Myglobin/Hemoglobin Structure & Function Flashcards

Hemoglobin
Terms Definitions
Simple Protein
contains only amino acids
Conjugated Protein
contains amino acids + non-amino acid component (prosthetic group)
Apoprotein
conjugated protein minus prosthetic group
Hemoglobin
oxygen carrying protein in red blood cells
Myoglobin
oxygen binding protein in muscle
For Mb or Hb, protein is
globin protein (composed of AA)
For Hb and Mb, prosthetic group is
heme group (protoporphyrin composed of 4 pyrrole rings)
Physiological role of Myoglobin:
a) found in heart and skeletal muscleb) serves as oxygen reservoir
Protein structure of Myoglobin
1) Compact, globular protein2) 75% alpha helix content3) 4 helices are terminated by Pro residues (Pro=helix breaker)4) is so compact that there is no room for water in its interior
globin fold
name given to the protein structure of Mb and Hb where the 8 helices create a pocket and wrap around the porphyrin ring
Heme pocket of globin protein
- serves to keep iron in 2+ oxidation state- this insures that reversible oxygen binding occurs
Ferrous
- when iron is in 2+ oxidation state- ferrohemoglobin or ferromyoglobin is formed and can reversibly bind oxygen
Ferric
- when iron is in 3+ oxidation state- ferrihemoglobin or ferrimyoglobin is formed and does not bind
Ferrihemoglobin
also called methemoglobin
Picket-fence iron porphyrin
- heme with synthetic carbon chains attached - synthetic chains create an artificial pocket on the exposed side of 2+ ferrous ion - can mimic oxygen binding of Mb and Hb
sliding dimers
α₁β₁ dimer and β₂α₂ - the two subunits of each dimer are tightly associated, but the two dimers are more loosely associated and slide by each other
"R"
- oxyHb- relaxed form- more compact- fewer salt bridges b/w mobile dimers- β subunits closer together- no cavity in tetramer- 33 Å b/w β chains
"T"
- deoxyHb- taut form- more extended- more salt bridges b/w mobile dimers- β subunits further apart- 40 Å b/w β chains
Cooperativity
binding of first ligand makes it easier to bind the second ligand etc.
Oxygen dissociation curve
plot of % Saturation of Hb or Mb on y-axis vs. partial pressure of oxygen (pO₂) on x-axis
Compare O₂ dissociation curves for Hb and Mb
1) Mb has higher O₂ affinity (Mb is half-saturated at 1 mm Hg, Hb is half-saturated at 26 mm Hg)2) Mb dissociation curve is hyperbolic, Hb dissociation curve is sigmoidal
Hill Plot
a diagnostic plot used to evaluate the extent of cooperativity which exists in a biological system- plot log (y/1-y) vs. log pO₂ - represents a linear transformation
Hill coefficient
slope (n) of the line at midpoint of binding- n=1, no cooperativity (binding sites are independent)- n > 1, positive cooperativity (sites are interdependent)- n increases with degree of cooperativity (max. value of n equals the # of binding sites that are interdependent)
Allosteric Effector
some molecule (other than O₂) that binds to a Hb allosteric site and alters Hb O₂ binding
Allosteric
other site
2 categories of allosteric protein sites
1) Ligand binding site2) Allosteric site
Ligand binding site
heme group where O₂ binds- "work site"
Allosteric site
another site on the protein where an allosteric effector binds and changes the affinity of the protein for its ligand- "regulatory site"
3 Allosteric Effectors of Hb
H+, CO₂, and 2,3-bisphophoglycerate (BPG)
Bohr Effect
lowering pH reduces the affinity of Hb for O₂
Lowering the pH does what to the Hb O₂ dissociation curve?
shifts it to the right
H+ bonding does what to the Hb O₂ dissociation curve?
make the curve more sigmoidal
3 functional groups of "T" with greater H+ affinity
β-142 histidine, α-122 histidine, & α-NH₂ group of α chain
Fetal Hb
HbF α₂γ₂
Adult Hb
HbA α₂β₂
Sequential model for allosteric interactions
subunits change conformation one at a time- Koshland proposed
Concerted model for allosteric interactions
subunits all change conformation together- monod, wyman, and changeux proposed
Homotropic Effects
effects due to allosteric interaction b/w identical ligands, as exemplified by the cooperative binding of O₂ to hemoglobin
Heterotropic Effects
effects due to allosteric interactions b/w different ligands, as exemplified by the decrease in O₂ binding elicited by H+, C0₂, and BPG
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