Biochemistry Lecture: Hemoglobin, Myoglobin, and Protein-Ligand Interactions

Course Administrative Updates & Announcements

• Lecture Schedule and Sequence Changes:

  • Hemoglobin and protein function topics were moved forward.

  • Coenzymes and vitamins topics were postponed to a later date to allow additional preparation time.

• Textbook Renumbering:

  • The material on protein function and hemoglobin was formerly designated as Chapter 9 in prior textbook editions.

  • In the current edition, this material is designated as Chapter 5.

• Exam Blackout Period:

  • An exam blackout period remains active because at least one student is scheduled to take the examination in the afternoon.

  • Discussion of exam content is strictly prohibited during this window.

• Quantitative Corrections:

  • The displacement distance of the iron atom upon oxygenation was previously listed on course slides as $0.4\,\text{Å}$.

  • The verified structural displacement distance is $0.04\,\text{Å}$.

Protein-Ligand Interaction Terminology

• Substrates vs. Ligands:

  • Enzymes: Catalytic proteins that bind specific molecules known as substrates, catalyze chemical transformations, and release converted products.

  • Non-Enzymatic Proteins: Proteins that bind molecules transiently without performing chemical modification or catalysis. The bound molecule is formally termed a ligand (never a substrate, as no chemical modification occurs).

• Binding Sites vs. Active Sites:

  • Non-enzymatic proteins feature a binding site where the ligand associates and dissociates.

  • Terms such as "active site" or "catalytic site" are restricted to enzymatic proteins that catalyze chemical reactions.

• Binding Models:

  • Neither the classic Lock-and-Key model nor the Induced Fit model is universally correct on its own.

  • Both models retain conceptual utility depending on the specific protein-ligand system analyzed.

Heme Structure & Iron Coordination Chemistry

• Heme Group Composition:

  • A functional heme group consists of an organic porphyrin ring system coordinated with a central iron(II) cation ($\text{Fe}^{2+}$).

  • The porphyrin ring sans iron comprises a conjugated heterocyclic framework of carbon, hydrogen, oxygen, and nitrogen atoms.

• Iron Oxidation States:

  • Iron must remain in the reduced ferrous state ($\text{Fe}^{2+}$) to bind molecular oxygen ($\text{O}_2$).

  • If iron is oxidized to the ferric state ($\text{Fe}^{3+}$), it loses its capability to bind oxygen, rendering the protein non-functional.

• Necessity of the Protein Framework:

  • Free heme in solution readily oxidizes upon contact with oxygen.

  • The surrounding polypeptide architecture of myoglobin or hemoglobin isolates and protects the heme group, preventing irreversible oxidation of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$.

• Erythrocyte Morphology:

  • Normal red blood cells (erythrocytes) are biconcave, flattened discs without central perforations.

  • This unique donut-like morphology maximizes surface-area-to-volume ratio for gas exchange and structural flexibility, which becomes impaired in pathologies such as sickle cell anemia.

Myoglobin Architectural Structure

• Secondary & Tertiary Fold:

  • Myoglobin is a monomeric, all-alpha-helical protein containing no beta strands.

  • It contains 8 distinct alpha helices designated sequentially as A through H, starting from the N-terminus.

• Structural Regions:

  • N-terminus: The initial amino acid region, followed by a short peptide segment leading directly into the A helix.

  • Helical Interconnections: Non-helical segments connecting helices are designated by the adjacent helices (e.g., AB loop/turn connects helix A and helix B; CD loop connects helix C and helix D).

  • Random Coils: Non-repetitive loop structures that maintain definite spatial conformations despite lacking standard secondary structures (alpha helices, beta sheets, or standard turns).

  • C-terminus: Located immediately after the terminal H helix.

• Heme Pocket Location:

  • The heme group resides in a hydrophobic pocket situated near the protein periphery.

  • Peripheral placement ensures molecular oxygen ($\text{O}_2$) can efficiently diffuse from the aqueous environment into the binding pocket.

Proximal and Distal Histidines

• Proximal Histidine:

  • Nomenclature: Designated as $\text{His-F8}$ (the 8th amino acid residue of the F helix) or $\text{His-93}$ (the 93rd amino acid from the N-terminus).

  • Coordination: Directly binds the central $\text{Fe}^{2+}$ ion on the backside of the heme ring via a coordinate covalent bond.

  • Bond Character: A coordinate covalent bond is a covalent linkage in which one participating atom (the nitrogen atom of the imidazole ring of histidine) donates both electrons to the bond pair, distinct from standard covalent bonds where each atom contributes one electron.

• Distal Histidine:

  • Nomenclature: Designated as $\text{His-E7}$ (the 7th amino acid residue of the E helix) or $\text{His-64}$ (the 64th amino acid from the N-terminus).

  • Coordination: Resides on the face of the heme ring opposite the proximal histidine; does not make direct contact with the $\text{Fe}^{2+}$ ion.

  • Function: Forms a crucial hydrogen bond with bound molecular oxygen ($\text{O}_2$), stabilizing the adduct and increasing oxygen-binding affinity.

• Protection Against Oxidation:

  • Operating in tandem, the proximal and distal histidines physically enclose the heme pocket.

  • This arrangement prevents oxygen molecules from simultaneously attacking both faces of the heme, which would otherwise oxidize $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$.

Structural Dynamics of Oxygen Binding & T to R Transition

• Deoxygenated Conformational State (T-State):

  • In the deoxygenated state, the porphyrin ring is non-planar and slightly puckered (saucer-like).

  • The central $\text{Fe}^{2+}$ ion has a larger ionic radius and sits slightly out of the plane of the porphyrin ring.

• Oxygenation Mechanism & Structural Cascade:

  • Binding of $\text{O}_2$ to $\text{Fe}^{2+}$ creates a coordinate covalent bond, driving a contraction of the iron atom's electron cloud.

  • Shrinkage of the atomic radius allows $\text{Fe}^{2+}$ to move into the plane of the porphyrin ring, pulling the porphyrin ring into a flat conformation.

  • The inward movement of $\text{Fe}^{2+}$ spans a distance of $0.04\,\text{Å}$.

• Spectroscopic Detection:

  • A spatial shift of $0.04\,\text{Å}$ is below the resolution threshold of standard X-ray diffraction techniques.

  • The displacement was experimentally verified using Electron Paramagnetic Resonance (EPR) spectroscopy, a technique analogous to Nuclear Magnetic Resonance (NMR) spectroscopy that targets unpaired electron spins.

• Allosteric Amplification Cascade:

  1. $\text{O}_2$ binds to $\text{Fe}^{2+}$.

  2. $\text{Fe}^{2+}$ contracts and translocates $0.04\,\text{Å}$ into the plane of the porphyrin ring.

  3. $\text{Fe}^{2+}$ drags the covalently attached proximal histidine ($\text{His-F8}$ / $\text{His-93}$).

  4. Movement of $\text{His-F8}$ exerts mechanical pull on the F helix.

  5. Displacement of the F helix triggers widespread quaternary rearrangement throughout the protein multimer, transitioning hemoglobin from the low-affinity T-state (tense) to the high-affinity R-state (relaxed).

Quaternary Structure of Hemoglobin

• Subunit Composition:

  • Adult hemoglobin ($\text{HbA}$) exists as a heterotetramer composed of four polypeptide chains: two alpha subunits and two beta subunits ($\alpha_2\beta_2$).

  • The $\beta$ subunit fold closely mirrors monomeric myoglobin; the $\alpha$ subunit fold is similarly homologous.

  • Structural differences between myoglobin and the $\beta$ subunit include a slightly longer A helix in myoglobin, alongside a shorter H helix and longer carboxyl-terminal tail in the $\beta$ subunit.

• Inter-Subunit Stabilization:

  • The four subunits assemble via a combination of hydrophobic and hydrophilic (polar/electrostatic) interactions.

  • Heme groups are positioned near the outer surface of each subunit to facilitate gas exchange.

• Quaternary Rearrangement During T $\rightarrow$ R Transition:

  • In the deoxygenated T-state, hemoglobin features a prominent central cavity.

  • Upon oxygen binding, the two $\beta$ subunits ($\beta_1$ and $\beta_2$) rotate and roll inward toward one another relative to the central axis, resulting in the narrowing and closure of the central cavity.

  • The carboxyl-terminal residue of the $\beta$ subunit, histidine $\text{His-HC3}$ (the 3rd residue following helix H), undergoes a spatial movement of approximately $10\,\text{Å}$.

  • This relocation allows $\text{His-HC3}$ to form salt bridges with nearby aspartate residues, stabilizing specific conformational states.

Quantitative Analysis of Ligand Binding: Fraction Bound Equation

• Equilibrium Expression:

  • Protein-ligand interaction is modeled as a dissociation reaction:     $PL \rightleftharpoons P + L$

  • The equilibrium dissociation constant ($K_d$) is formulated as:     $K_d = \frac{[P][L]}{[PL]}$

  • Here, $[P]$ represents free protein, $[L]$ represents free ligand, and $[PL]$ represents the protein-ligand complex.

• Fraction Bound ($Y$):

  • Defined as the ratio of occupied binding sites to total available binding sites:     $Y = \frac{\text{Occupied Sites}}{\text{Total Sites}} = \frac{[PL]}{[PL] + [P]}$

  • Expressed in terms of measurable quantities ($[L]$ and $K_d$):     $Y = \frac{[L]}{[L] + K_d}$

  • In classic biochemical literature, the capital Greek letter theta ($\theta$) is frequently utilized in place of $Y$ to represent fraction bound.

• Physical Significance of $K_d$:

  • $K_d$ corresponds to the free ligand concentration at which exactly $50\%$ of total binding sites are occupied ($Y = 0.50$).

  • Lower values of $K_d$ reflect higher binding affinity (tighter binding), yielding a steeper initial curve at low ligand concentrations.

• Gas Phase Adaptation ($P_{50}$):

  • For gaseous ligands such as oxygen, partial pressure ($P_{\text{O}_2}$) substitutes for molar concentration ($[L]$).

  • The dissociation constant $K_d$ is replaced by $P_{50}$, defined as the partial pressure of oxygen at which $50\%$ of binding sites are saturated:     $Y = \frac{P_{\text{O}_2}}{P_{\text{O}_2} + P_{50}}$

Binding Curves & Physiological Oxygen Delivery

• Myoglobin Saturation Characteristics:

  • Exhibits a hyperbolic binding curve due to its monomeric structure lacking cooperative interactions.

  • Possesses an extremely high affinity for oxygen (very low $P_{50}$).

  • In arterial lung capillaries ($P_{\text{O}_2} \approx 100\,\text{torr}$ or $12.5\,\text{kPa}$), myoglobin becomes virtually fully saturated ($\approx 100\%$).

  • In tissue capillary beds ($P_{\text{O}_2} \approx 20\text{--}40\,\text{torr}$), myoglobin retains approximately $93\%$ of its bound oxygen, releasing only $\approx 7\%$.

  • Consequently, myoglobin is ineffective as an oxygen transport protein and functions instead as an oxygen storage reserve.

• Hemoglobin Saturation Characteristics:

  • Exhibits a sigmoidal (S-shaped) binding curve resulting from allosteric conformational transitions.

  • Achieves high fractional saturation in the lungs ($P_{\text{O}_2} \approx 100\,\text{torr}$).

  • Undergoes cooperative unloading in peripheral tissues, delivering approximately $66\%$ of its total oxygen-carrying capacity.

• Hypothetical Non-Cooperative States:

  • Locked R-State (High Affinity): Behaves like myoglobin—saturates completely in the lungs but releases minimal oxygen in tissue beds.

  • Locked T-State (Low Affinity): Achieves only $\approx 50\%$ saturation in the lungs and delivers only $\approx 20\text{--}25\%$ of oxygen to peripheral tissues.

Allosteric Regulation in Hemoglobin

• Homotropic Allosteric Regulation:

  • Hemoglobin oxygenation represents a classic model of homotropic allosteric regulation.

  • The primary ligand ($\text{O}_2$) acts simultaneously as the allosteric effector/regulator.

• Mechanism of Cooperativity:

  • The binding of the first $\text{O}_2$ molecule to a subunit in the T-state lowers the energetic barrier for the T $\rightarrow$ R conformational transition.

  • This structural realignment increases the binding affinity for oxygen at all remaining vacant heme sites within the tetramer.

Questions & Discussion

• Q: What is the exact difference between a standard covalent bond and a coordinate covalent bond?

  • Response: In a standard covalent bond, each participating atom contributes one electron to the shared pair. In a coordinate covalent bond, one participating atom contributes both electrons to the shared pair.

• Q: Given a fraction bound $Y = 0.50$ at a free ligand concentration of $[L] = 0.50\,\text{M}$, what is the value of $K_d$?

  • Student Answer Attempt 1: $0.125$

  • Correction & Verification: $0.50\,\text{M}$. Substituting values into the fraction bound equation:     $0.50 = \frac{0.50\,\text{M}}{0.50\,\text{M} + K_d}$     $0.50(0.50\,\text{M} + K_d) = 0.50\,\text{M}$     $0.25\,\text{M} + 0.50(K_d) = 0.50\,\text{M}$     $0.50(K_d) = 0.25\,\text{M}$     $K_d = 0.50\,\text{M}$

• Classroom Infrastructure Note:

  • Adjustment of classroom projection equipment required activating the left projector display during the review of catalytic versus binding site terminology.