Hemoglobin

Hemoglobin and Myoglobin

Overview of Hemoglobin and Myoglobin

• Both hemoglobin and myoglobin are proteins responsible for oxygen binding.

• Hemoglobin is primarily found in blood, while myoglobin is located in skeletal and cardiac muscles.

Hemoglobin

• Function: Oxygen transporter in the blood.

• Structure:

  • Globular protein, roughly spherical in shape.

  • Tetramer composed of four polypeptide chains:

  • Two identical alpha chains (α1 & α2)

  • Two identical beta chains (β1 & β2)

  • Chain Length:

    • α-chain: 141 amino acids

    • β-chain: 146 amino acids

  • Each polypeptide consists of eight alpha-helix sections labeled from A-H, with connecting regions named accordingly (e.g., A-B, B-C).

  • Specific amino acids in the helical sections are numbered (e.g., Histidine at F-8).

• Prosthetic Group:

  • Contains a heme group, a non-protein component that binds oxygen.

  • The heme group is composed of a porphyrin ring with a central Fe2+ atom, which binds with oxygen.

  • Iron in heme is coordinated with four nitrogen atoms from pyrroles in the plane; the fifth position is occupied by the imidazole side chain of His-F-8, and oxygen binds as the sixth ligand.

Myoglobin

• Function: Oxygen storer in skeletal and cardiac muscles.

• Structure:

  • Globular protein composed of a single polypeptide chain.

  • Contains 153 amino acids and consists primarily of eight alpha helices.

  • Releases oxygen during extreme oxygen deprivation situations, such as exercise.

Heme Group and Oxygen Binding

• When oxygen binds to heme, the iron atom shifts position to 0.02 nm above the porphyrin ring, changing the conformation of hemoglobin, which enhances its ability to bind to oxygen.

• Cooperativity:

  • Binding of oxygen to one subunit of hemoglobin increases the affinity of the other subunits for oxygen, known as positive cooperativity.

Biological Function and Conformational Changes

• The conformational alteration that occurs during oxygen binding is crucial for hemoglobin's biological function.

• Hemoglobin Structures:

  • Two types of contacts exist in hemoglobin:

  • Packing Contacts: Shift during conformational changes.

  • Sliding Contacts: Change during structural transitions.

  • Hemoglobin exhibits two conformations:

  • T-state (Tense/Taut):

    • Deoxyhemoglobin state; oxygen is only accessible to the alpha-chain heme, restricted by steric hindrance.

  • R-state (Relaxed):

    • Oxyhemoglobin state where all heme groups are oxygenated, without steric hindrance.

Oxygen Transport Dynamics

• Oxygen is delivered to cells through a combination of diffusion and binding with hemoglobin.

• Chemical Forms of Oxygen Transport in Blood:

  • A small fraction of oxygen is dissolved in blood (0.31 mL per 100 mL blood).

  • The majority is transported as oxyhemoglobin in red blood cells (RBC).

• At arterial pH (7.44), oxygen binding shifts in response to H+ ions to facilitate oxygen release in tissues (venous pH = 7.35).

• Equations:

  • For oxyhemoglobin release:
    $ext{HHb} + ext{O}<em>2 ightleftharpoons ext{HbO}</em>2 + ext{H}^+$

  • For red blood cell oxygen dissociation:
    $ext{O}<em>2 + ext{Hb} ightleftharpoons ext{HbO}</em>2 + ext{H}^+$

Oxygen Binding Kinetics

• Oxygen binding to hemoglobin is sigmoidal due to allosteric interactions, enhancing the affinity for additional oxygen as each subunit binds.

• Myoglobin, in contrast, exhibits a hyperbolic binding curve as it does not participate in cooperativity.

• The P50 value indicates the partial pressure of oxygen at which hemoglobin is 50% saturated, with lung partial pressure typically around 100 mm Hg (100% saturation) and peripheral tissues around 40 mm Hg (approximately 75% saturation).

Clinical Aspects of Oxygen Saturation

• Hypoxemia: Defined as below-normal levels of oxygen in the arteries, can be caused by conditions such as anemia, ARDS, asthma, congenital heart defects, COPD, etc.

• Oxygen Delivery Capacity:

  • At a pulmonary partial pressure of 100 torr, hemoglobin can exhibit high cooperativity in loading oxygen. If cooperativity were absent, only 79% may remain bound under similar conditions.

• This difference exemplifies the efficiency of hemoglobin in oxygen transport, achieved through positive cooperativity.

Bohr Effect and Oxygen Regulation

• Active tissues produce CO2 and H+, altering the pH and favoring O2 release due to the Bohr effect.

• Hemoglobin can buffer H+ ions, impacting oxygen binding affinity and enhancing release in metabolically active tissues.

Mutations and Genetic Disorders

• Sickle-cell Anemia: This genetic disorder results from a mutation where valine replaces glutamate in the β chain of hemoglobin (referred to as hemoglobin S or HbS). Symptoms include chronic pain and increased risk of infections, with treatment options including medications and blood transfusions.

• Statistics: Affects approximately 100,000 Americans, particularly prevalent among African Americans (1 in 365 births) and Hispanic Americans (1 in 16,300 births).

Diagnosis of Anemia

• Common symptoms include fatigue, irritability, headaches, difficulty concentrating.

• Diagnosis may involve a complete blood count (CBC) and oxygen saturation measurement using a pulse oximeter, which should ideally fall between 95% and 100%. Values below 90% are considered a clinical emergency.

Sample Questions for Assessment

1. What is the oxidation state of iron in hemoglobin and methemoglobin?

2. Fetal hemoglobin is a tetramer containing what types of protein confirmation?

   • A. α2, γ2, and β2

   • B. α2 and β2

   • C. α2 and γ2

   • D. γ2 and β2

3. True or False: Myoglobin is a tetramer.

4. True or False: Oxygen loading to hemoglobin is partial pressure dependent.

5. What mutation causes sickle-cell anemia?

   • E. Valine replaces glutamate at position 6 on the surface of the beta chain.

   • F. Glutamate replaces valine at position 6 on the surface of the beta chain.