Lecture 5 - Myoglobin and Hemoglobin

What process do myoglobin and hemoglobin play a part in?

Oxygen transport and storage

Why do we need myo and hemoglobin?

1. O₂ has limited solubility in water

2. O₂ oxidizes and creates free radicals = toxic to us

3. Though Fe²⁺ binds O₂, it is not soluble either

4. Heme allows Fe²⁺ to bind O₂ without the negatives

Myoglobin primary function

Store oxygen in muscles, and when O₂ level is high

Hemoglobin primary function

• Transport oxygen from lungs to tissues

  • Can bind in lung when oxygen is high

  • Can release in tissues when oxygen is low

Where is myoglobin found?

Muscles/tissues

Where is hemoglobin found?

Lungs or tissues; erythrocytes (RBCs)

Myoglobin structure

• Globular protein with 8 a-helices connected by loops

  • A Helix, B Helix connected at AB corner

Amphipathic helices

Helices with both non-polar and polar faces (facing inside and outside respectively)

Prosthetic Group

Non-protein group that binds to protein and is required for function

What type of group is Heme?

Prosthetic group; it is required in myoglobin and hemoglobin for function

What role does Heme play?

1. Solubilize Fe

2. Prevent Fe²⁺ oxidation by O₂

Heme structure

4 nitrogens in a porphyrin ring, coordinated to an iron atom

How many bonds can Fe form?

Six bonds (4 with nitrogen, 1 with proximal Histidine, 1 with O)

Proximal Histidine

Histidine that binds directly to Fe in Heme

Distal Histidine

Hydrogen bonds with oxygen

What happens to Heme upon oxygenation?

It flattens; conformation change

Deoxy-Mb

5 Fe ligands (bonds); 4 from Heme and 1 from protein (proximal Histidine)

Oxy-Mb

Oxygen is the 6th ligand to iron

What is Q?

Fractional saturation with oxygen (.75 = 75%)

P(O₂)

Concentration of oxygen measured by pressure

P₅₀

O₂ needed to saturate myoglobin/hemoglobin 50%

Why does myoglobin bind O₂ effectively?

It has a high affinity; doesn’t let go easily though

Unique property of histidine

It can bind metals like Fe

Mb and Hb are…

Homologous proteins (share properties due to shared sequences)

How many hemes does hemoglobin have?

4 (compared to 1 in myoglobin)

Expected curve for myoglobin

Hyperbolic (efficient at binding, inefficient at release)

Expect curve for hemoglobin

Sigmoidal (cooperative binding; good at binding and release)

Cooperative Binding

Change in protein’s affinity for ligan as a function of ligand concentration

What makes hemoglobin good at transport?

It can transition from high to low affinity (cooperative binding)

Cooperativity is also known as…

Allostery

Requirements for cooperativity

1. Multiple, connected binding sites

2. Communication between sites

3. Quaternary structure (usually)

Hill plot

Graph that allows us to analyze cooperativity (index - nH - of cooperativity)

Slope of 1 on Hill Plot means…

No cooperativity

Increasing number of binding sites…

Increases index of cooperativity

If nH > 1…

Positive cooperativity

If nH < 1…

Negative cooperativity

Positive cooperativity

Binding of one ligand increases the likelihood of binding to another (Hemoglobin)

Negative cooperativity

Binding of one ligand decreases likelihood of another binding

Important of Hemoglobin’s Allostery

Allows to bind weakly to O₂ when oxygen is low

Allows to bind strongly to O₂ when oxygen is high

Unique Structure of Hemoglobin

Dimer of 2 Dimers

a1 to a2 and b1 to b2

Few contacts of polar salt bridges (weakest)

a1 to b1 and a2 to b2 (vertical)

VERY strong due to many contacts = not easily altered

a1 to b2 and a2 to b1

Fairly strong but can be altered