Proteins II- Myoglobin and Hemoglobin

Question 1

Generally, what is myoglobin used for in mammals?

Answer 1

O2 storage, increases effective solubility of O2 in muscle cells.

Question 2

What are the two parts of myoglobin?

Answer 2

Heme and Globin protein

Question 3

What makes up the globular protein part of myoglobin

Answer 3

153 residues form to make 8 alpha helices, labelled A through H.

E and F helices form a hydrophobic pocket where the heme sits.

A-D Helices form the prophyrin ring ( actually part of the heme)

Question 4

What holds the heme to the globin in mb

Answer 4

heme group is integrated in a prophyrin ring, which is held in a hydrophobic pocket of the globin. HisF8, which is part of the hydrophobic pocket, binds to the iron part of the heme

Question 5

What holds the heterocyclic prophoyrin ring together and what is the ring made up of?

Answer 5

held together via methene bridges, made up of 4 pyrrole groups with the alpha helices labelled A to D.

Question 6

What makes up the heme

Answer 6

FeII and porphyrin ring

Question 7

Where is the iron located?

Answer 7

in the middle of the heme, held in place by chelation of the ring; the ironII binds to teh N atoms of the porphyrin ring.

Question 8

Function of HisF8

Answer 8

binds to the iron of the heme via COVALENT bonds

Question 9

Function of HisE7

Answer 9

Distal histidine. Hydrogen bonds to the o2 atom to prevent it from irreversibly damaging the Fe II via oxidation.
sterically prevents optimal binding of CO, reducing affinity. CO cannot form a perfect angle.

Question 10

How many ligands does the iron in the heme have?

Answer 10

6.
4 pyrrole rings that make up the porphyrin ring
1 covalent bond to proximal histidine F8
1 pseudo-attachment to oxygen molecule. ( not actually attached)

Question 11

What is the distal his called in mb? in beta Hb? in alpha hb?

Answer 11

distal in mb: His64
Distal in beta Hb: His63
Distal in alpha Hb: His58

Question 12

What do hydrophobic side chains in the globin structure do? What specific amino acids hold the heme in place?

Answer 12

creates hydrophobic pocket

• made of nonpolar amino acids
• Valine E2 and Phenylalanine CD1 hold the heme in place (holds the porphyrin ring)

Question 13

What happens when Fe2 gets oxidized? what prevents this from happening?

Answer 13

Globin protects iron from irreversible oxidation

• when exposed to oxygen, the Fe2 atom is irreversibly oxidized to Fe3 and cannot bind to another oxygen.
• the close approach of oxygen to Fe2 is prevented by distal histidine; holds the o2 away from Fe2 via hydrogen bonds.

Question 14

What causes the browning of old meat?

Answer 14

When Fe2 oxidizes to Fe3, metMb or metHb is formed- there is no oxygen in the blood because Fe3 has no affinity for it anymore.

Question 15

where would you find neuroglobin?

Answer 15

in the brain, retina, and endocrine tissues

-protects neurons from inadequate blood flow

Question 16

What shape is the myoglobin binding curve?

Answer 16

Hyperbolic Binding Curve

Question 17

When will a molecule exhibit a hyperbolic binding curve

Answer 17

when ligands interact independently with their binding sites. (there is no cooperativity)

Question 18

How many components make up hemoglobin

Answer 18

Hb is a tetramer. 2 alpha units and 2 beta units make 2 alpha-beta dimers that allow rotation

Question 19

T state

Answer 19

state where Hb has low affinity for O2. alpha beta dimers are held together via hydrophobic interactions, salt bridges and Hbonding are present.

Question 20

R state

Answer 20

state where Hb has high affinity for O2. O2 binds to the Hb. tetramer breaks the salt bridges and releases H+ ions and constricts itself. changing shape.

Question 21

Define Allosteric

Answer 21

when the binding of one ligand affects the binding affinity of the other ligands

Question 22

Why is hemoglobin a cooperative protein?

Answer 22

because it is allosteric. O2 binding to one subunit increases the O2 affinity of the remaining subunits

Question 23

What shape is the oxygen binding curve for hemoglobin/

Answer 23

sigmoidal. As the PO2 decreases, the affinity for oxygen decreases (when there is less o2 in the surroundings, the hb releases the O2 ie, in muscles). As the PO2 increases, the affinity for oxygen increases (when there is more O2 in the surroundings, the HB binds O2, ie in the lungs)

Question 24

starting from t state, describe the conformational changes when oxygen binds to the heme

Answer 24

T state: the iron is in the middle of the porphyrin ring, and the entire heme is connected to the globin via the proximal histidine. When no o2 is bound, the heme domes out towards the HisF8, causing the Fe to pop out of the plane of the ring because the N bonds are too long.

Transition: when O2 binds to the distal histidinevia hydrogen bonding, the Fe-N bonds shorten, and the iron begins to lie flat in the plane of the ring.

Because the Fe is covalently bonded to the HIsF8, the His F8, and thus the rest of the globin, shifts and constricts.

As one part of the tetramer shifts, the rest of the tetramer also shifts (coorperativity)

During T–> R transition, the salt bridges and ion pairs get destroyed as the subunits shifts. Carboxyllic group at beta terminus gets de protonated and releases H+ ions

Question 25

what is bohr effect

Answer 25

increasing the pH increases Hb affinity for oxygen.

Decreasing the pH decreases Hb affinity for oxygen

Question 26

Describe Bohr effect in the capillaries

Answer 26

in the muscles, where respiration takes place, CO2 is produced. It dissolves to form bicarbonate, but protons are also released. (HCO3 + H+), thus everytime respiration occurs, the pH drops mroe and more. Combined with the fact that HB gives off its own H+ when it accepts oxygen, the pH generally drops According to Bohr, the decrease in pH stimulates the decrease in O2 affinity of Hb. the protons in the environment bind with the Hb, reforming the salt bridges and ion pairs, stabilizing T state and causing the Hb to unload the oxygen into the deoxygenated (acidic) tissues.

Question 27

Describe Bohr effect in the lungs

Answer 27

when O2 conc is high and the CO2 conc. is low, causing a higher and more stable pH, the Hb converts from T state to R state, releasing protons and breaks salt bridges, and accepting an oxygen due to its higher affinity. Protons recombine with the bicarbonate in the blood to eliminate CO2, which is transfered to the lungs and exhaled..

Question 28

How does ion pairing and salt bridges stabilize the t state?

Answer 28

ion pairing increases pk values, stabilizing the structure. increase pk= less acidic=less likely to deprotonate (change structure)=more stable. When the ion pairs and salt bridges break, the pk lowers, and the structure changes conformation to R state, and pk is lower.

Question 29

Which amino acids are involved in salt bridge formation?

Answer 29

His146 and Asp84.

Question 30

What state does BPG stabilize?

Answer 30

stabilizes T state by decreasing affinity for O2 when PO2 is low.

Question 31

How does BPG interact with Hb? Why does it only bind to deoxygenated Hb?

Answer 31

BPG has 8 negative charges that interact with N terminus of the beta helices. BPG forms Hbonds and form salt bridges with 8 positive cahrges on Nterminus in the middle of the Hb.

BPG only can bind to deoxygenated state because when Hb is in R state, the Hb constricts and salt bridges are gone, pushing the BPG out.

Question 32

How does BPG help with people in higher altitudes?

Answer 32

body produces more BPG and RBS. The O2 binding curve shifts more right,, decreasing the affinity for O2 when PO2 is low, allowing more O2 to be released into the muscles.

Question 33

Difference between adult and fetal hemoglobin

Answer 33

Adult: A2B2
Fetus: A2G2

Question 34

Why does fetal hemoglobin have a lower affinity for BPG than Adults?

Answer 34

BPG binds to adult hemoglobin more tightly.

Fetal hemoglobin has serine 143 instead of histidine 143. In adults, his143 participates in salt bridge formation, and its positive charges interact with the negative charges on the BPG molecule, allowing tighter binding. However ,inn fetal Hb, the serine is neutral, and there is no salt bridge/ ion interactions between BPG and serine, lowering the strength of the bond.

Question 35

Why do you get dizzy if you respire rapidly?

Answer 35

when you breathe too fast, you release the CO2, which lowers the release of O2. CO2 typically triggers O2 release because the presence of CO2=decrease ph=decrease affinity=Hb lets go of oxygen, which goes to the muscles. therefor, if you do not have enough CO2, no O2 will be released.

Question 36

How does CO2 act as an allosteric affector?

Answer 36

CO2 decreases affinity and stabilizes T state via ph decrease.

CO2 also binds to N terminus of amino groups to form carbamates. The negative charge of the carbamates helps stabilize salt bridges.

Question 37

How does Cl- act as an allosteric affector?

Answer 37

acts as a chloride-bicarbonate exchanger. Chloride move into the red blood cell, allowing HCO3- to move out into the plasma via gradient. HCO3- then has an opportunity to combine with H+ to form CO2, which can then get exhaled.

Question 38

What would happen if a histidine (B146) was mutated to an aspartate?

Answer 38

his is positively charged and participates in salt bridge formation. aspartate is neutral, and thus cannot participate in salt bridge formation, destabilizing the T state.

Because T state is not favored, O2 affinity goes up, and more protons would be released. You will lose contribution of Bohr Effect because O2 will start binding too much– the affinity is higher and thus the R state is favored.

Decreasing ph no longer contributes to T state stabilization because O2 affinity is too high

Question 39

What mutation causes sickle cell anemia?

Answer 39

B6 glutamate to valine

Question 40

How does sickle cell anemia protect against malaria?

Answer 40

malaria parasite uses up O2, which stabilizes T state, decreases pH, which also stabilizes T state, and increases potassium concentration, which weakens RPB membranes.

The cells begin to sickle even more, and the liver removes it.

Question 41

Why would runners train at high altitude instead of sealevel?

Answer 41

at high altitude, your body would make more bpg, which acts as an allosteric effector that stabilizes T state. With the increase amount of BPG in the blood, more oxygen will be released by Hb due to its decreasing affinity.

Question 42

Does hyperventilation prior to a dive help increase O2 concentration?

Answer 42

no. hyperventilation gets rid of CO2. this may even decrease O2 release because CO2 acts as an allosteric effector by forming H+ and bicarbonate, lowering the pH and thus decreasing the affinity for O2 and stabilizing T state. If you get rid of your CO2, there will be nothing driving the Hb to release the O2 to the tissues, locking everything in R state.

Generally, it will not increase the concentration of O2 in the body because the body has no problem getting oxygen, it has a problem releasing it. In the lungs (Arterial blood) , Hbs are most likely already saturated with oxygen on their own, they don’t need to “increase concentration.”

Question 43

Crocodile Hb does not bind to BPG. Their Hb preferentially binds to HCO3- instead. How does this help the crocodile obtain its dinner?

Answer 43

HCO3- is created when CO2 is produced, ie, when the crocodile is holding its breath, along with H+. HCO3- and H+ act as allosteric affectors, as they stabilize T state. Typically, only H+ can bind to the hb, which forms the salt bridges and causes the Hb to release some of the oxygen, but the HCo3- does not interact with the Hb. The crocodile, however, can bind HCO3- as well as H+, allowing even more O2 to be released to the tissues, up to 100% of the O2 can be released. more CO2 will be converted to HCO3- as the crocodile holds its breath underwater, causing Hb decrease its affinity more and more, releasing more and more O2.

Question 44

What state is the hemoglobin in when a person has sickle cell anemia?

Answer 44

Hb locked into t state because valine forms hydrophobic polymers that lock into the center of hydrophobic pockets, decreasing affinity for O2.

Question 45

A person with sickle cell anemia often has elevated levels of BPG. Is this good or bad?

Answer 45

Good because they get increased Oxygen output by decreasing the affinity for O2, which is beneficial because generally a person with sickle cell anemia has a hard time getting oxygen to the tissues. It is also bad because BPG stabilizes T state, which promotes more valine polymerization, making the disease worse by forming more hydrophobic knobs, which fit into other hydrophobic pockets.

Question 46

Increasing the volume of air that reaches the lungs and takes part in gas exchange will cause blood to :

Answer 46

increase, because the partial pressure of CO2 in the blood will decrease.

Question 47

Which are not ligands to the porphyrin ring?

1) HisF8
2) N
3) O
4) HisE7

Answer 47

His E7. Distal histidine does not truly bind to the ring. it binds to the oxygen via Hbonds

Question 48

A prosthetic group of a protein is a non-protein structure that is:

Answer 48

permanently associated with the protein

Question 49

Why can’t you just have Hb? Why do you need a Blood cell?

Answer 49

1) RBC is the site for BPG production, which stabilizes T state and allows Hb to release O2 to tissues.
2) RBC has repair mechanisms and enzymes
3) RBS protects Hb
4) RBC hold the Hb together. If the alpha beta dimers, spread out too much, it can cause damage to the liver.
5) RBC is the site of chloride-HCO3- exchange. Cl transfers into RBC, allowing HCO3 to move out into the plasma, where it combines with water for form CO2, where it diffuses from plasma to lungs and gets exhaled.
6) Site where carbonic anhydrase is present. Allows efficient conversion of Co2 to HCo3- and H+, and back to CO2, where it can be exhaled. Helps stabilize T state by decreasing ph levels by converting Co2 to Hco3- efficiently.

Question 50

An individual Hb subunits and myoglobin subunits share the same ___structure but have different ____ structures.

Answer 50

secondary/tertiary, different primary.

Question 51

How would allosteric affectors effect mb?

Answer 51

they won;t. Mb is not an allosteric protein and does not exhibit cooperativity. It displays a hyperbolic binding curve, its affinity is not affected by anything, it does not change.

Question 52

Where is BPG most concentrated?

Answer 52

BPG concentration is constant between lungs and tissues. It simply only works in the tissues when it binds to the Hb to release oxygen.

Question 53

What part of the Hb does BPG bind to?

Answer 53

beta helices, in the center cleft. forms salt bridges with beta-lys82, his143, His2.