Hemoglobin and Myoglobin

Question 1

Heme

Answer 1

Iron-chelated porphyrin prosthetic group in hemoglobin and myoglobin. Heme B is principle heme in oxygen-binding proteins.

Question 2

What is Heme B composed of?

Answer 2

Protoporphyrin IX (contains 4 linked pyrrole groups) and ferrous iron (Fe2+)

Question 3

Hemoglobin structure

Answer 3

Globular, tetrameric protein (4 non-covalently linked subunits: 2 _ type and 2 _ type) with hydrophobic residue patches which facilitate quarternary structure. Eight helices connected by _ turns-E and F helices bind to heme group. 2 polar carboxylic groups interact with Fe2+

Question 4

What parts of the hemoglobin chain are invariant?

Answer 4

Proximal His (stronger bond than distal His because closer), distal His, and hydrophobic heme binding residues

Question 5

Myoglobin structure

Answer 5

Single polypeptide chain protein and one O2 binding site. Eight _ helices connected by _ turns (just like hemoglobin).

Question 6

Apoprotein form of hemoglobin/myoglobin

Answer 6

Does not contain heme and cannot bind oxygen

Question 7

What happens to the amino acid stucture when oxygen molecule binds?

Answer 7

Oxygen molecule binds to the ferrous ion replacing the distal His

Question 8

Which chromosomes contain the genes for hemoglobin?

Answer 8

3 _ genes are on chromosome 16 and 4 _ genes are on chromosome 11

Question 9

_-type subunits

Answer 9

_1, _2, _

Question 10

_-type subunits

Answer 10

, , _, G, A

Question 11

HbF chain composition

Answer 11

_2_2

Question 12

HbA1 chain composition

Answer 12

_2_2

Question 13

HbA2 chain composition

Answer 13

_2_2

Question 14

HbGrowler chain composition

Answer 14

_2_2

Question 15

HbPortland chain composition

Answer 15

_2_2

Question 16

Hemoglobin T-state (Perutz model)

Answer 16

Tense state where ferrous ion out of plane of heme due to steric hinderance

Question 17

Hemoglobin R-state (Perutz model)

Answer 17

Relaxed state where ferrous ion moves to the center of the porphyrin ring. Energetically more favorable state.

Question 18

Steps involved in T-state to R-state transition

Answer 18

1. Oxygen binds 2. Iron atom moves 3. Proximal His moves 4. F helix moves 5. Entire globin tertiary structure moves

Question 19

BPG (sometimes called DPG)

Answer 19

2,3-biphosphoglycerate synthesized from glucose in glycolysis. Has high affinity for T-state Hb. Produced in high quantities in RBCs. Binding of BPG to Hb shifts equilibrium towards T-state promoting release of oxygen. One molecule of BPG binds to positively charged cavity formed by _ chains.

Question 20

Hypoxia

Answer 20

Can be caused by anemia, smoking, or high altitude. BPG levels increase, promoting release of oxygen to tissues. In anemia RBC levels lower than normal so oxygen transport is low. Smoking/CO poisoning blocks binding of oxygen to Hb (CO will displace oxygen with 1000% higher affinity). In high altitude oxygen pressure is lower and Hb will not saturate 100%–body responds by making more Hb and BPG.

Question 21

“Oxygen trap”

Answer 21

Storage of blood in presence of anticoagulant causes lower levels of BPG in RBCs so poor oxygen release.

Question 22

HbF effects on mother

Answer 22

HbF has higher oxygen binding affinity than HbA. _ subunits do not bind BPG well so oxygen binding curve shifts to the left of the mother’s. This facilitates flow of oxygen from mother to fetus.

Question 23

Where is the oxygen dissociation curve the steepest?

Answer 23

At concentrations in the tissues so oxygen delivery to respond to small changes in oxygen concentration.

Question 24

Myoglobin oxygen dissociation curve

Answer 24

Hyperbolic curve (saturation=Y). Because of this myoglobin would be a poor carrier of oxygen, but good for storage. Fully saturated in tissue.

Question 25

Hemoglobin oxygen dissociation curve

Answer 25

Sigmoidal curve. This is because of cooperative binding of the 4 subunits.

Question 26

Alkaline Bohr Effect

Answer 26

When Hb binds to oxygen with 100% saturation in the lungs, pH is higher than periphery and switches to R state. Ionic interactions between subunits break and release protons. Opposite happens in tissues.

Question 27

Ways that Hb can transport CO2

Answer 27

1. Chemically bound to free amino terminus of globin subunit in carbamate form (this stabilizes T-state and helps unload oxygen) 2. Coupled to proton transfer in the bicarbonate buffer system

Question 28

Hyperventilation

Answer 28

Causes decrease in CO2 concentrations in blood which results in left shift in the oxygen dissociation curve and less oxygen delivered (via Bohr effect).

Question 29

Blood transfusion problems

Answer 29

Stored blood loses BPG and cannot release all of its oxygen, quickly loses NO so inefficient gas exchange, delivers extra Hb thus iron so risk of transfusional hemosiderosis

Question 30

Ways that Hb can transport NO

Answer 30

1. Reversibly binds to Hb and gets delivered to receptors of cells of the blood vessel wall. This facilitates transfer of gases between blood and tissues. 2. NO bound to R-state and released when Hb releases oxygen and changes to T-state. This saves NO from rapid destruction in blood.

Question 31

Nitric oxide (NO)

Answer 31

Very potent vasodialator

Question 32

HbS mutation

Answer 32

_6 (Glu-->Val) causes sickle cell anemia

Question 33

HbC mutation

Answer 33

_6 (Glu-->Lys) causes mild anemia and crystals in RBCs

Question 34

HbM mutation

Answer 34

_58 (HbM-Boston) or _63 (HbM-Saskatoon) (His-->Tyr) causes methemoglobin, cyanosis

Question 35

Methemoglobinemia

Answer 35

Most frequent genetic mutation occurs at distal or proximal His. If genetic either the enzyme methemoglobin reductase or the globin is mutated. If acquired not enough reducing agent (excess of oxidizing agent). Ferrous iron gets oxidized to ferric and cannot bind oxygen. Treat with IV ascorbate and methylene blue.

Question 36

Thallasemia

Answer 36

Autosomal recessive disorder where one of the globin genes not expressed properly, leads to anemia. Abnormal quarternary structures result depending on mutation.

Question 37

Effects on oxygen binding of 1. oxygen concentration 2. CO2 concentration 3. low pH 4. BPG concentration 5. temperaure

Answer 37

1. High O2 promotes oxygen binding 2. High CO2 weakens oxygen binding 3. Weakens oxygen binding 4. High BPG weakens oxygen binding 5. High temp weakens oxygen binding