The 3 dimensional structure is determined by its ____ structure

Primary

Hemoglobin vs Myoglobin? What do they bind to?

Hemoglobin: binds O2 in lungs and releases it in tissues Myoglobin: binds O2 in muscle cells

Where do you get O2 during intense exercise?

Myoglobin acts as a reserve

What is the dissociation equation?

Kd = [X][Y] / [XY]

If you have an increase in Kd, would you expect a more shallow or steep curve?

Shallow

What would a shift right mean for Kd?

Weaker affinity of ligand for protein

What does Kd indicate?

Concentration of the ligand where % saturation is 50%

Where is the heme group located in myoglobin?

Between helices E and F

What is the geometry of heme?

Circular and planar

Describe the poryphrin ring of heme

- Ring contains Fe ion w/ 4 N atoms around it - 2 substituents at bottom of ring are polar charge groups (the rest are non polar aliphatic groups)

What occurs at the 5th coordination position?

His residue binding

What occurs at the 6th coordination position?

O2 binding

What is the His at the 5th coordination position called?

Proximal His or HisF8

What does the proximal His do? 2

- Bind heme into heme-binding pocket - Prevent oxidation of Fe atom

What does the distal His do? 2

- Increase O2 binding affinity - Lower affinity for other molecules

T/F: Helices E/F move during O2 binding

False, they have a constant affinity

What happens if carbon monoxide is present?

CO will bind to Fe - Myoglobin not available to bind to O2

Describe the quaternary structure of hemoglobin

- 2 alpha subunits - 2 beta subunits - Heterotetramer

Describe the tertiary structure of alpha and beta globin, and myoglobin

3 polypeptides w/ 8 alpha helices w/ heme binding pocket between helices E/F - homologous proteins

How many polypeptide chains does hemoglobin have & how many O2 can they bind? myoglobin?

Hemoglobin: 4 chains, 4 O2/Hb Myoglobin: 1 chain, 1 O2/Mb

What are conservative substitutions?

Have minor effects on structure/function

What are critical substitutions?

Change structure and function

How do alpha and beta subunit of Hb bind O2?

Same as Mb - O2 at 6th coordination position of an Fe ion in heme ring

What does hyperbolic curve say about affinity? Hb or Mb?

Constant affinity - Kd doesnt change - Mb

What does sigmoidal curve say about affinity? Hb or Mb?

Cooperative binding affinity - ligand affinity changes as more ligand bind - Hb

How does Hb change its affinity for O2?

Tense state: low affinity Relaxed state: high affinity

Does the tense state include dexoy or oxy Hb? What size is the central cavity

- Deoxy - Large

What are effectors?

Compounds which alter their affinity at other binding sites after binding to something

What is homoallosteric?

Binding of effector affects further binding of same compound

What is heteroallosteric?

Binding of effector affects further binding of diff compound

What are activators? inhibitors?

Activators: increase binding affinity Inhibitors: decrease binding affinity

What is positive cooperativity?

Binding of a ligand/substrate at one site increases affinity of other sites for same ligand

O2 is a _________ activator of Hb

Homoallosteric

Explain the T state events in O2 binding: Fe moves into plane of heme 3

- HisF8 moves w/ Fe - Helix F moves - Subunit interface changes

Explain the T state events in O2 binding: Subunit interface changes 2

- Helix F/His F8/Fe movement - O2 binding site becomes high affinity (R)

What are the 3 allosteric effectors on Hb?

- O2 (homo activator) - 2,3-BPG (hetero inhibitor) - H+ (hetero inhibitor)

Increased BPG means what for O2 binding?

Decreased

Does BPG bind to oxy or deoxyhemoglobin? why?

Deoxy because oxy's central cavity is too small

Does metabolism increase or decrease pH?

Decrease - Generates p+

What does a lower pH mean for AA side chains? and for BPG? O2?

Protonation of side chains - Enhanced BPG binding - Decreased O binding

What is the favoured state in the lungs? Why?

- R state - high ppO2 and pH

Actively respiring tissues have a _____ pH Favoured state?

Low pH - T state is favoured = O2 released

What kind of substitution do sickle cell anemia cells have?

Critical - Glu6 is replaced w/ Val

Describe the cell appearance of sickle cells

- In Hb there is small hydrophobic patch exposed between helices E/F during T->R - Val (hydrophobic) binds = Hb aggregates

Does His143 increase or decrease BPG affinity?

Decrease = Increase O2 affinity