- 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)

Biochem - Globins Flashcards by M Sheena

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

reduce it

How well did you know this?

Not at all

Perfectly

2 reasons for O2 transport proteins:

reduces O2 activity
gets more solubility in the blood

How well did you know this?

Not at all

Perfectly

Oxidation vs reduction

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

How well did you know this?

Not at all

Perfectly

Oxidant vs reductant

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

How well did you know this?

Not at all

Perfectly

Example of powerful oxidant

oxygen

How well did you know this?

Not at all

Perfectly

How do hemoglobin and myoglobin affect oxygens reactivity?

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

How well did you know this?

Not at all

Perfectly

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

protons
buffer

How well did you know this?

Not at all

Perfectly

What are heme iron’s two oxidation states?

Fe2+ (ferrous)
Fe3+ (ferric)

How well did you know this?

Not at all

Perfectly

How many bonds can ferrous iron form?

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

How well did you know this?

Not at all

Perfectly

Carbon monoxide binds
_____ more tightly than oxygen

200-fold

How well did you know this?

Not at all

Perfectly

T/F: Ferric iron doesnt bind to oxygen?

True

How well did you know this?

Not at all

Perfectly

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

prosthetic

How well did you know this?

Not at all

Perfectly

What is an example of prosthetic groups?

vitamins

How well did you know this?

Not at all

Perfectly

What is the heme prosthetic group derived from?

precursors in the body (NOT vitamins)

How well did you know this?

Not at all

Perfectly

Heme structure

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)

How well did you know this?

Not at all

Perfectly

Myoglobin structure:

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

How well did you know this?

Not at all

Perfectly

Heme and myoglobin:

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)

How well did you know this?

Not at all

Perfectly

Which residue does histidine bind to the heme iron?

Residue position 93

How well did you know this?

Not at all

Perfectly

Histidine in helix F:

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

How well did you know this?

Not at all

Perfectly

Histidine in helix E:

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

How well did you know this?

Not at all

Perfectly

Hemoglobin structure:

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

How well did you know this?

Not at all

Perfectly

How many oxygen can hemoglobin vs myoglobin bind?

H: 4 oxygens
M: 1 oxygen

How well did you know this?

Not at all

Perfectly

What is the sequence identity btwn Hb and Mb?

17%

What is the sequence identity btwn α and b globins?

40%

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

tertiary

Forms of Hemoglobin in the Human Circulatory System:

- 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

Forms of Hemoglobin in the Fetal/Embryonic Circulatory System:

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

Fetal development and clinical manifestations

- 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

Quaternary Structure of Hemoglobin:

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

Myoglobin-O2 binding equation

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

Myoglobin-O2 binding continued

- 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)

Myoglobin-O2 binding shifts

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

Why is Mb binding simpler than Hb binding?

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

Hemoglobin-O2 binding equation

N = cooperativity (hill coefficient)

Does Hb display positive or negative cooperativity?

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

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

100-fold

What are the 2 reasons positive cooperativity is ideal?

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

O2 binding properties - Hb/Mb

- 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)

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

15 degrees

Hb movement at the atomic level

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

Another name for DeoxyHb

T state

Another name for OxyHb

R state

Histidine in DeoxyHb vs OxyHb

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

Relationship between O2 and protons in Hb

- 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

pH effect: protons and O2

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

What happens when CO2 binds to Hb?

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

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

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

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

- lower - higher

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

TRUE

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

- gene duplications - random mutational drift

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

bad: sickle cell disease good: resistance to malaria

What is the malaria parasite known as?

Plasmodium falciparum

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

- 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)

Tendency to polymerize is favored by which state?

- T state

What factors favor polymerization ("sickling")?

- High proton concentration - Low O2 - Low pH