Myoglobin and Hemoglobin: Oxygen Binding and Allostery

Myoglobin and Hemoglobin: Oxygen Binding and Allostery

Learning Objectives

• Explain the roles of myoglobin and hemoglobin in oxygen transport.

• Define apoprotein and holoprotein.

• Describe the key chemical features involved in oxygen binding by myoglobin and hemoglobin.

• Determine the dissociation constant ($K_d$) and calculate the percentage of ligand binding for proteins from their binding curves.

• Define allostery, positive cooperativity, and negative cooperativity, and recognize these properties from protein binding/activity data.

• Describe the allosteric effectors of hemoglobin and their physiological relevance to oxygen and carbon dioxide transport.

Oxygen Transport: Myoglobin vs. Hemoglobin

Roles in Oxygen Transport

Myoglobin and hemoglobin are essential proteins for oxygen transport and storage in vertebrates. Their distinct functions reflect their structural and biochemical properties.

• Myoglobin: Stores oxygen in muscle tissues, facilitating oxygen supply during intense muscular activity.

• Hemoglobin: Absorbs oxygen in the lungs and delivers it to tissues throughout the body via the bloodstream.

• Hemoglobin binds oxygen cooperatively, meaning its affinity for oxygen increases as more oxygen molecules bind.

• Hemoglobin is a classic example of allosteric regulation.

Comparison of Oxygen Binding

Oxygen is transported from the lungs (or gills) to tissues, where it is required for aerobic metabolism in mitochondria to generate ATP. The process involves both myoglobin and hemoglobin:

• Oxygen diffuses into tissues or is transported by myoglobin in muscles.

• Hemoglobin carries oxygen through the blood in red blood cells (erythrocytes).

• Each red blood cell contains approximately 300 million molecules of hemoglobin, greatly increasing the oxygen-carrying capacity of blood.

• Carbon dioxide (CO2) is carried back to the lungs by hemoglobin or in plasma as bicarbonate (HCO3-).

Protein Structure and Oxygen Binding

Globin Family and Heme Group

Myoglobin and hemoglobin are members of the globin protein family. Both proteins bind oxygen via a heme prosthetic group, but not all globins are used for oxygen transport.

• Heme group: A tightly bound, non-peptide structure consisting of a planar porphyrin ring with a central iron (Fe2+) atom.

• The iron atom is coordinated by nitrogen atoms from the porphyrin and a histidine residue from the protein.

• Oxygen binds reversibly to the iron atom, allowing for oxygen delivery and release.

Apoprotein vs. Holoprotein

• Apoprotein: The protein portion without its prosthetic group (e.g., myoglobin or hemoglobin without heme).

• Holoprotein: The complete protein with its prosthetic group bound (e.g., myoglobin or hemoglobin with heme).

Oxygen Binding Mechanism

• Oxygen binds reversibly to the heme group, allowing myoglobin and hemoglobin to supply oxygen to tissues.

• Binding involves a histidine residue (often His-64 in myoglobin) and the iron atom of the heme.

• Other molecules, such as carbon monoxide (CO), can also bind the heme group. CO binds much more tightly and can displace oxygen, leading to toxicity.

Ligand Binding Equations and Curves

Fractional Saturation and Binding Curves

The fraction of protein bound to ligand (oxygen) can be described mathematically. This is essential for understanding protein-ligand interactions.

• Fractional saturation ($Y$): The fraction of total protein bound to ligand.

• For myoglobin, the binding curve is hyperbolic, indicating non-cooperative binding.

• For hemoglobin, the binding curve is sigmoidal, reflecting cooperative binding and allostery.

Ligand Binding Equation

The general equation for the fraction of protein bound to ligand is:

• $Y = \frac{[L]}{K_d + [L]}$

• Where $Y$ is the fraction of protein bound, $[L]$ is the ligand concentration, and $K_d$ is the dissociation constant.

Myoglobin Oxygen Binding

• At a partial pressure of oxygen ($P_{O_2}$) of 30 mm Hg, myoglobin is ~90% saturated.

• Myoglobin binding curve is hyperbolic, indicating high affinity and non-cooperative binding.

• Equation for myoglobin oxygen binding:

$Y_{O_2} = \frac{[O_2]}{K + [O_2]}$

• $K$ is the concentration of oxygen required to bind 50% of myoglobin (analogous to $K_d$).

Hemoglobin Oxygen Binding

• Hemoglobin consists of four subunits (2 alpha and 2 beta chains), each with a heme group.

• Hemoglobin binding curve is sigmoidal, indicating cooperative binding.

• At $P_{O_2}$ of 100 mm Hg (lungs), hemoglobin is ~98% saturated; at 30 mm Hg (tissues), ~50% saturated.

Allostery and Cooperativity

Definitions

• Allostery: The regulation of a protein's activity by the binding of a ligand at a site other than the active site, affecting the binding of other ligands.

• Positive cooperativity: Binding of one ligand increases the affinity for subsequent ligands.

• Negative cooperativity: Binding of one ligand decreases the affinity for subsequent ligands.

Binding Curve Shapes

• No cooperativity: Hyperbolic binding curve (e.g., myoglobin).

• Positive cooperativity: Sigmoidal binding curve (e.g., hemoglobin).

• Negative cooperativity: Binding curve with reduced activity as more ligand binds.

Monod-Wyman-Changeaux (MWC) Model

• Hemoglobin exists in two states:

  • Tense (T) state: Low affinity for oxygen.

  • Relaxed (R) state: High affinity for oxygen.

• Binding of oxygen to one subunit promotes a conformational change to the R state, increasing affinity at other sites.

Allosteric Effectors of Hemoglobin

Types of Effectors

• Homotropic effectors: Affect their own binding (e.g., oxygen).

• Heterotropic effectors: Bind at a regulatory site distant from the active site (e.g., 2,3-bisphosphoglycerate (2,3-BPG), H+, CO2).

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

• 2,3-BPG binds to hemoglobin, stabilizing the T state and reducing oxygen affinity.

• Promotes oxygen release in peripheral tissues.

Bohr Effect

• Lower pH (higher H+ concentration) decreases hemoglobin's affinity for oxygen, facilitating oxygen release.

• CO2 accumulation in blood lowers pH via the reaction: $CO_2 + H_2O \rightarrow HCO_3^- + H^+$

• This effect increases the efficiency of oxygen unloading in tissues.

Summary Table: Myoglobin vs. Hemoglobin

Property	Myoglobin	Hemoglobin
Location	Muscle tissue	Red blood cells
Structure	Single polypeptide chain	Four polypeptide chains (tetramer)
Oxygen Binding	Non-cooperative (hyperbolic curve)	Cooperative (sigmoidal curve)
Function	Oxygen storage	Oxygen transport
Allosteric Regulation	None	Present (affected by 2,3-BPG, H+, CO2)

Key Equations

• Fractional saturation (general): $Y = \frac{[L]}{K_d + [L]}$

• Bohr effect (CO2 hydration): $CO_2 + H_2O \rightarrow HCO_3^- + H^+$

Example Application

• At high altitude, increased 2,3-BPG levels in red blood cells promote oxygen release to tissues.

• During exercise, myoglobin supplies oxygen to muscle mitochondria for ATP production.

Additional info: The above notes expand on the provided slides and images, filling in academic context and definitions for clarity and completeness.