Myoglobin and hemoglobin are proteins that transport oxygen in the body. Myoglobin transports and stores oxygen in muscles. It has a globin protein that surrounds a heme group, protecting it and allowing oxygen binding. Hemoglobin transports oxygen in blood and differs from myoglobin in being a tetramer of four globin chains that bind oxygen cooperatively. The binding of oxygen to one chain increases the affinity of the other chains. This allows hemoglobin to efficiently load and release oxygen in the lungs and tissues respectively.

1. Myoglobin and Hemoglobin
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

2. Myoglobin & Hemoglobin
Objectives
 Identify biological functions.
 Identify parts of Mb & their roles in
O2 transport.
 Identify how Hb differs from Mb.

3. Myoglobin
 O2 transport, storage
in cells.
 Two parts: protein
and heme prosthetic
group.
 Protein:
 155 amino acids, ~ 17
kDa.
 Compact, globin fold.
 75% helix.

4. Heme
 Heme is composed of porphyrin and Fe2+.
 Porphyrin is non-polar, except two proprionate groups.
 Porphyrin binds O2.

5. Heme Ligands
 Fe is 6-coordinate.
 Four N of heme group
 One N from proximal His.
 One from H2O or O2.

6. Heme – Protein Interaction
 Heme stabilizes
protein fold.
 Binds through hf
interactions
 Prooprionate groups
on surface.
carboxyl group of
proprionate.

7. Myoglobin
 Protein completely
surrounds heme.
 Function of protein:
 ↑ heme solubility.
 ↓ heme oxidation
(Fe2+ → Fe3+)
 metmyoglobin inactive.
 ↓ CO binding.

8. O2 vs. CO2 binding.
 CO binds
tightly; linear.
 O2 binds less
tightly, bent
structure.
 Distal His forces
bent binding of
both, weakens
CO2 binding.
Distal His
Proximal
His
Fe
C
O
Fe
O
O
Fe
C
O

9. Hemoglobin
 Tetrameric protein
 Dimer of dimers, (ab)2
 a,b chains resemble Mb.
 Each chain contains, heme,
prox. histidine.
 Each binds 1 eq. O2.
 ab dimer interface different
from aa, bb interface.
 Marked by salt bridges that
stabilize the deoxy structure.

10. 0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120
pO
2,
mm Hg
Y,
fractional
saturation
4° Structure of Hb alters O2 binding.
 Interactions between
dimers alters oxygen
binding.
 Direct plot shows Hb
has lower affinity than
Mb.
 Sets up delivery
system.
 O2 bound by Hb in
lungs; released in
tissues.
Mb Hb

11. Cooperative regulation
Hb oxygen binding:
 Start binding with given affinity in deoxy state, subsequent binding
enhances affinity.
 Defines positive cooperative regulation.
 Only seen in multi-domain proteins.
 Hill coefficient ~ number of interacting subunits. Advantage: binding
is more sensitive to small changes in [ligand].

12. Molecular Model
 In deoxy state, Fe out
of heme plane;
domed.
 Bind O2, moves Fe.
 This moves proximal
His and its helix.
 Moving helix alters
a/b interface.
 Deoxy = Tense (T)
 Oxy = Relaxed (R)
Fe
N N
N
O
O

Fe
N N
N

0.7A
His-F8
proximal side
distal side
N
N
C
O
Fe
Fe
CO
F Helix.
T
R

13. H146 D94
Bohr Effect
 Oxygen affinity sensitive to pH.
 ↓ pH; ↑ pO2,50 (lowers sensitivity).
 D94 ↔ H146 salt bridge in T
state only.
 Excess H+ forms salt bridge,
favors deoxy state.
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120
pH = 7.5
pH = 7.2
pH = 6.8
log pO2
log
Q

14. CO2
 Produced during aerobic
metabolism.
 Reacts with N terminal
amino; carbamylation
reaction.
 Negative charge forms salt
bridge with aR141,
stabilizes deoxy state.
R-NH2 + CO2 R-NH-CO2
- + H+
In tissues,
High CO2
In lungs,
Low CO2
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120
pH = 7.5
pH = 7.2
pH = 6.8
log pO2
log
Q

15. CO2 is Coupled to Bohr Effect
In Tissue
 CO2 is bound by Hb or
converted to bicarbonate by
carbonic anhydrase.
 Buffers blood pH.
 Hb binds 2H+ / 4 O2 released, also
buffers (Bohr effect).
In Lungs
 Low pCO2; reaction reverses;
 CO2 and H+ released from Hb,
 pO2,50 decreased (increased oxygen
affinity).
From Lange’s Biochemistry

16. 2,3-BPG
 Side product of
glycolysis.
 indicates active
respiration, need O2.
 Binds cationic region in
T-form.
 Favors deoxy, releases O2
to tissues.
 [2,3-BPG] is high,
responsible for observed
pO2,50 of 27 mm Hg.
 stripped Hb has pO2, 50 ~
8 mm Hg.

17. Another look at 2,3-BPG
 BPG acts as a
“wedge” and drives
the R state to the T
state.
 Forces release of
bound O2 in active
tissue.
 BPG increases at
high altitude.