MCB Chapter 9: Myoglobin and Hemoglobin

MCB 2000S

Myoglobin Protein

a single polypeptide chain (3*)

Role: a single heme group that binds oxygen. (in muscle tissue)

Hemoglobin Protein

Multiple Polypeptide subunits (4*)

• 4 pp chains: 2 alpha (identical) & 2 Beta (identical) chains that transport oxygen in the blood.

• Has 4 heme groups that bind 4 different O2

• Transports & Protects O2 from the hydrophilic environment

Heme Group

Prosthetics Group that assists protein (Mg & Hg) when binding to oxygen

Structure

• Organic (protoporphyrin) C,N,H,O

• Inorganic (Iron: Fe2+) responsible for binding to protein & O2

Proximal Histidine

an amino acid in myoglobin and hemoglobin that coordinates the iron atom in the heme group, stabilizing the binding of oxygen.

(lower coordinate site)

Resonance stabilization

• Iron is too big/ electronegative so the addition of O2 (more electronegative) pulls iron up into the heme and makes it smaller. (Resonance stabilizing)

• From ferrous ion - Ferric Ion (less stable)

Extra

• Ferric ion needs another Amino acid to make - charge more stable (Distal Histidine)

Hemoglobin Binds how different?

Positive Cooperatively

• When 1 site of hemoglobin binds to 1 oxygen it makes it easier for other oxygen to bind to the rest of its 3 sites as hemoglobin goes under a 3D conformational change (shift)

• R-state - T-state

R-state & T-state of hemoglobin

T-state (Tense)

• Deoxyhemoglobin state because Iron is so big and its lowers Affinity.

R-state (Rest)

• Oxyhemoglobin because O2 made Iron have a higher affinity.

This all causing cooperation

O2 binding curve

Myoglobin

• Hyperbolic curve

• High affinity for o2

• Will only drop o2 fully in very low pressure

• Good for tissue

Hemoglobin

• Sigmoidal

• Low affinity for 02

• T state makes 1st o2 hard to bind after r state easy bind

• Release o2 in tissue, takes from lungs

• smart response

2,3 BPG (long term)

Hemoglobin regulator in tetramer

• Decreases hemoglobin affinity for oxygen (stabilizes t-state)

• Makes letting go of oxygen better in tissue

• (Right shift)

• Helpful for storing in the lungs

• During low 02 conditions

• From Red blood cells

• Negative charges all over

Bohr Effect (short term)

• Hemoglobin regulator

• When acidic and high co2

• They cause a shape change in Hb and decrease o2 affinity

• To let go of oxygen more easily

• best at low ph. and high co2

Fetal and Maternal HB

Fetus needs O2 from mother

• Needs a higher affinity for o2

• Don’t have a beta subunit (His143), but it has a gamma subunit replacing it (serine)

• Mutation His143Ser

• Left shift

• Low Km, High affinity