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Biochemistry - HBF1 > Lecture 03 > Flashcards

Lecture 03 Flashcards

(54 cards)

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1
Q

Describe the structure of the globin family of proteins

A

3D globin fold where 8 alpha-helical segments twist around each other

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2
Q

How many monomers does myoglobin (Mb) have?

A

1

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3
Q

How many monomers does hemoglobin (Hb) have?

A

4

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4
Q

How many heme units are present in Mb and Hb?

A

1 and 4; one per monomer

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5
Q

Describe the bonds to iron +2 in the center of heme

A

4 bonds to the nitrogens of protoporphyrin IX rings
1 to attach heme to globin protein
1 to molecular oxygen

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6
Q

How does heme attach to the globin protein?

A

Proximal histidine (HisF8)

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7
Q

What is the significance of distal histidine (HisE7)?

A

Distal histidine influences how ligands bind to the iron at the center of the heme

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8
Q

Where does oxygen bind relative to HisE7?

A

Proximal

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9
Q

What is the geometry of the free heme:O2 complex?

A

Bent

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10
Q

What is the geometry of the free heme:CO complex?

A

Linear

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11
Q

Why is carbon monoxide so dangerous?

A

The binding affinity of globin proteins for CO is 250x higher than for O2. This means oxygen cannot bind and therefore cannot be transported.

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12
Q

How does HisE7 reduce the Hb’s affinity for CO?

A

The steric hindrance caused by distal histidine forces CO to bind at an angle which is not ideal. On the other hand, the bent geometry is ideal for oxygen

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13
Q

What are the two resonance forms of globin-bound O2?

A

Ferrous iron with dioxygen and ferric iron with superoxide anion

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14
Q

Ferrous vs. ferric

A

Ferrous - 2+ oxidation state
Ferric - 3+ oxidation state

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15
Q

What is superoxide?

A

Free radical species that is harmful to cells (causes unwanted side reactions)

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16
Q

What oxidation state of iron does oxygen bind to?

A

Ferrous - 2+

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17
Q

What is the role of HisE7 with respect to the resonance forms of globin-bound O2?

A

Distal histidine stabilizes the resonance form the superoxide anion so that superoxide is less likely to be released.
If superoxide departs, iron is no longer able to bind molecular oxygen

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18
Q

What are the oxidized versions of Mb and Hb called?

A

metMb and metHb

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19
Q

What percentages of metHb are dangerous

A

50% for complications, 70% for death

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20
Q

Where is Mb found?

A

Cardiac and skeletal muscle

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21
Q

What is the function of Mb?

A

Stores and delivers oxygen to mitochondria for oxidative phosphorylation as needed

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22
Q

What is the function of Hb?

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A

Transports oxygen from the lungs to the peripheral tissues

23
Q

What is allostery?

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A

When two or more ligand-binding sites are physically separate yet functionally related.

24
Q

Homotropic allosteric effects/cooperativity

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A

When the effecter and affected ligands are the same type of molecule

25
Heterotropic allosteric effects
Effecter and affected ligands are different types of molecules
26
Positive vs. negative cooperativity
First ligand binding either increases or decreases affinity for the second
27
Positive vs. negative allosteric effectors
Regulatory ligands has either a positive or negative effect on activity site
28
What kind of allostery is relevent for Hb and Mb?
Positive cooperativity for Hb Mb is not allosteric
29
Describe the binding curve of nonspecific binding (no defined binding site)
Linear
30
Describe the binding curve of a protein with one specific binding site
Rectangular hyperbola
31
Describe the binding curve for a protein with allostery
Sigmoidal
32
What is Kd/P50?
The equilibrium constant for the ligand dissociation reaction; indicates the free ligand concentration at half-saturation
33
What is the relationship between P50 and binding affinity?
The smaller the P50, the higher the binding affinity
34
Why is it important that Hb's affinity for O2 is lower?
That is how it's easier for oxygen to be released into the tissues
35
What does the Hill coefficient represent?
Approximately how many binding sites interact
36
What value of n represents 0 cooperativity or the presence of 1 binding site only?
n = 1
37
What value of n represents positive cooperativity?
n > 1
38
What value of n represents negative cooperativity?
n < 1
39
What is the value of the Hill coefficient for Hb?
2.8
40
What are two conformers of Hb?
T state (deoxyHb) and R state (oxyHb)
41
What is the main structural difference between the 2 conformers of Hb?
T state has a large central cavity, R state does not
42
Which conformer of Hb has a high affinity for oxygen?
R state, oxyHb
43
How does Hb transition from the T state to the R state?
In the deoxy state, iron is not in the heme plane. After O2 binds to the first subunit, it pulls iron into the heme plane along with proximal His, and the helix moves as a result. This change then leads to the T to R transition of the whole protein
44
How do allosteric effecters shift the binding curve?
Activators shift the binding curve to the left, causing tighter binding Inhibitors shift the binding curve to the right, leading to weaker binding
45
Name the 4 principle allosteric effectors of Hb
2,3-BPG Protons CO2 Cl-
46
What is the role of 2,3-BPG relevant to Hb?
It stabilizes the T state of Hb and decreases Hb's affinity for oxygen
47
Why is the presence of 2,3-BPG especially important at higher altitudes?
More oxygen is released. At higher altitudes, barometric pressure is lower which means that not all of the oxygen available in the air will be dissolved in the blood
48
Describe the Bohr effect
The presence of increased CO2/decreased pH (pH below 7.6) in tissues decreases Hb's affinity for oxygen and releases it into said tissues.
49
How does low pH affect Hb?
Acidic conditions protonates an imidazole (histidine) side chain and allows for the T state to be stabilized.
50
What is the chloride shift?
High CO2 concentration causes high chloride concentration in RBCs. This is because bicarbonate is released to the lungs to shift the equilibrium in favor of CO2 that gets released with expiration. Chloride shifts maintain ion balance in the tissues after bicarbonate is lost
51
How does chloride effect Hb binding affinity?
Chloride neutralizes a positive charge, therefore eliminating a salt bridge and destabilizing the R state, causing oxygen to be released.
52
How else does CO2 affect Hb binding?
It can form a reversible covalent adduct (carbaminoHb) that stabilizes the T state
53
What is carbamylated Hb?
Hb with an isocyanate (HNCO) group attached; drived from urea and is indicative of uremia caused by kidney failure
54
What is Hb A1c?
Glycated Hb; caused by increase glucose concentration in the blood and indicative of diabetes

Biochemistry - HBF1 (5 decks)

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