Study Guide for Hemoglobin and Myoglobin

Allostery in Hemoglobin

Overview of Hemoglobin and Myoglobin Structures

  • Tertiary and Quaternary Structures:

    • Hemoglobin (Hb) and myoglobin (Mb) have distinct tertiary and quaternary structures.

    • Hb is a tetramer composed of two alpha (α) and two beta (β) chains, with a total of 4 heme groups.

    • Mb is a monomeric protein with a single heme group, smaller in structure compared to Hb.

Oxygen Binding Curves

  • Oxygen Binding For Mb and Hb:

    • Mb obeys Michaelis-Menten kinetics but is not an enzyme.

    • The oxygen binding curve for Hb is sigmoidal, indicating cooperative binding.

  • Conformational Changes:

    • Binding of oxygen leads to small changes in the tertiary structure of myoglobin and larger changes in quaternary structure of hemoglobin.

    • Oxygen binding alters the geometry of the heme group, causing conformational shifts.

Cooperative Binding and Models

  • Cooperativity:

    • Describes how the binding of one oxygen molecule influences the binding of subsequent oxygen molecules.

    • Both Monod-Wyman-Changeux (MWC) model and Koshland-Némethy-Filmer (KNF) model can explain cooperativity.

  • Bohr Effect:

    • The affinity of Hb for oxygen decreases at lower pH (increased H+ concentration).

Allosteric Effectors and Their Mechanisms

  • 2,3-Bisphosphoglycerate (2,3-BPG):

    • Acts as a negative allosteric effector for hemoglobin, stabilizing the T (tense) state, reducing oxygen affinity.

    • Binds to a site distinct from the heme iron where oxygen binds.

Differences Between Fetal and Adult Hemoglobin

  • Fetal Hemoglobin (HbF):

    • Composed of two alpha and two gamma (γ) chains (α2γ2 structure).

    • Has a higher O2 affinity compared to adult Hb (Hbf), primarily due to less binding of 2,3-BPG.

    • Unique amino acid substitutions contribute to its properties.

Conformational Changes Upon Oxygen Binding

  • Oxygen Binding to Heme Group:

    • Iron (Fe) within the heme interacts with five/six ligands, including the imidazole side chain of His F8.

    • When O2 binds, there’s a change in position of Fe, which then drags connected helices and results in a shift in interactions between subunits.

Physiological Effects and Context

  • Impact of H+:

    • An increase in H+ concentration promotes O2 dissociation (Bohr effect), facilitating O2 delivery in tissues.

    • Histidine 146’s protonation leads to a transition from R state to T state, decreasing oxygen affinity.

  • Influence of CO2 on Hemoglobin:

    • Increased CO2 production in tissues results in proton production, further driving the release of oxygen.

    • CO2 hydration reaction: CO<em>2+H</em>2O⇄H++HCO3−CO<em>2 + H</em>2O ⇄ H^+ + HCO3^-.

Summary of Cooperative Binding

  • Functionality of Mb vs. Hb:

    • Mb’s function as an oxygen-storage protein emphasizes its affinity for O2 under all pressures.

    • Hb’s design allows for the binding in lungs (high pO2) and release in tissues (low pO2).

  • Cooperativity Factors:

    • The transition of Hb from deoxy (T state) to oxy (R state) is crucial for its function, especially in O2 delivery.

Key Definitions

  • K systems: Change in the concentration of substrate that gives half-maximal binding (K0.5); changes presence of allosteric effectors but not Vmax.

  • V systems: Demonstrates how Vmax is affected by allosteric effectors while K0.5 remains unchanged.

  • Allosteric Enzymes: Typically oligomeric, exhibiting cooperative substrate binding kinetics; regulated positively or negatively by effectors.

Quiz Questions

  • Various quizzes contained in the transcript illustrate the principles of allostery, cooperative binding, the Bohr effect, and impact of allosteric regulators.