Hemoglobin – Comprehensive Notes

Hemoglobin – Comprehensive Study Notes

1) Hemoglobin: Overview

  • High-efficiency transporter of oxygen (O₂) and carbon dioxide (CO₂)

  • Facilitates gas exchange between lungs and tissues

  • Also contributes to maintaining acid–base balance

  • Makes up about 95% of red blood cell (RBC) cytoplasmic contents

  • Free Hb has a short life outside of the RBC; Hb is carried in RBCs to protect from denaturation and loss

  • Biconcave RBC shape provides maximum surface area for gas exchange

2) Hemoglobin Synthesis

  • Hemoglobin is a large, globular protein composed of:

    • 4 globin chains

    • Each globin chain attached via iron to a heme molecule

  • Synthesis occurs mainly during the nucleated stage of RBC maturation

  • Requirements for production:

    • Adequate globin synthesis

    • Adequate synthesis of protoporphyrins (heme precursor)

    • Adequate iron supply

  • Total Hb synthesis involves coordination of globin and heme production (see figures and pathways below)

3) Heme Synthesis (Key Points)

  • Synthesis occurs in mitochondria and cytoplasm of RBC precursors

  • Core reaction:

    • Protoporphyrin IX + Fe^{2+}
      ightarrow ext{Heme}

    • Each iron reversibly binds to one O₂ molecule when in the ferrous state

  • If iron is oxidized to ferric state (Fe^{3+}), it can no longer bind O₂

  • The synthesis pathway proceeds through several intermediates and enzymes (see Pathway below)

  • Heme is a prosthetic group bound to globin chains to form Hb

4) Iron Supply and Delivery

  • About two-thirds (2/3) of body iron is bound in heme

  • Iron undergoes redox cycling between ferric (Fe^{3+}) and ferrous (Fe^{2+}) forms during transport and utilization

  • Plasma iron transport is mediated by transferrin

5) Heme Synthesis & Regulation — Pathway Outline

  • Location: Mitochondria and Cytosol

  • Steps (glossing the sequence):
    1) Glycine + Succinyl CoA → ALA via ALA synthase (mitochondria)
    2) ALA → Porphobilinogen (PBG) via ALA dehydratase (cytosol)
    3) PBG → Hydroxymethylbilane via PBG deaminase
    4) Hydroxymethylbilane → Uroporphyrinogen III synthase
    5) Uroporphyrinogen III decarboxylase → Coproporphyrinogen III
    6) Coproporphyrinogen oxidase → Protoporphyrinogen IX
    7) Protoporphyrinogen oxidase → Protoporphyrinogen IX/III (context-dependent terminology)
    8) Protoporphyrinogen IX → Protoporphyrin IX
    9) Ferrochelatase + Fe^{2+} → Heme

  • Key points:

    • The pathway yields heme (with Fe^{2+}) as a final product that binds to globin chains

    • The process is regulated; synthesis is coordinated with globin production

  • Note: The figures illustrate the core flow and participation of both mitochondrion and cytosol

6) Hemoglobin – Final Product

  • 4 globin chains + 4 heme groups + 4 iron ions = 1 hemoglobin molecule

  • Visual reference: final product schematic from Keohane/Harmening figures

  • Formula representation:
    extHb=4extglobinchains+4exthemegroups+4extFe2+extionsext{Hb} = 4 ext{ globin chains} + 4 ext{ heme groups} + 4 ext{ Fe}^{2+} ext{ ions}

7) Globin Chains and Genes

  • Globin chains come in 6 types coded by six genes

  • Each globin chain length (amino acids):

    • α (alpha): 141 aa

    • β (beta): 146 aa

    • γ (gamma): 146 aa

    • δ (delta): 146 aa

    • ε (epsilon): 146 aa

    • ζ (zeta): 141 aa

  • Chromosomal location:

    • α-like genes on chromosome 16

    • β-like genes on chromosome 11

  • Globin synthesis: occurs via mRNA transcription in the nucleus, ribosome translation in cytosol, and assembly with heme to form heterodimers; two heterodimers form the Hb tetramer

  • Figure references: Globin chains overview and synthesis steps (Figures 8.3–8.5 in Harmening/Keohane)

8) Globin Ontogeny and Hemoglobin Types

  • Hemoglobin composition changes with gestational and postnatal age due to switching of expressed globin genes

  • Mechanism: promoter sequences and hematopoietic organ changes drive expression patterns

  • Adult Hb types:

    • HbA: Hb = 2 (α2β2) > 95%

    • HbA2: α2δ2 < 3.5%

    • HbF: α2γ2 1–2%

  • Fetal/newborn Hb types:

    • HbF: α2γ2 60–90%

    • HbA: α2β2 0–40%

  • Embryonic Hb types:

    • Gower I:
      extGowerI: ζ<em>2ϵ</em>2ext{Gower I: } \ \zeta<em>2 \epsilon</em>2

    • Gower II: extGowerII:α<em>2ϵ</em>2ext{Gower II: } \alpha<em>2 \epsilon</em>2

    • Portland: extPortland:ζ<em>2γ</em>2ext{Portland: } \zeta<em>2 \gamma</em>2

  • Site of hematopoiesis shifts from yolk sac → liver/spleen → bone marrow during development (Figure 8.6)

  • Schematic timeline (from intrauterine life to infancy shown in figure): globin chain synthesis and site of production change over time

9) Hemoglobin Structure — Globin and Heme Architecture

  • Globin is a globular protein; Hb is a tetramer with 2 α and 2 non-α chains; each globin chain has 8 helices per chain

  • Each Hb tetramer contains 4 heme groups; each heme contains an iron ion (Fe) that binds O₂

  • The heme group is held between helices within the globin structure

  • Visuals: Hb structure figures (Figure 8.4, 8.5) illustrate the tetrameric arrangement and heme positioning

10) Oxygen Transport and the Oxygen Dissociation Curve

  • Hb must bind O₂ readily in the lungs and unload it efficiently in tissues

  • The Hb-O₂ affinity is governed by partial pressure of O₂ (pO₂)

  • Oxygen Dissociation Curve:

    • Plot: % O₂ saturation vs. pO₂

    • Shape: Sigmoid (S-shaped) due to cooperativity

    • At low PO₂, Hb has low affinity; at high PO₂, Hb becomes highly saturated

    • Cooperativity: once one O₂ binds, the remaining sites become easier to saturate

  • Normal reference: P₅₀ (the PO₂ at 50% saturation) ≈ 26–30 mmHg under standard conditions

  • Reading the curve:

    • P_{50} ext{ – partial pressure of oxygen at which Hb is }50 ext{% saturated}

    • Under normal conditions (37°C, pH 7.4): P50extis26extto30extmmHgP_{50} ext{ is } 26 ext{ to }30 ext{ mmHg}

  • Curve shifts indicate changes in Hb affinity for O₂ (cooperativity maintained)

11) Curve Shifts: Left vs Right

  • Left shift (increased Hb-O₂ affinity):

    • Causes (from the text): decrease in temperature; increase in pH (Bohr effect is cited in the opposite direction for pH changes; here the note says increased pH) ; decreased 2,3-BPG

    • Implication: O₂ loading in lungs is favored, unloading in tissues is reduced

  • Right shift (decreased Hb-O₂ affinity):

    • Causes: increased temperature; decreased pH (Bohr effect); increased 2,3-BPG

    • Implication: O₂ unloading in tissues is favored, loading in lungs is less favorable

  • The curve illustrations commonly show left vs right shifts corresponding to these condition changes (Figures 3-14 and 3-15 references)

12) 2,3-Bisphosphoglycerate (2,3-BPG)

  • A red blood cell (RBC) organic phosphate produced during RBC metabolism

  • Allosteric modifier of hemoglobin

  • Structural states:

    • Deoxygenated Hb is in the tense (T) state

    • Oxygenated Hb is in the relaxed (R) state

  • 2,3-BPG binds between the β chains of hemoglobin, promoting a conformational change that reduces Hb's affinity for O₂

  • Its presence shifts the curve to the right (lower O₂ affinity at a given PO₂) and stabilizes the T state

  • Diagram reference: 2,3-BPG enters between β chains (Figure 3-14)

13) Bohr Effect

  • A shift in the O₂ dissociation curve caused by a change in pH

  • Mechanism: Increased CO₂ lowers pH, increasing the need for O₂ delivery to tissues

  • Hb acts as a buffer, helping to maintain acid-base balance (Keohane Keohane/Keohane-Keohane reference)

  • Practical implication: In tissues with high CO₂ (high acidity), Hb releases O₂ more readily (right shift)

14) Factors Affecting Hb-O₂ Affinity (Summary)

  • Temperature: ↑ temperature → right shift; ↓ temperature → left shift

  • RBC 2,3-BPG: ↑ 2,3-BPG → right shift; ↓ 2,3-BPG → left shift

  • pH: ↓ pH (acidic) → right shift (Bohr effect); ↑ pH (alkaline) → left shift

  • Overall: Right shift → decreased O₂ affinity (more O₂ delivery to tissues); Left shift → increased O₂ affinity (less O₂ delivery)

  • Expressed succinctly:

    • Right shift: extO<em>2extaffinityightarrowextdecreases,extP</em>50<br>ightarrowextincreasesext{O}<em>2 ext{ affinity} ightarrow ext{decreases}, ext{ P}</em>{50} <br>ightarrow ext{increases}

    • Left shift: extO<em>2extaffinityightarrowextincreases,extP</em>50<br>ightarrowextdecreasesext{O}<em>2 ext{ affinity} ightarrow ext{increases}, ext{ P}</em>{50} <br>ightarrow ext{decreases}

15) 2,3-BPG in Detail

  • 2,3-BPG is formed during RBC metabolism

  • It modulates Hb’s affinity for O₂ by stabilizing the T state

  • It is an allosteric modifier and directly affects the O₂ unloading capacity of Hb

  • It sits between the β chains of Hb in the deoxygenated state, promoting conformational changes that reduce O₂ affinity when deoxygenated

16) Dyshemoglobins

  • Types and features:

    • Methemoglobin (metHb): Reversible binding of Fe^{3+} instead of Fe^{2+}

    • Cannot bind O₂

    • Normal levels < 1%

    • Increased amounts can be acquired or hereditary; toxic when > 50%

    • Carboxyhemoglobin (COHb): Reversible binding of CO to heme iron

    • CO affinity is about 200× that of O₂

    • Toxic when > 40%

    • Sulfhemoglobin (sulfHb): Irreversible binding of sulfur to heme

    • Caused by drugs or exposure to sulfur-containing chemicals

    • Toxic when > 0.5%

17) Summary Points

  • Hemoglobin composition: globin portion, protoporphyrin ring, and iron

  • Oxygen dissociation is regulated by pH, pO₂, temperature, and 2,3-BPG

  • Left-shifted Hb-O₂ curve = increased O₂ delivery to tissues (not as readily releasing O₂ at tissues); Right-shifted curve = decreased O₂ delivery to tissues (more O₂ delivered readily when needed)

  • Dyshemoglobins: carboxyhemoglobin (COHb), methemoglobin (metHb), sulfhemoglobin (sulfHb)

  • Developmental changes in Hb composition reflect the switching of globin gene expression across gestation and postnatal life

18) Notable Figures/References Mentioned

  • Figure 8.5 and Figure 8.6 (Keohane 2025) illustrate Hb synthesis and ontogeny

  • Figure 8.3 (Hb chains) and Table 8.1 (Hb globin chains) summarize globin genes and amino acid lengths

  • Figure 3-14 and Figure 3-15 (Harmening, 2009) illustrate O₂ dissociation curve and shifts

  • Hb final product and heme illustrations from Harmening/Keohane sources

19) Quick Reference Formulas and Key Values

  • Hb stoichiometry (final product):
    extHb=4extglobinchains+4exthemegroups+4extFe2+extionsext{Hb} = 4 ext{ globin chains} + 4 ext{ heme groups} + 4 ext{ Fe}^{2+} ext{ ions}

  • Hb types in adults:

    • ext{HbA}: ext{α}2 ext{β}2 ext{ (> 95%)}

    • ext{HbA2}: ext{α}2 ext{δ}2 ext{ (< 3.5%)}

    • ext{HbF}: ext{α}2 ext{γ}2 ext{ (1-2%)}

  • Hb types in newborns:

    • ext{HbF}: ext{α}2 ext{γ}2 ext{ (60-90%)}

    • ext{HbA}: ext{α}2 ext{β}2 ext{ (0-40%)}

  • Embryonic Hb types:

    • Gower I: extζ<em>2extε</em>2ext{ζ}<em>2 ext{ε}</em>2

    • Gower II: extα<em>2extε</em>2ext{α}<em>2 ext{ε}</em>2

    • Portland: extζ<em>2extγ</em>2ext{ζ}<em>2 ext{γ}</em>2

  • Normal P₅₀: P50ext(normal)<br>ightarrow26−30extmmHgP_{50} ext{ (normal)} <br>ightarrow 26-30 ext{ mmHg}

  • O₂ affinity shifts:

    • Left shift: extO<em>2extaffinityext↑,extP</em>50ext↓ext{O}<em>2 ext{ affinity} ext{↑}, ext{ P}</em>{50} ext{↓}

    • Right shift: extO<em>2extaffinityext↓,extP</em>50ext↑ext{O}<em>2 ext{ affinity} ext{↓}, ext{ P}</em>{50} ext{↑}

  • Left shift causes: extLowertemp,exthigherpH,extlower2,3−BPGext{Lower temp}, ext{ higher pH}, ext{ lower 2,3-BPG}

  • Right shift causes: extHighertemp,extlowerpH,exthigher2,3−BPGext{Higher temp}, ext{ lower pH}, ext{ higher 2,3-BPG}

  • Dyshemoglobin thresholds:

    • MetHb toxic > >50\ ext{%}

    • COHb toxic > >40%>40\%

    • SulfHb toxic > >0.5%>0.5\%

  • For deeper study, refer to the cited figures (8.3–8.6, 3-14, 3-15) and Harmening/Keohane texts cited in the slide deck.