BC4 Hemoglobin & Myoglobin

Question 1

Myoglobin

Answer 1

heart and skeletal muscle (small amount in smooth)

binds and stores O2, releasing when needed.

MOnomer 75% alpha helices

hydrophobic interactions to stabilize

tertiary results in globin fol, crevice between E and F for heme binding

Question 2

hemoglobin

Answer 2

erythrocytes

bind and transport O2 from lungs to tissue

CO2 from tissue to lungs

tetramer, two alpha globin and two beta globin polypeptides. or two ab dimers

HbA, a2b2 90% of total

HbF a2g2 <2% but higher affinity (fetal)

Question 3

Heme Group

Answer 3

Prosthetic group that binds O2

protoporphyrin ring with ferrous iron 2+ in the center with six coordination sites (need Fe + heme for easy to release O2)

4 filled with N of the planar ring, and two other sites bind H2O in free heme.

Fe3+ makes it methemoglobin, NOT ACTIVE FOR BINDING

FIFTH coordination site binds to histidine, on alpha helix of the protein (F helix)

SIXTH coordination site binds O2 molecule. O2 is in turn stabilized by a distal histidine residue within the E helix.

Question 4

Myoglobin dissociation curve

Answer 4

hyperbolic, Mb binds tightly for storage

lungs…always bound (never in lungs)
95% saturated (pO2 110)

tissues 90% saturated (pO2-30)

only releases under stress (pO2 10)

Question 5

Hb Dissociation curve

Answer 5

sigmoidal

lower affinity

lungs: 95%
tissues: 50%

arterial and venus pressure releases about 45% of O2

Question 6

Hb T & R

Answer 6

T- tense R- relaxed
T-deoxyhemoglobin, low affinity of O2
R-oxyhemoglobin, high affinity

upon binding O2, Fe2+ atom moves into the plane of the porphyrin ring (change in tertiary structure and quaternary)

Fe pulls proximal histidine (F helix) with it, and in turn rotates one ab dimer 15 degrees relative to the other. B

Beta subunits come closer together, and more compact hemeoglobin can bind easier.

by 4th, 100fold greater binding affinity

Question 7

Hb affinity by tissue

Answer 7

in lungs, high pO2, high binding affinity

in the tissue, low pO2, low affinity

Question 8

Allosteric regulation

Answer 8

regulation of protein activity through reversible binding of molecules named effectors

only proteins with quart. structure can have allosteric activity

HOMOTROPIC: identical to true ligand

HETEROTROPIC: different from true ligand
-usually bind in different site from true ligand

Question 9

Negative Allosteric Effectors

Answer 9

(i) stabilize T state (ii)shift the binding curve to the right (iii)promote release of O2

2,3 BPG-byproduct of glycolysis.  
-abundant in blood (5mM)
-stabilizes T confirmation by binding in cavity, holding it open, kicks out O2 and changes R state
-negative charged ionic 
-allows more O2 release in tissue
CANNOT BIND TO FETAL

CO2
-binds to N terminus of Hb. goes from + to - carbamate
-ionic bonds then stabilize t state
-20% Co2 transported to lungs on Hb, 75% as HCO3, 5% as Co2
arterial blood low, veinous blood high affinity

H+ (Bhor)
-decreasing pH, deoxy becomes stabilized.
-pH drops, Hb releases O2
-Hb a buffer
-binds to histidine
lungs: H+ increases amount of CO2 exhaled
pO2 high ->release CO2 -> H2CO3 -> carbonic anhydrase -> H2) and Co2

Question 10

Positive Allosteric effector

Answer 10

Carbon Monoxide & O2
CO
heme binds CO with higher affinity than O2

distal histidine hinders this connection, so that its only 210 fold higher (25000)

1. reduces O2 transport
2. increases affinity for O2

poisoning -> O2 therapy