Lesson 6: Protein Structure and Function - Myoglobin and Hemoglobin

Question 1

where is myoglobin found

Answer 1

• skeletal muscle –> job is to “store”

Question 2

myoglobin

Answer 2

• tertiary structure
• 8 helical segments
• porphyrin ring with a central Fe+
• proximal and distal His residues are important in O2 binding

Question 3

what is hemoglobin’s job

Answer 3

transport O2 form lungs and deliver to tissues

Question 4

hemoglobin

Answer 4

• composed of 4 polypeptide subunits; therefore quaternary structure
• composed of 2 alpha and 2 bet subunits
• each subunit is structurally similar to an individual Mb ( each 4 chains on Hb has a folded subunity structure similar to that of Mb and each carries a heme)
• porphyrin ring

Question 5

F helix of Hb contains what

Answer 5

proximal His residue

Question 6

E helix of Hb contains what

Answer 6

distal residue

Question 7

the distal His of Hb binds to what
(also give the number of residue)

Answer 7

distal His 64 forms hydrogen bond with the bound O2

Question 8

where is the oxygenation of myoglobin and hemoglobin bound through ()
- what are critical for this binding

Answer 8

• 6th coordination bond
• 2 histidines (proximal and distal) are critical to this binding

Question 9

what is the shape of myoglobin curve

Answer 9

hyperbolic

Question 10

what does the curve shape and saturation of Mb graph indicate

Answer 10

indicats that there is a single binding site for O2 and binding can be described by a single binding constant

Question 11

P50:

Answer 11

pressure where 50% of molecules are saturated

Question 12

what relationship is there between P50 and oxygen binding affinity

Answer 12

INVERSE RELATIONSHIP with binding affinity

Question 13

ex:
if Mb had a low p50 value—-

Answer 13

• high affinity for O2
• means that it takes a small amount of O2 to reach 50% mark
  — indication of binding affinity

Question 14

O2 path (step 1)

Answer 14

hemoglobin transports O2 from the lungs to the respiring tissues where it is used for aerobic metabolism in the mitochondria

Question 15

O2 path (step 2)

Answer 15

inside cells, dissolved O2 diffuses freely or is bound to myoglobin, which aids transport of O2 to mitochondria. Myoglobin can also store O2 for later use (as in deep-diving mammals)

Question 16

O2 path (step 3)

Answer 16

CO2 produced by oxidative processes in the tissues is carried back to the lungs by hemoglobin, or in the plasma as HCO3-, and released

Question 17

hyperbolic =

Answer 17

single binding site - no cooperativity

Question 18

sigmoidal =

Answer 18

• cooperative bindng (cooperativity) –> subunits are working together
• =must be >1 ligand binding site
• binding of ligand to 1 binding site influences binding of ligand to other sites

Question 19

why does fetal HbF have a lower P50 value than HbA

Answer 19

as O2 is exchanged in the placenta, the fetal Hb needs to “compete” with maternal tissues for available O2. Hence it needs to have a higher affinity and a lower p50 value

Question 20

HbF

Answer 20

Fetal Hb –> (a2) (y2)

Question 21

HbA

Answer 21

adult Hb —> (a2) (b2)

Question 22

synthesis of normal Hb from globin genes: yolk sac, liver, spleen

Answer 22

in utero, gene expression unregulates alpha and gamma. Yolk sac, liver, and spleen synthesize fetal Hb`

Question 23

synthesis of normal Hb from globin genes: bone marrow

Answer 23

gene expression shifts postnatally. Bone marrow synthesizes adult Hb

Question 24

T-state

Answer 24

• low O2 affinity state
• deoxy state (oxygen is unbound)
• puckered porphyrin ring plane, large and basic central cavity
• tense state

Question 25

R - state

Answer 25

• high O2 affinity state
• oxy state (oxygen is bound)
• flat porphyrin ring upon O2 binding
• stimulates a series of large scale conformational changes
• relaxed state

Question 26

when no ligands are bound,

Answer 26

T state is more stable-
- less stable upon oxygenation –> higher in energy

Question 27

when 4 ligands are bound

Answer 27

R state is more stable
- more stable upon oxygenation –> lower in energy

Question 28

without oxygen…

Answer 28

reduced oxygen is no longer planar because the ion is to large and the heme is puckered
- this is called deoxyhemoglobin

Question 29

puckered heme

Answer 29

no O2 binding

Question 30

planar heme

Answer 30

there is O2 binding

Question 31

where does the H -bond form in the deoxyhemoglobin (T state)

Answer 31

between Tyr 42’s OH and Asp 99’’s O-

Question 32

where does the H-bond form in the oxyhemoglobin (R state)

Answer 32

asp 94’s O and asn 102’s nH

Question 33

concerted and sequential models of O2 binding (step 1)

Answer 33

1st O2 (ligand) binds

Question 34

concerted and sequential models of O2 binding (step 2)

Answer 34

2nd subunit undergoes T–> and binds 2nd O2

Question 35

concerted and sequential models of O2 binding (step 3)

Answer 35

3rd subunit undergoes T—-> R and bonds 3rd O2

Question 36

concerted and sequential models of O2 binding (step 4)

Answer 36

4th subunit undergoes T ——->R and binds 4th O2

Question 37

what does the binding of O2 to Hb stimulate

Answer 37

the release of H+ from Hb (bohr effect)

Question 38

N-terminus:

Answer 38

bohr position

Question 39

His 146

Answer 39

bohr position

Question 40

the bohr effect

Answer 40

1.) protonation of “bohr positions” (amino termini of alpha; His 146 of beta) stabilizes T state in absence of O2

2.) Binding of O2 releases H+ from “bohr positions”, thereby shifting equilibrium to R state

Question 41

pH 7.6 vs 6.8

Answer 41

• at pH7.6, Hb is about 99 percent saturated in lungs and 75 percent saturated in the tissue –> inefficient O2 transfer
• at pH 6.8, Hb is 99 percent saturated in the lungs and around 35 percent saturated in the tissue
  ^^^^^ much more efficient O2 transfer at this pH

Question 42

the efficiency of oxygen unloading () significantly as the pH drops

Answer 42

increases

Question 43

in oxygenated Hb,

Answer 43

• r state favored
• favors deprotonation of Bohr positions; therefore His more acidic
  (lower pKr)

Question 44

in dexogyenated Hb,

Answer 44

• t state favored
• favors protonation of bohr positions; therefore His is more basic (higher pKr)

Question 45

2,3-biphosphohlycerate (BPG) modulates the conformational changes within Hb

Answer 45

A.) highly purified (stripped) Hb has a higher affinity for O2 than Hb in whole blood (similar to Mb!!!)

B.) blood must contain some other compound that affects O2 binding to Hb

C.) the metabolite BPG is that compound

TAKE AWAY ——–»»» BPG stabilizes T-state

Question 46

stripped Hb

Answer 46

dangerously R state favored

Question 47

blood Hb

Answer 47

bpg stabilizes t state so O2 can be released

Question 48

2,3 - BPG Binding: high O2 favors Oxy.

Answer 48

shift T-> R.
“squeeze out” BPG

Question 49

2,3 - BPG Binding: Heterotrophic negative allosteric modifier

Answer 49

BPG binds.
Shift R —> T

Question 50

O2 is a positive allosteric effector of Hb

Answer 50

O2 binding helps shift T–> R

Question 51

example of homotropic allostery:

Answer 51

binding of O2 promotes binding of the same ligand (O2)

Question 52

positive allostery b/c binding of O2 () binding of the ligand O2 to other sites on the molecule

Answer 52

increases

—– add more O2, increase binding capautor

Question 53

what are the Bohr positions and how are they different from proximal His, Distal His, and His residues at central cavity

Answer 53

Bohr Positions:
- 2 His 146 residues of B chains
2 N-termini of alpha chains

His 2 and His 143 - central cavity residues
4 proximal and 4 distal His residues