binds deoxyhemoglobin and reduces its oxygen affinity

Question 19

When does 2,3-BPG concentration increase in the body?

Answer 19

At high altitude and during pregnancy

Question 20

Why does fetal hemoglobin bind oxygen more strongly than adult hemoglobin?

Answer 20

Because it has lower affinity for 2,3-BPG

Question 21

Where does 2,3-BPG bind on hemoglobin?

Answer 21

In the central cavity between the β subunits of deoxyhemoglobin

Question 22

What is allosteric regulation?

Answer 22

Regulation where binding at one site affects binding at a different site

Question 23

How does 2,3-BPG exhibit allosteric regulation?

Answer 23

By binding to a non-active site and stabilizing the T state

Question 24

What happens to the oxygen-binding curve of hemoglobin in the presence of 2,3-BPG?

Answer 24

It shifts to the right, indicating reduced oxygen affinity

Question 25

What is the physiological advantage of hemoglobin's sigmoidal oxygen-binding curve?

Answer 25

Efficient oxygen loading in the lungs and unloading in tissues

Question 26

What mutation causes sickle cell anemia?

Answer 26

A Glu → Val mutation in the β-globin chain

Question 27

How does the sickle cell mutation affect hemoglobin structure?

Answer 27

It promotes hydrophobic interactions leading to polymerization and cell deformation

Question 28

Why is hemoglobin more functionally versatile than myoglobin?

Answer 28

Because its quaternary structure allows cooperative and allosteric behavior

Question 29

What are ionic bonds in hemoglobin's T state responsible for?

Answer 29

Stabilizing the deoxygenated form of hemoglobin

Question 30

Why does the T state have a lower oxygen affinity?

Answer 30

Because of salt bridges and ionic bonds that constrain the heme conformation

Question 31

How does oxygen binding disrupt T state interactions?

Answer 31

It causes a shift in subunit positions, breaking ionic bonds

Question 32

Why is the R state favored at high pO₂?

Answer 32

Because oxygen binding promotes the conformational shift to the relaxed state

Question 33

What is meant by 'oxygen delivery efficiency' in hemoglobin?

Answer 33

The difference in saturation between lungs and tissues, enhanced by the sigmoidal curve

Question 34

What effect does low pH have on hemoglobin’s affinity for oxygen?

Answer 34

It lowers affinity, promoting oxygen release (Bohr effect)

Question 35

What causes the formation of carbamates in hemoglobin?

Answer 35

CO₂ reacts with amino groups of globin chains

Question 36

How does carbamate formation aid in CO₂ transport?

Answer 36

By enabling CO₂ carriage in the blood and enhancing oxygen unloading

Question 37

What structural feature enables hemoglobin's cooperative behavior?

Answer 37

Inter-subunit interactions that respond to ligand binding

Question 38

Why can hemoglobin be considered an allosteric enzyme?

Answer 38

Because ligand binding alters its conformation and function at other sites

Question 39

What is the role of histidine in the Bohr effect?

Answer 39

Acts as a proton acceptor that stabilizes the T state when protonated

Question 40

Why is fetal hemoglobin adapted for maternal-fetal oxygen transfer?

Answer 40

Due to its reduced interaction with 2,3-BPG and higher O₂ affinity

Question 41

How does hemoglobin adapt to high altitude?

Answer 41

By increasing 2,3-BPG levels, lowering oxygen affinity and improving tissue delivery

Question 42

What does a right-shifted oxygen dissociation curve indicate?

Answer 42

Decreased oxygen affinity and enhanced oxygen release

Question 43

What is meant by hemoglobin saturation?

Answer 43

The percentage of heme sites occupied by oxygen

Question 44

What type of graph represents cooperative oxygen binding?

Answer 44

A sigmoidal saturation curve

Question 45

What structural property is lost in sickle hemoglobin aggregates?

Answer 45

Solubility and normal quaternary arrangement

Question 46

How does sickle cell anemia illustrate the importance of primary structure?

Answer 46

A single amino acid change leads to pathogenic quaternary structure

Question 47

What happens to oxygen delivery in individuals with increased 2,3-BPG?

Answer 47

It improves due to 𝒓𝒆𝒅𝒖𝒄𝒆𝒅 haemoglobin O₂ affinity

Question 48

What is the functional consequence of losing hemoglobin quaternary structure?

Answer 48

Loss of cooperative and allosteric regulation, as in myoglobin

Question 49

Which concept defines affinity?

Answer 49

p50

Question 50

What is p50?

Answer 50

O₂ levels required to achieve 50% saturation

Question 51

Define “cooperativity”

Answer 51

A phenomenon characteristic of proteins with multiple binding sites where the affinity of the binding sites for a ligand is increased/decreased upon the binding of a ligand to a binding site