Lecture 6: Hemoglobin

Question 1

Porphyrin

Answer 1

Heme-containing protein

Question 2

Heme prosthetic group

Answer 2

Planar porphyrin ring with ferrous (Fe2+) ion in center ring

Question 3

Hemoglobin vs myoglobin

Answer 3

Myoglobin: heme-containing monomer with hyperbolic O2 curve that stores oxygen in muscle.

Hemoglobin: heme-containing tetramer with sigmoidal O2 curve that transports oxygen in RBCs.

Question 4

HbA structure

Answer 4

Tetramer with 2 alpha chains and 2 beta chains (2 alpha-beta dimers). Transports 4x O2

Question 5

HbF structure

Answer 5

Tetramer with 2 alpha chains and 2 gamma chains (instead of beta)

Question 6

Hb T and R states

Answer 6

Taut (T) = deoxyHb form
Relaxed (R) = oxyHb form

Question 7

How does oxygen binding affect the structure of hemoglobin?

Answer 7

O2 binding pulls the Fe2+ more into the heme ring plane. This movement ruptures some of the polar bonds between the Hb dimers and makes further O2 bonding more stable

Question 8

Hb vs Mb oxygen binding curves

Answer 8

Mb has a hyperbolic curve and is always higher affinity vs Hb. Hb’s sigmoidal curve reflects cooperative binding behavior.

Question 9

Under what allosteric conditions is T Hb most stable?

Answer 9

Taut Hb (low O2 affinity) is stabilized by low pH, low pO2, high pCO2, and high 2,3-BPG concentrations. Opposite stabilizes R Hb

Question 10

Bohr effect

Answer 10

Release of oxygen by Hb is enhanced by low pH and high pCO2. High CO2 also creates more bicarb + H+, lowering pH even more.

Question 11

Why is allosteric regulation of Hb useful?

Answer 11

Metabolically active tissues produce 2,3-BPG, CO2, and H+ and demand more oxygen. Allosteric regulation of Hb O2 affinity supports rapid oxygen unloading/delivery in areas that need it most.

Question 12

What is HbF’s O2 dissociation curve like?

Answer 12

HbF also has a sigmoidal O2 curve, but is slightly left-shifted compared to HbA. Higher O2 affinity enables HbF to take oxygen from HbA across the placenta

Question 13

Thalassemia

Answer 13

Loss of or reduction in production of alpha or beta chain. Results in low levels of functional Hb and more RBC turnover.

Question 13

When does fetal hemoglobin go away?

Answer 13

Fetal hemoglobin persists at high levels until around 6 months post-natal.

Question 13

Sickle cell disease

Answer 13

Caused by Glu->Val at position 6 on beta chain. Deforms RBC into inflexible sickle shape. Blocks microvessels, causes local anoxia/lactic acid build up, self-associates other RBCs to sickle, causes vaso-occlusive crises.

Question 14

HbC disease

Answer 14

Glu->Lys at position 6 on beta chain. Mild, chronic hemolytic anemia

Question 15

Degrees of severity of alpha-thalassemia

Answer 15

1 deletion = silent carrier
2 deletions = minor
3 deletions = HbH disease
4 deletions = Hb Bart; fatal hypoxia by neonatal age

Question 16

HbH disease

Answer 16

Form of alpha-thalassemia where beta-4 Hb tetramers (HbH) bind O2 with high affinity and no cooperativity. RBCs precipitate out over time.

Question 17

HbE disease

Answer 17

Form of beta-thalassemia with a single point mutation in the beta chain. Alpha-4 tetramers lack cooperativity and are insoluble

Question 18

Degrees of severity of beta-thalassemia

Answer 18

Beta-thalassemia minor: 1 of 2 beta genes is mutated, mild.

Beta-thalassemia major: no functional beta genes. Also Mediterranean/Cooley’s anemia. Abnormal growth, development, and bone formation (chipmunk face) and iron deposition on organs due to transfusions/RBC turnover.

Question 19

Hemoglobin A1c

Answer 19

Hb A1c is irreversibly glycated Hb. Normally 4-6% of adult Hb, extent depends on blood glucose. Serves as indicator for average blood glucose over time.

Question 20

Methemoglobinemia

Answer 20

Oxidation of ferrous (Fe2+) to ferric iron (Fe3+) in heme. Results in high O2 affinity + no O2 delivery. Normally <1%, levels >50-60% fatal.

Question 21

Maintenance of methemoglobin

Answer 21

1. H2O2 cleared by reduced glutathione (GSH)
2. NADH/NADPH reduce MetHb via Methemoglobin Reductase enzyme.

Question 22

Congenital methemoglobinemia

Answer 22

Caused by globin mutations stabilize ferric iron and make it more resistant to reduction by MR. Alternatively, mutations in enzymes that reduce metHb to Hb.

Question 23

Acquired methemoglobinemia

Answer 23

Caused by use/exposure to oxidizing drugs, chemicals, or toxins. Increased metHb production overwhelms physiological reduction mechanisms.