6.2 Myoglobin and Hemoglobin - Gillie

What are the characteristics of myoglobin structure?

•Monomer

•Small and compact:

-153 amino acids

Where is myoglobin found and its function?

Found in muscle tissue

-Oxygen storage

What is myoglobin composed of?

Composed of 8 α-helices

-Helices A - H

What does myoglobin contain?

Prosthetic heme group

Does myoglobin reversibly bind O2?

Yes

When is myoglobin put onto the prosthetic heme group?

After it is made

What is the structure of heme group?

Porphyrin ring system

Where is the heme group placed in myoglobin?

Placed in hydrophobic pocket between helices E and F

What atom is at the center of the heme group?

Iron atom

What does the iron atom at the the center of the heme group coordinate?

Coordinate covalent bonds with electronegative atoms

What are the oxidation states of Fe?

Fe^2+ or Fe^3+

Which oxidation state of iron reversibly binds O2?

Fe^2+

How many coordination sites does Fe^2+ have and what do they bind?

6 coordination sites:

–4 to Pyrrole N’s in the heme

–1 to Proximal Histidine in protein

–1 to O2 → reversible

What changes in Fe^2+ coordination if there is a distal His?

H-bonds to O2

-On E helix of protein

How is Ka related to binding affinity?

Directly

higher ka = stronger binding

lower ka = weaker binding

How is Kd related to binding affinity?

Inversely

higher kd = weaker binding

lower kd = stronger binding

What is the shape of a binding curve?

sigmoidal 9s-shaped)

What is cooperative binding?

Binding of one ligand to one site increases binding affinity for another ligand at another site

How does cooperative binding work in reverse?

Release of one ligand to one site decrease binding affinity for another ligand at another site

What are the two states of hemoglobin?

deoxyhemoglobin and oxyhemoglobin

Which form of hemoglobin has O2 bound?

oxyhemoglobin

What is the shape of the heme ring shape in deoxyhemoglobin?

Not planar

How far out of plane is the Fe in the heme ring in deoxyhemoglobin?

~0.6 Å out of plane

What is the shape of the heme ring shape in oxyhemoglobin?

Planar

What is the conformation of the heme ring in deoxyhemoglobin?

T or tense

What is the conformation of the heme ring in oxyhemoglobin?

R or relaxed

What happens during the conformational shape of deoxyhemoglobin to oxyhemoglobin?

-O2 binding pulls Fe into plane of heme

•His F8 is pulled with Fe

•Entire F helix moves

What are two models of hemoglobin binding?

Concerted and sequential

What happens in the concerted hemoglobin binding model?

•Protein found in either all T or all R

-Binding to a single subunit helps stabilize the R-state conformation

-More of the protein complexes are found in the R state

What happens in the sequential hemoglobin binding model?

•Protein found in a mix of T and R

-Binding doesn't convert the entire complex to the R state

-Alters the affinity of adjacent subunits

What are the two allosteric effectors?

Positive effectors and negative effectors

What do positive effectors do?

-Increase Hb affinity for O2

-Shift equilibrium toward R state

•Example: O2

What do negative effectors do?

-Decrease Hb affinity for O2

-Shift equilibrium toward T state

•Examples: 2,3-BPG, H+, CO2

What does the Bohr effect decrease?

O2 binding affinity