Myoglobin and Hemoglobin: Structure, Function, Allosteric Regulation, and Variants

Overview and Physiological Significance of Myoglobin and Hemoglobin

• Primary Biological Roles of Myoglobin and Hemoglobin:

  • Myoglobin ($\text{Mb}$): Functions to store and transport oxygen ($O_2$) within mammalian muscle tissue. It is a monomeric protein present in muscular cells.
  • Hemoglobin ($\text{Hb}$): Dissolved inside red blood cells (erythrocytes) to transport oxygen ($O_2$) from the lungs to peripheral active tissues throughout the body. Additionally, $\text{Hb}$ plays a vital role in transporting protons ($H^+$) and carbon dioxide ($CO_2$) from metabolizing tissues and muscles back to the lungs.

• Historical and Structural "Firsts":

  • $\text{Mb}$ and $\text{Hb}$ were the first two proteins to have their complete three-dimensional structures determined at atomic resolution using X-ray crystallography.
  • They were the first proteins to be definitively associated with a specific physiological function.
  • $\text{Hb}$ was the first protein for which a specific molecular basis for a hereditary disease was identified (sickle cell anemia).

• Insights Provided by the Study of $\text{Mb}$ and $\text{Hb}$:

  • Structure-Function Relationships: Demonstrates how tertiary structure (in $\text{Mb}$ and $\text{Hb}$ subunits) and quaternary structure (in tetrameric $\text{Hb}$) dictate biological and physiological function.
  • Pathophysiology: Provides a clear molecular explanation for genetic disorders such as sickle cell disease.
  • Carbon Monoxide ($CO$) Poisoning: Explains the molecular mechanism of $CO$ poisoning. Preferential binding of $CO$ to the heme groups of $\text{Mb}$ and $\text{Hb}$ over $O_2$ leads to severe physiological asphyxiation.
  • Physiological Adaptation: Illustrates how organisms adapt to high-altitude environments and how oxygen is efficiently transferred from maternal blood across the placenta to the fetus.

Structural and Chemical Characteristics of Myoglobin and Hemoglobin

• Structural Characteristics of Myoglobin ($\text{Mb}$):

  • Composed of a single polypeptide chain containing $153$ amino acid residues.
  • Contains a single heme prosthetic group and binds $1$ molecule of oxygen ($O_2$) per $\text{Mb}$ molecule.
  • Subunit structure consists entirely of several \begin{equation*}\alpha\end{equation*}-helices linked by connecting loops, containing zero \begin{equation*}\beta\end{equation*}-sheets.
  • Globular structure where non-polar (hydrophobic) residues are buried inside the protein core, and polar (hydrophilic) residues are located on the exterior surface.

• Structural Characteristics of Hemoglobin ($\text{Hb}$):

  • Tetrameric protein composed of $4$ polypeptide chains: $2$ \begin{equation*}\alpha\end{equation*}-chains (each $141$ amino acids long) and $2$ \begin{equation*}\beta\end{equation*}-chains (each $146$ amino acids long).
  • Note: The designations "\begin{equation*}\alpha\end{equation*}-chains" and "\begin{equation*}\beta\end{equation*}-chains" represent distinct subunit types and do not refer to secondary structures such as \begin{equation*}\alpha\end{equation*}-helices or \begin{equation*}\beta\end{equation*}-sheets.
  • Binds $4$ oxygen ($O_2$) molecules per tetramer ($1$ oxygen molecule per individual monomer/subunit).
  • Subunits consist of multiple \begin{equation*}\alpha\end{equation*}-helices connected by loops with no \begin{equation*}\beta\end{equation*}-sheets.
  • Globular tetramer with non-polar residues sequestered in the interior and polar residues positioned on the exterior surface.
  • Organized as an asymmetrical tetrahedron ($\alpha\alpha\beta\beta$) composed of two dimeric pairs: an $\alpha_1\beta_1$ dimer and an $\alpha_2\beta_2$ dimer.

• Oxygenation States:

  • Deoxy Form: Deoxy$\text{Mb}$ and deoxy$\text{Hb}$ (unbound, containing no $O_2$).
  • Oxy Form: Oxy$\text{Mb}$ (bound to $1$ $O_2$ molecule) and oxy$\text{Hb}$ (bound to $4$ $O_2$ molecules).

• Chemistry of Oxygen Binding:

  • The primary chemical function of $\text{Mb}$ and $\text{Hb}$ is to reversibly bind oxygen at the central ferrous iron ($Fe^{2+}$) group of the prosthetic heme ring.
  • The protein structure surrounding the heme permits oxygenation (reversible binding of $O_2$ to $Fe^{2+}$) while preventing oxidation of ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$) and reduction of oxygen.
  • Critical Histidine Residues: Two key histidine residues inside the hydrophobic interior are essential for oxygen binding:
    • $\text{His-64}$ (distal histidine).
    • $\text{His-93}$ (proximal histidine, designated as $\text{His F8}$), which is covalently attached to Helix F of the protein backbone.
  • Atomic Coordinates and Structural Shift:
    • Key amino acids surrounding the heme pocket include $\text{Leu F7}$, $\text{Leu F4}$, $\text{His F8}$, $\text{Val FG5}$, and $\text{Leu FG3}$.
    • In the deoxygenated state, the $Fe^{2+}$ ion sits outside the plane of the porphyrin ring (puckered conformation).
    • Upon $O_2$ binding, the $Fe^{2+}$ ion is pulled back into the plane of the porphyrin ring (planar conformation), causing a physical displacement of approximately $0.6\,\text{\AA}$.

Oxygen Binding Kinetics, Hyperbolic vs. Sigmoidal Behavior, and Transport

• Binding Behavior and Curves:

  • Myoglobin ($\text{Mb}$): Displays a hyperbolic oxygen saturation curve.
    • Indicates that each oxygen-binding site is completely independent of all other binding sites.
    • Binding or releasing an $O_2$ molecule by one $\text{Mb}$ protein has no effect on oxygen binding or release by any other $\text{Mb}$ molecule.
  • Hemoglobin ($\text{Hb}$): Displays a sigmoidal ("S"-shaped) oxygen saturation curve.
    • Indicates positive cooperativity: the binding of one $O_2$ molecule to a subunit increases the binding affinity for subsequent $O_2$ molecules across the same $\text{Hb}$ tetramer.
    • Conversely, as surrounding oxygen pressure drops, the loss of one $O_2$ molecule from fully oxygenated $\text{Hb}$ facilitates the sequential release of the remaining bound $O_2$ molecules.

• Understanding $\text{P}_{50}$ Values:

  • $\text{P}_{50}$ is the partial pressure of oxygen ($pO_2$) required to achieve $50\%$ oxygen saturation of a protein pool.
  • Affinity Relationship: A lower $\text{P}_{50}$ value indicates higher oxygen affinity (tight binding), whereas a higher $\text{P}_{50}$ value indicates lower oxygen affinity.
  • $\text{Mb}$ $\text{P}_{50}$: Ranging from $2.8\,\text{torrs}$ to $4\,\text{torrs}$. Because $\text{Mb}$ becomes half-saturated at extremely low oxygen pressures, it binds oxygen tightly and can easily capture $O_2$ released by $\text{Hb}$.
  • $\text{Hb}$ $\text{P}_{50}$: Approximately $26\,\text{torrs}$ (in the presence of physiological $2,3\text{-BPG}$). It requires significantly higher surrounding $pO_2$ levels to achieve half-saturation.

• Physiological Oxygen Transport Dynamics:

  • Lungs: Surrounding oxygen pressure is high ($pO_2 = 100\,\text{torrs}$). Under these conditions, $\text{Hb}$ becomes fully saturated, forming oxy$\text{Hb}$.
  • Capillaries of Active Muscle: Surrounding oxygen pressure drops ($pO_2 = 20\,\text{torrs} \text{ to } 30\,\text{torrs}$). Under these conditions, $\text{Hb}$ efficiently unloads its bound oxygen.
  • Deep Active Muscle Tissue: Internal oxygen levels fall even lower ($pO_2 = 1\,\text{torrs} \text{ to } 2\,\text{torrs}$). At these minimal pressures, $\text{Mb}$ surrenders its stored oxygen, allowing $O_2$ to diffuse into cellular mitochondria for oxidative phosphorylation.

Structural Basis of Cooperativity: T State and R State Transitions

• Quaternary Conformational States:

  • T State (Taut / Tense Form): Represents deoxygenated hemoglobin (deoxy$\text{Hb}$), where all four binding sites are empty. Characterized by low oxygen affinity. C-terminal residues ($\beta\text{His-146}$ and $\alpha\text{Arg-141}$) participate in salt bridges (electrostatic interactions) that stabilize this rigid conformation.
  • R State (Relaxed Form): Represents fully oxygenated hemoglobin (oxy$\text{Hb}$), where all four binding sites hold $O_2$. Characterized by high oxygen affinity. Inter-subunit salt bridges are broken.
  • Intermediate States: Tetramers carrying $1$, $2$, or $3$ $O_2$ molecules represent transient, structurally unstable intermediate states.

• Molecular Mechanism of the T to R Transition:

  • In the T state, oxygen binding to the heme of $\beta$-subunits is physically blocked, primarily by the side chain of residue $\text{Val-67}$. Consequently, initial $O_2$ binding occurs at the $\alpha$-subunits.
  • When oxygen binds to the $\alpha$-subunit heme inside each $\alpha\beta$ dimer (totaling $2$ $O_2$ molecules per $\text{Hb}$ tetramer), the iron atom is pulled into the plane of the porphyrin ring, flattening the ring.
  • This movement pulls $\text{His-93}$ ($\text{His F8}$) and its associated $\alpha$-helix, generating conformational strain across the tightly packed helical framework.
  • As cumulative strain breaks key inter-subunit electrostatic interactions (salt bridges), the $\alpha_1\beta_1$ dimer rotates by approximately $15^\circ$ relative to the $\alpha_2\beta_2$ dimer.
  • This global quaternary shift alters contact points between $\alpha_1$ and $\beta_2$ (as well as $\alpha_2$ and $\beta_1$) while preserving the stable internal interfaces within $\alpha_1\beta_1$ and $\alpha_2\beta_2$.
  • The $15^\circ$ rotation shifts $\text{Val-67}$ away from the $\beta$-subunit binding pocket, transitioning the protein to the high-affinity R state and allowing $\beta$-subunits to rapidly bind $O_2$.
  • R to T State Transition: In low oxygen environments (tissues) and in the presence of allosteric regulators, oxygen dissociates (unloading from $\beta$-subunits first), $\text{Val-67}$ moves back into position, salt bridges reform, and $\text{Hb}$ reverts to the T state.

Allosteric Regulation of Oxygen Binding

• Allosteric Mechanism: Allosteric regulation occurs when effector molecules bind at a specific site separate from the functional oxygen-binding heme sites to modulate protein activity. Allosteric effectors alter oxygen binding to tetrameric $\text{Hb}$, but have no effect on monomeric $\text{Mb}$.

• 1. 2,3-Bisphospho-D-glycerate ($2,3\text{-BPG}$):

  • Mechanism: Highly negatively charged metabolite present in human blood containing carboxylate and phosphate groups ($\text{CO}_2^-$, $\text{HC-O-PO}_3^{2-}$, $\text{H}_2\text{C-O-PO}_3^{2-}$).
  • Binds electrostatically within a central cavity present exclusively between the subunits of deoxy$\text{Hb}$ (T state).
  • Interacts with positively charged amino acid residues from both $\beta$-subunits:
    • $\beta_1\text{-Lys-82}$ and $\beta_2\text{-Lys-82}$
    • $\beta_1\text{-His-2}$ and $\beta_2\text{-His-2}$
    • $\beta_1\text{-His-143}$ and $\beta_2\text{-His-143}$
  • By stabilizing the T state, $2,3\text{-BPG}$ lowers the overall oxygen affinity of $\text{Hb}$, promoting oxygen unloading in tissues.
  • In the R state, narrowing of the central cavity and misalignment of positive residues prevent tight $2,3\text{-BPG}$ binding.
  • Fetal Hemoglobin ($\text{HbF}$):
    • $\text{HbF}$ consists of $2$ $\alpha$-subunits and $2$ $\gamma$-subunits ($\alpha_2\gamma_2$), replacing the adult $\beta$-subunits.
    • The $\gamma$-subunit sequence substitutes Serine for Histidine at position $143$ ($\gamma\text{Ser-143}$ instead of $\beta\text{His-143}$).
    • Loss of two positive charges weakens $2,3\text{-BPG}$ binding. Consequently, $\text{HbF}$ has a higher oxygen affinity than maternal adult hemoglobin ($\text{HbA}$), enabling directionally preferential oxygen transfer from maternal oxyhemoglobin to fetal deoxyhemoglobin.

• 2. The Bohr Effect ($H^+$ / pH):

  • Refers to the regulation of oxygen binding by hydrogen ion concentration ($H^+$ / pH).
  • An increase in $H^+$ concentration (lowered pH) stabilizes the T state and forces $\text{Hb}$ to release bound $O_2$
  • A decrease in $H^+$ concentration (raised pH) favors the R state and forces $\text{Hb}$ to bind $O_2$
  • Molecular Basis: Lower pH leads to protonation of the N-termini of $\alpha$-chains and $\text{His-146}$ of $\beta$-chains ($\beta\text{His-146}$). Protonated $\beta\text{His-146}$ forms a stabilizing electrostatic salt bridge with $\beta\text{Asp-94}$, locking the protein in the T state ("Taut"). Upon oxygenation to the R state, $\beta\text{His-146}$ and $\beta\text{Asp-94}$ move apart, breaking the salt bridge and releasing protons ($H^+$).
  • Physiological Coordination:
    • Actively Metabolizing Tissue: Lower pH due to metabolic $H^+$ production causes $\text{Hb}$ to release $O_2$ and bind $H^+$.
    • Lungs: Higher pH causes $\text{Hb}$ to bind $O_2$ and release $H^+$.

• 3. Carbon Dioxide ($CO_2$):

  • Elevated $CO_2$ concentrations force $\text{Hb}$ to release oxygen.
  • $\text{Hb}$ transports $CO_2$ directly by forming carbamate adducts: $CO_2$ reacts with uncharged terminal amino groups of $\text{Hb}$ chains to yield negatively charged carbamates ($\text{-NH-COO}^-$).
  • In the T state, the negative carbamate charge on an $\alpha_2$ chain is stabilized by proximity to the positively charged guanidino group of $\alpha_1\text{Arg-141}$ (forming a T-state salt bridge).
  • Upon transition to the R state, the carbamate and $\alpha_1\text{Arg-141}$ side chains move apart, destabilizing the carbamate and causing $CO_2$ release.
  • Summary of Net Physiological Exchange:
    • In Tissues: $\text{Hb}$ releases $O_2$ and binds $CO_2$ and $H^+$.
    • In Lungs: $\text{Hb}$ binds $O_2$ and releases $CO_2$ and $H^+$.

Hemoglobin Variants, Pathophysiology, and Clinical Therapies

• Sickle Cell Disease (SCD / $\text{HbS}$):

  • Molecular Etiology: Non-contagious inherited disorder caused by a point mutation at residue $6$ of the $\beta$-chain, substituting Glutamate with Valine ($\beta6\text{Glu} \rightarrow \text{Val}$).
  • Pathophysiology: Replaces a polar, charged surface residue with a nonpolar, hydrophobic residue, creating a hydrophobic patch on the protein surface. This patch fits into a complementary hydrophobic pocket exposed on the surface of deoxygenated $\text{HbS}$ molecules (absent in oxygenated $\text{HbS}$).
  • Under high concentrations of deoxy$\text{HbS}$ (following oxygen delivery in microvascular capillaries), hydrophobic interactions drive $\text{HbS}$ molecules to polymerize into rigid fibrous strands.
  • Fibrous polymers distort red blood cells into rigid sickle or crescent shapes, causing microvascular blockages (vaso-occlusive crises or VOCs) that impede capillary blood flow, resulting in severe oxygen deprivation and tissue damage.
  • Genetics:
    • Homozygous ($\text{HbS/HbS}$): Inherits two mutant genes; produces exclusively $\text{HbS}$. Symptoms are absent in early infancy until fetal hemoglobin ($\text{HbF}$) is completely replaced by $\text{HbS}$. Characterized by severe anemia, heightened infection susceptibility, and vascular complications.
    • Factors Exacerbating Sickling: Any condition increasing the proportion of deoxy$\text{HbS}$, including high altitude, flying in non-pressurized aircraft, elevated $CO_2$ levels, decreased pH (e.g., from physical exercise), or increased $2,3\text{-BPG}$ concentrations.
    • Heterozygous (Sickle Cell Trait, $\text{HbA/HbS}$): Inherits one normal gene and one mutant gene. Produces both $\text{HbA}$ and $\text{HbS}$, generally maintaining a normal life span.
  • Malaria Resistance: Both homozygotes and heterozygotes possess increased clinical resistance to Plasmodium falciparum, the parasite responsible for malaria. Because infected red blood cells exhibit shortened lifespans and accelerated clearance, Plasmodium falciparum cannot complete its intracellular developmental cycle.

• Pharmacological and Medical Therapies for SCD:

  • Hydroxyurea: Stimulates production of fetal hemoglobin ($\text{HbF}$), reducing the relative concentration of $\text{HbS}$ and preventing sickling. Must be handled with extreme care due to severe risks, including carcinogenicity and severe myelosuppression (decreased bone marrow activity leading to reduced RBCs, WBCs, and platelets).
  • L-Glutamine (Endari®): Decreases red blood cell susceptibility to oxidative stress, reducing the overall frequency of sickle cell crises.
  • Voxelotor (Oxbryta®): Binds $\text{HbS}$ to increase its oxygen affinity, maintaining the protein in its oxygenated state (which does not sickle). Received accelerated FDA approval in $2019$, but was voluntarily withdrawn globally on September 25, 2024, by Pfizer due to clinical trials indicating that overall risks outweighed benefits, specifically showing elevated rates of vaso-occlusive crises (VOCs) and mortality.
  • Bone Marrow Transplant: The only curative treatment for sickle cell disease; rarely performed due to substantial clinical risks.

• Key Literature References:

  • Nature Communications 1:104 pp. 1–7 (2010) — Sickle cell trait gene and malaria resistance.
  • American Journal of Hematology, Vol 87, pp. 340–346 (2012) — Clinical implications of sickle cell trait.
  • Biochemistry, Vol 57:5, pp. 470–471 (2018) — Mechanism of Endari.
  • ACS Med. Chem. Lett. 2017, 8, 321–326; Drugs (2020) 80:209–215; Cureus 12(2): e7105 (2020) — Development and evaluation of Voxelotor.

• Other Notable Hemoglobin Variants:

  • Approximately $5\%$ of the global population carries an inherited hemoglobin disorder. Variants are frequently named after the geographic location where they were first identified:
    • $\text{Hb (Ohio)}$ ($\beta141\text{Ala} \rightarrow \text{Asp}$): Decreases the structural stability of the $\beta\text{His-146}$ to $\beta\text{Asp-94}$ salt pair, altering oxygen binding kinetics.
    • $\text{Hb Milledgeville}$ ($\alpha44\text{Pro} \rightarrow \text{Leu}$): Alters residue $\alpha44\text{Pro}$ involved in forming the $\alpha_1\beta_2$ interface in the deoxy form, destabilizing the deoxy state and shifting oxygen affinity.
    • $\text{Hb Providence}$ ($\beta\text{Lys-44} \rightarrow \text{Asn}$): The $\text{Asn}$ variant can undergo side-chain deamidation (loss of $\text{-NH}_2$) to convert into $\text{Hb Providence Asp}$, altering oxygen affinity.
    • $\text{Hb Syracuse}$ ($\beta146\text{His} \rightarrow \text{Pro}$): Mutates the key C-terminal histidine residue involved in Bohr effect ion pairing, impairing normal response to allosteric effectors and modifying oxygen affinity.