Myoglobin and Hemoglobin Structure and Function

Myoglobin and Hemoglobin Structure and Function

• Introduction
  • Understanding myoglobin and hemoglobin is crucial due to their roles in oxygen transport and storage.

Deoxyhemoglobin vs Oxyhemoglobin

• Structural Differences
  • O2 binding alters the structure of the hemoglobin (Hb) tetramer.
  • There are distinct structures for deoxy- (T state) and oxyhemoglobin (R state).
  • Conformational changes in one subunit affect other subunits.

Changes in Oxygen Affinity

• Cooperativity
  • Affinity changes require a protein with multiple binding sites that interact with each other, termed cooperativity.
  • Positive Cooperativity: First binding event increases affinity at remaining sites, identifiable by sigmoidal binding curves.
  • Negative Cooperativity: First binding event reduces affinity at remaining sites (rare).

Cooperative Binding in Hemoglobin

• Oxygen Transport
  • Hemoglobin's affinity for O2 must vary with partial pressure of oxygen (pO2) to ensure effective transport.
  • Sigmoidal binding curve indicates cooperative binding behavior.

Oxygen Binding Curve Analysis

• Sigmoidal Curve
  • Indicates cooperativity among binding sites; one binding event influences others.
  • T state has a lower affinity while R state has a higher affinity for oxygen.

Hill Equation and Ligand Binding

• Quantitative Description
  • For a protein with n binding sites: P + nL \rightleftharpoons PLn (Hemoglobin: Hb + nO2 \rightleftharpoons Hb(O2)n ).
  • The Hill equation relates pO2 to fractional saturation Y:
    Y = \frac{(pO2)^n}{(p{50})^n + (pO_2)^n}

Hill Coefficients

• Interpretation
  • Hill coefficient n_H indicates cooperativity:
  • n_H = 1 : No cooperativity (independent binding).
  • n_H > 1 : Positive cooperativity (increased affinity).
  • n_H < 1 : Negative cooperativity (decreased affinity).

Structural Changes During Oxygen Binding

• R and T States
  • R State: Higher affinity, stabilized when O2 is bound.
  • T State: More stable in absence of O2; involves higher number of ion pairs.
  • O2 binding triggers conformational change from T to R state.

Ion Pairs and Stability

• T State Stabilization
  • Stabilized by ion pairs at the α1β2 and α2β1 interfaces.
  • Conformational change involves breaking these ion pairs during O2 binding.

Heme and F Helix Movement

• Conformational Changes
  • Movement of amino acids surrounding heme causes structural transitions during T to R.
  • Changes in position of the F helix occur as a result of O2 binding.

Allosteric Regulation**

• Functional Implications
  • Allosteric proteins like hemoglobin are influenced by modifiers that bind at one site and affect others.
  • Understand how these interactions regulate ligand binding and the protein's overall behavior.

Example Problem

• Calculating Hill Coefficient
  • If ligand binding to a newly discovered protein increases affinity at other sites, the Hill coefficient n_H is:
  • Answer: Greater than 1, indicating positive cooperativity.