Biochem - Globins

Question 1

What do hemoglobin and myoglobin do to O2’s reactivity?

Answer 1

reduce it

Question 2

2 reasons for O2 transport proteins:

Answer 2

• reduces O2 activity
• gets more solubility in the blood

Question 3

Oxidation vs reduction

Answer 3

O: loss of e- (increase in oxidation #)
R: gain of e- (decrease in oxidation #)

Question 4

Oxidant vs reductant

Answer 4

O: accepts e-‘s (oxidation state decreases)
R: donates e-‘s (oxidation state increases)

Question 5

Example of powerful oxidant

Answer 5

oxygen

Question 6

How do hemoglobin and myoglobin affect oxygens reactivity?

Answer 6

reduce it by forming a protective environment around it so it cant react

Question 7

Hemoglobin picks up _____, so it acts as a biological _____

Answer 7

protons
buffer

Question 8

What are heme iron’s two oxidation states?

Answer 8

Fe2+ (ferrous)
Fe3+ (ferric)

Question 9

How many bonds can ferrous iron form?

Answer 9

6 bonds
- 4 bonds with the nitrogens of the pyrrole rings
- 1 bond with the histidine of the globin chain
- 1 bond with oxygen

Question 10

Carbon monoxide binds
_____ more tightly than oxygen

Answer 10

200-fold

Question 11

T/F: Ferric iron doesnt bind to oxygen?

Answer 11

True

Question 12

O2 doesn’t bind to protein components, it binds to _____ groups

Answer 12

prosthetic

Question 13

What is an example of prosthetic groups?

Answer 13

vitamins

Question 14

What is the heme prosthetic group derived from?

Answer 14

precursors in the body (NOT vitamins)

Question 15

Heme structure

Answer 15

• forms 4 pyrroles (because of the 4 nitrogens)
• 1 central iron
• flat
• bulk of molecule is hydrophobic/nonpolar
• one edge that is polar (because of the proprionic acid groups)

Question 16

Myoglobin structure:

Answer 16

• 153 amino acids (80% in α helix form)
• overall nonpolar molecule
• has 1 polar surface (between histidines)

Question 17

Heme and myoglobin:

Answer 17

• Heme slides into a pocket between E and F helices
• Histidine in helix F binds heme iron (proximal His)
• Histidine in helix E binds O2 (distal His)
• the surface between the 2 histidine portions is POLAR (prevents H2O from entering and oxidizing the iron)

Question 18

Which residue does histidine bind to the heme iron?

Answer 18

Residue position 93

Question 19

Histidine in helix F:

Answer 19

• proximal His (more anterior when looking at a 3D structure)
• binds heme iron

Question 20

Histidine in helix E:

Answer 20

• distal His (more posterior when looking at a 3D structure)
• binds O2
• actually makes contact with O2, unlike prox. histidine

Question 21

Hemoglobin structure:

Answer 21

• heterotetramer
• 2α and 2β subunits (so hemoglobin can bind up to 4 O2)

Question 22

How many oxygen can hemoglobin vs myoglobin bind?

Answer 22

H: 4 oxygens
M: 1 oxygen

Question 23

What is the sequence identity btwn Hb and Mb?

Answer 23

17%

Question 24

What is the sequence identity btwn α and b globins?

Answer 24

40%

Question 25

Structural conservation between globin chains is greatest at which structural level?

Answer 25

tertiary

Question 26

Forms of Hemoglobin in the Human Circulatory System:

Answer 26

- 92.5% HbA1 α2b2 (alpha 2 beta 2) - 2.5% HbA2 α2δ2 (alpha 2 delta 2) - 5% HbA1c α2 (b-NH-Glc)2 overall: 95% adult hemoglobin is in blood, 5% is HbA1

Question 27

Forms of Hemoglobin in the Fetal/Embryonic Circulatory System:

Answer 27

Fetus: - HbF α2γ2 (alpha 2 gamma 2) Embryo: - Hb Gower ζ2ε2 (zeta 2 epsilon 2) - Hb Portland ζ2γ2 (zeta 2 gamma 2)

Question 28

Fetal development and clinical manifestations

Answer 28

- when beta chains replace gamma chains at birth, visible manifestations become observed (not affected during embryonic development, because no beta chains in the embryo) - disorder to alpha chains is expressed during fetal development

Question 29

Quaternary Structure of Hemoglobin:

Answer 29

- non covalent assembly of polypeptide chains - has a distinctive hole in middle of structure - when treated with urea, becomes 2 alpha-beta dimer "halfs"

Question 30

Myoglobin-O2 binding equation

Answer 30

Y = fraction of Mb with O2 bound pO2 = partial pressure of O2 Kd = equilibrium dissociation constant note: if pO2 > Kd : Y=1 (this is why shape of binding curve plateau's at 100% binding) note: if pO2 < Kd : Y = pO2/Kd

Question 31

Myoglobin-O2 binding continued

Answer 31

- Kd and affinity are inversely related - Low Kd = high binding strength (affinity) - Low K-1 (slow and small) = high affinity (results in more bound species)

Question 32

Myoglobin-O2 binding shifts

Answer 32

if curve shifts to right: - Kd increases (bc bigger pO2 value) - affinity for O2 decreases if curve shifts to left: - Kd decreases - affinity for O2 increases

Question 33

Why is Mb binding simpler than Hb binding?

Answer 33

- Mb is a single O2 binding to a single polypeptide chain - Hb binding equation has exponents - Hb shape is sigmoidal

Question 34

Hemoglobin-O2 binding equation

Answer 34

N = cooperativity (hill coefficient)

Question 35

Does Hb display positive or negative cooperativity?

Answer 35

positive cooperativity (when the binding on one ligand stimulates the binding of another)

Question 36

In Hb, the last O2 bind with _____ greater affinity than the first O2

Answer 36

100-fold

Question 37

What are the 2 reasons positive cooperativity is ideal?

Answer 37

- it fully loads partial pressures in the lung - it significantly unloads partial pressure in tissues

Question 38

O2 binding properties - Hb/Mb

Answer 38

- Hb releases O2 to tissues - Mb within the cells of the tissues picks it up - at the level where Hb releases O2, Mb binds (bc it has high affinity at those partial pressures)

Question 39

When O2 binds to DeoxyHb, how many degrees is the orientation shifted to become OxyHb?

Answer 39

15 degrees

Question 40

Hb movement at the atomic level

Answer 40

Deoxy state: iron is "puckered out" Oxy state: iron is flush (gets pulled into the ring)

Question 41

Another name for DeoxyHb

Answer 41

T state

Question 42

Another name for OxyHb

Answer 42

R state

Question 43

Histidine in DeoxyHb vs OxyHb

Answer 43

Deoxy: + charged His pKa 8 (high pKa = weak acid = stronger binding to proton) Oxy: neutral His pKa 6 (normal His pKa = releases the proton)

Question 44

Relationship between O2 and protons in Hb

Answer 44

- inversely related - O2 binds to Hb, protons come off - protons bind to Hb, O2 comes off overall: protons influence the affinity of Hb for O2

Question 45

pH effect: protons and O2

Answer 45

* High proton concentrations decrease pH values - proton binds -> decreased pH - decreased pH = right shift - right shift = increased Kd - increased Kd = lower affinity for O2

Question 46

What happens when CO2 binds to Hb?

Answer 46

- right shift - lower affinity O2 - is used as a way to get more O2 to tissues

Question 47

Where does BPG bind? What is net charge of BPG? What does it do? How does BPG affect affinity for O2?

Answer 47

- central cavity of Hb - -5 - stabilizes deoxyHb - lowers affinity for O2

Question 48

Fetal Hb has a _____ affinity for BPG and a _____ affinity for O2 when compared to maternal Hb

Answer 48

- lower - higher

Question 49

T/F: Fetal Hb and maternal Hb have a similar relationship to Hb and Mb

Answer 49

TRUE

Question 50

The globin gene family is thought to have evolved through a series of...

Answer 50

- gene duplications - random mutational drift

Question 51

Sick cell hemoglobin (HbS) is a balanced polymorphism with good and bad characteristics

Answer 51

bad: sickle cell disease good: resistance to malaria

Question 52

What is the malaria parasite known as?

Answer 52

Plasmodium falciparum

Question 53

In HbS, what amino acid can substitute Glutamic acid at position 6 of beta globin chains?

Answer 53

- Valine (non-polar substituting polar) - Val can fit between Leu and Phe hydrophobic pockets - Causes Hb to go through polymerization (distorts RBC, causes the "sickle" shape)

Question 54

Tendency to polymerize is favored by which state?

Answer 54

- T state

Question 55

What factors favor polymerization ("sickling")?

Answer 55

- High proton concentration - Low O2 - Low pH