Hemoglobin Notes

Hemoglobin Structure and Function

• Overview: Hemoglobin enables oxygen transport in blood; Myoglobin is a related heme protein with different physiological roles.

Myoglobin vs. Hemoglobin

• Myoglobin binds oxygen at the heme group with a relatively simple, non-cooperative binding profile; hemoglobin binds O₂ cooperatively due to its quaternary structure.
• Structural context (from figures referenced): Myoglobin vs. hemoglobin graphs show fractional saturation (Y) versus pO₂; Myoglobin saturates at lower pO₂, whereas hemoglobin shows a sigmoidal (cooperative) curve as pO₂ increases.
• Visual cues from figures: Myoglobin tends to have a higher affinity at lower pO₂ than hemoglobin, facilitating rapid uptake in muscle tissue; hemoglobin’s cooperative binding allows efficient loading in lungs and release in tissues.

Hemoglobin Structure

• Hemoglobin is a tetramer composed of two α and two β subunits; each subunit contains a bound heme group.
• Quaternary organization: Best described as a pair of identical αβ dimers (α1β1 and α2β2).
• In deoxyhemoglobin, the molecule is in the T state (tense state) with tight inter-dimer interfaces; this conformation predisposes to lower affinity for O₂.
• Oxygen binding triggers a transition to the R state (relaxed state) via structural changes that propagate to other subunits.
• The transition involves rotation of the αβ dimers relative to one another, resulting in the R state.
• The iron (Fe) ion moves into the plane of the heme when O₂ binds; the proximal histidine (a component of an α-helix) moves with the iron.
• The net effect is cooperative binding, with positive cooperativity for O₂ binding.

Heme Group Details

• Protoporphyrin ring coordinates an Fe atom at its center via four nitrogens of the heme
• The Fe atom has two additional coordination sites:
  • 5th coordination site: imidazole nitrogen of a proximal histidine
  • 6th coordination site: oxygen (O₂) binding site when O₂ is bound
• Consequences: Binding of O₂ to Fe alters the geometry of the heme and communicates to the protein, triggering the T→R transition.

Conformational Changes and Quantitative Details

• The Fe atom moves approximately $\boxed{0.4 ext{ Å}}$ from the plane of the porphyrin ring when going from deoxy to oxy form.
• The iron position change is accompanied by movement of the proximal histidine; this transmits conformational change to the rest of the molecule.
• The 15° rotation between the αβ dimers is a characteristic rotation observed between deoxy (T) and oxy (R) forms.
• The T to R transition is triggered by oxygen binding and is accompanied by subunit rearrangements that increase O₂ affinity in the presence of O₂.

Allosteric Regulation of Oxygen Binding

• 2,3-Bisphosphoglycerate (2,3-BPG) is a critical allosteric regulator:
  • Stabilizes the T state and thus promotes O₂ release in tissues.
  • Binds to a pocket that exists only when hemoglobin is in the T state.
  • This interaction lowers Hb’s affinity for O₂, facilitating delivery to tissues.
• The Bohr effect (involving pH and CO₂):
  • CO₂ and H⁺ produced by respiring tissues promote O₂ release by stabilizing the T state.
  • The result is a rightward shift of the Hb-O₂ dissociation curve (higher pO₂ required for the same saturation) under conditions of low pH or high CO₂.
  • This effect enhances O₂ delivery to actively respiring tissues.
• Structural basis of the Bohr effect includes carbamate formation at the amino termini of subunits, which introduces negative charges that promote ionic interactions at the dimer-dimer interface (favoring the T-state), and His146 protonation at lower pH stabilizes interactions with Asp94, reinforcing the T-state.

Fetal Hemoglobin (HbF)

• HbF composition: α₂γ₂ instead of α₂β₂ (γ chains replace β chains).
• HbF does not bind 2,3-BPG as well as adult Hb, resulting in a higher affinity for O₂.
• Higher oxygen affinity is advantageous in the placenta, enabling fetal Hb to bind O₂ released from maternal Hb.
• Reference example: NCBI/PDB Motif discussion for HbF vs HbA, with structural and binding implications.

Hemoglobin Variants and Comparative Aspects

• Bar-headed goose Hb vs. human Hb: Notable amino acid substitutions at specific positions (example positions shown include Met55, Val33, Leu55, Ile33, Pro119, Ala119, Glu26, Arg30; exact substitutions vary per sequence comparison).
• These substitutions can influence properties such as oxygen affinity and thermal stability, illustrating how sequence variation can adapt Hb function to different physiological conditions.

Normal Adult Hemoglobin: True/False Style Takeaways

• In the deoxy form, the iron atom is within the plane of the heme ring. ⇒ True
• 2,3-BPG binds to a negatively charged pocket at the dimer-dimer interface. ⇒ True
• Only the beta subunits contain heme groups. ⇒ False (both α and β subunits have heme)
• Oxygen binds to hemoglobin at the iron atom of the heme ring. ⇒ True

Why Hemoglobin Delivers Oxygen Better Than Myoglobin

• Option assessment (conceptual):
  • Myoglobin has high affinity for O₂ and binds readily in the lungs, aiding uptake into muscle tissue.
  • Hemoglobin provides cooperative binding and allostery, enabling efficient loading in lungs and efficient unloading in tissues across varying pO₂ conditions.
  • Correct conceptual statement: Hemoglobin delivers O₂ effectively due to its cooperative binding and allosteric regulation; it binds O₂ with relatively lower affinity at tissue pO₂ to promote release, while myoglobin binds more tightly and is better for storage in muscles.
• From the provided choices, the statement that best aligns with Hb's behavior in tissues is: "Hemoglobin binds oxygen with low affinity at partial pressures in the tissues" (i.e., the Hb-O₂ curve shifts to release O₂ where tissue pO₂ is low).

Effects of Diabetic Ketoacidosis (Lower pH) on Hb-O₂ Transport

• In lower blood pH, hemoglobin tends to hold onto O₂ less effectively in distal tissues, promoting release to tissues that need it.
• Practical implication: Acidosis enhances oxygen delivery by promoting O₂ release from Hb.

Carbon Monoxide (CO) Binding to Hb

• CO binds to the heme iron with ~240× higher affinity than O₂ and stabilizes the R-state.
• Effect on oxygen-binding curve: Shifts the curve to lower pO₂ for oxygen binding (leftward shift), meaning higher apparent affinity and reduced O₂ delivery to tissues due to impaired release.
• Clinical relevance: CO poisoning reduces O₂ delivery by both occupying Hb and hindering O₂ release.

Sickle Cell Anemia (HbS)

• Genetic basis: Mutation at position 6 of the β chain (Glu → Val; Glu6Val).
• Consequences: HbS tends to polymerize in the deoxy (T) state, forming aggregates that deform red blood cells.
• Genotype-phenotype distinctions:
  • Sickle cell disease (HbSS): both β chains mutated, often fatal.
  • Sickle cell trait (HbAS): one mutated allele; typically asymptomatic.
• HbS polymerization occurs when HbS is deoxygenated, leading to sickled, rigid red blood cells that can occlude capillaries.

HbSC Disease and Fiber Formation

• HbSC disease results from a compound heterozygous state where one β-chain carries the HbS mutation and the other carries the HbC mutation.
• Resulting fibers and sickling can still cause occlusion of capillaries, albeit with different severity compared to HbSS.
• The sickled cells contribute to tissue damage due to impaired blood flow.

Visual and Reference Notes

• Figures referenced (e.g., Figure 9.2, Figure 9.3, Figure 9.4, Figure 9.8) illustrate Y vs. pO₂ curves, structural transitions, and Fe movements.
• For deeper reading and structural visuals, see sources noted in the transcript, including standard biochemistry texts and PDB references.

Quick Reference Equations and Key Notations

• Fractional saturation: $Y = \frac{[Hb ext{-}O<em>2]}{[Hb]</em>{ ext{total}}}$

• Fe movement upon O₂ binding: ext{Fe displacement}
  ightarrow oxed{0.4 ext{ Å}}

• Heme coordination:

  • 5th coordination: $ext{proximal histidine (imidazole)}$
  • 6th coordination: $ext{O}_2 ext{ binding site}$

• Bohr effect: lowering pH or increasing CO₂ shifts the O₂ dissociation curve to the right (relative to pO₂) and stabilizes the T-state, promoting O₂ release in tissues.

• 2,3-BPG binding: Stabilizes T-state by occupying a pocket present only in T-state Hb.

• Fetal Hb: $ext{HbF} = ext{α}<em>2 ext{γ}</em>2$; reduced binding of 2,3-BPG → higher O₂ affinity.

• Sickle cell mutation: ext{β} ext{ chain position 6}: ext{Glu}
  ightarrow ext{Val}

• Notes on variants (Bar-headed goose vs human): Key substitutions at multiple sites (examples include Met55 vs Leu55, Val33 vs Ile33, Pro119 vs Ala119, Glu26 vs Glu26, Arg30 vs Arg30) that affect Hb properties relevant to high-altitude flight adaptation in geese.

• Summary theme: Hemoglobin’s function arises from its regulated allostery (T/R states), cooperative substrate binding, and modulating factors (2,3-BPG, pH/CO₂, CO, fetal vs adult Hb) to optimize O₂ loading in lungs and unloading in tissues under physiological conditions.