Oxygen Storage in Muscle Tissue

1

• Oxygen Storage in Muscle Tissue

• Myoglobin (Mb)
• Originally isolated from sperm whales
• 10X abundance greater in aquatic- than
  terrestrial-mammals
• Mb knockout mice exhibit normal exercise
  capacity
• Detoxification of reactive nitric oxide a
  signaling molecule

2

• Myoglobin Oxygen Binding Curve

Quantification of Myoglobins O2-Binding
Behavior Mb dissociation MbO2 ? Mb
O2 Dissociation constant K MbO2/MbO2
Myoglobin-O2 term MbO2
MbO2/K Fractional saturation Y
MbO2/(Mb MbO2)Substitute for MbO2 Y
MbO2/K/(Mb MbO2/K)Simplify Y
O2/(K O2) O2 partial pressure Y
pO2/(K pO2)
3
Myoglobin Protein Structure

• Single polypeptide chain with 8 alpha helices
  (A-H)
• Prosthetic heme group only Fe (II)
• Central Fe (II) atom bound to 4 porphyrin N
  atoms
• N of His F8 and O2 binds below and above the Fe
  (II)

4
Globin Evolution
Homologous proteins with a common
ancestor Genetic mutations with invariant
residues
Adult hemoglobin
Embryonic hemoglobin
5
Hemoglobin Oxygen Transport in Mammals

• Hemoglobin features
• Located in red blood cells 4 subunits
• Cooperative oxygen binding (allosteric protein)
• Deoxygenated T-form and Oxygenated R-form

6
Hemoglobin Cooperativity Enhances O2 Delivery

• Proximal His binds Fe (II)
• Distal His disfavors CO binding
• O2 ? Fe (II) shift from out of plane to
  porphyrin plane

Limitations with Hemoglobin as an oxygen
transporter?
7
Conformational Change in Hemoglobin
Which is the T and R configuration? Is this
enzyme conversion concerted or sequential?
8
2,3-Bisphosphoglycerate Stabilizes Hemoglobin T
Configuration
Central cavity size difference T versus R?
Amino acids that comprise the central cavity?
9
Hemoglobin Central Cavity with 2,3-Bisphosphoglyce
rate
How does 2,3-BPG affect T versus R configuration?
10
Bohr Effect Protons and Carbon Dioxide Promote
the T State

• Stabilizing the T state increases hemoglobin
  O2 release
• T - more sigmodial
• R - more hyperbolic
• T ? R H

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T-State Stabilization by Salt Bridge Formation
How is CO2 linked with H concentration?
12
Carbon Dioxide Acidifies the Hemoglobin
Environment
13
Tissue-Specific Hemoglobin Affinity for Oxygen
What is the favorable hemoglobin state (T/R) in
capillaries near muscle tissue and lungs?
14
Adjusted Hemoglobin-Oxygen Affinity with
Environmental Changes
How must fetal hemoglobin oxygen binding vary
from the mother?
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Adjusted Hemoglobin-Oxygen Affinity with
Environmental Changes
Fetal hemoglobin (a2?2) has a Ser substitution
for His Two less positive charges in central
cavity means what?
16
Hemoglobin Mutation Sickle-Cell Anemia

• Sickle-Cell Features
• Lower hemoglobin (Hb) solubility for Hb S
  than Hb A (normal)
• Amino acid substitution Glu ? Val
• Reduced T-state solubility
• O2 affinity and allosteric properties
  unaffected
• Heterozygotes asymptomatic

17
Copper Oxygen Carrier Present in Lobster
Hemoglobin
Do changes in the oxygen-binding prosthetic group
preclude cooperative O2 binding?
18
Site-Directed Mutagenesis Experimentation

• Convert proximal His ? Gly
• Exogenous imidazole binds with porphyrin iron

• Would modified hemoglobin
• exhibit cooperative binding?
• (ii) generate Fe (III)?

19
Substitution for 2,3-Bisphosphoglycerate in Bird
Hemoglobin
Which compound(s) are well suited to bind in the
central cavity of bird hemoglobin?
20
Hemoglobin Fractional Saturation Curve Shift
Curve matching Increase in CO2 Increase in
2,3-BPG Increase in pH Loss of 4 structure
Physiological CO2 and 2,3-BPG at pH 7