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Define the terms heme and hemeprotein, and explain the structural
features of the heme iron atom that allows for reversible oxygen binding.
o heme: iron porphyrin structure; large, aromatic tetrapyrrole with
iron atom in the Fe++ (ferrous) state (allows reversible O2 binding)
o hemeprotein: proteins that contain a heme group; eg: Hb, Mb
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Compare and contrast the general structural features of hemoglobin versus
myoglobin, in terms of primary, secondary, tertiary and quaternary
structure.
o Hb (transport)
 Quaternary:
 tetrameric
 2 alpha/2 beta chains
 alpha/beta dimers associate via hydrophobic
interaction, H-bond, ion pairs
 Tertiary:
 globular
 heme prosthetic group
 2ndary:
 alpha helices
o Mb (storage)
 No quaternary structure (monomeric)
 Tertiary:
 globular
 heme prosthetic group
 2ndary:
 8 alpha helices
Draw and label the sigmoidal oxygen binding curve for hemoglobin and
the hyperbolic oxygen binding curve for myoglobin.

Explain the structural basis for the oxygen binding properties of myoglobin
versus hemoglobin, focusing on the positive cooperativity of oxygen
binding to hemoglobin. Include the definition of the term “allosteric” as it
pertains to hemoglobin and oxygen binding.
o at any partial pressure of O2, you will have a higher % O2 sat for Mb
than Hb because of positive cooperativity of Hb
o Positive cooperativity:
 first O2 binds
 heme Fe drawn into plane of porphyrin ring
 causes conformational changes, which transmit to other
subunits, making it easier for O2 to bind
 transmitted by “protein-protein” contacts
 as O2 binds Hb, adjacent subunits’ affinity for O2 increases
 as O2 binds Hb, adjacent subunits’ affinity for O2 decreases
o Positive cooperativity is good for Hb because it has to pick up O2 in
lungs and release it in the tissues; don’t want it to have such a high

affinity that it won’t release in the tissues- but don’t want it to have
such low affinity that won’t pick up efficiently in lungs
o can see this in the sigmoidal binding curve of Hb
o when look at binding curve for Mb, it’s not sigmoidal bc there is no
cooperativity; this is a good thing because Mb is responsible mainly
for O2 storage
o If Hb demonstrated binding similar to Mb, would not be able to
release sufficient O2 to tissues
o allosteric proteins: binding of ligand at one site affects binding
strength at another
 allosteric modulators make it easier/harder for O2 to bind Hb
Explain why myoglobin and hemoglobin structure / oxygen affinity /
oxygen binding properties are appropriate to meet tissue oxygen needs in
working muscle versus resting muscle.
o Basics
 O2 6th ligand, reversibly binds
 Fe must be in reduced form (ferrous, Fe++) in order for O2 to
bind; so body must prevent oxidation of Fe2+Fe3+
 positive cooperativity increases affinity for O2 as more O2 bind
Hb
 T state Hb= low affinity state (this form mostly in tissues)
 R state Hb= high affinity state (this form mostly in lungs)
o Working mm:
 consuming O2, producing CO2 (lowers pH)
 producing a lot of lactic acid via anaerobic respiration (lowers
pH)
 so- have excess H+ in working mm
 H+ can bind allosterically to Hb at a His residue
 this His will then form an ion pair with Asp residue, stabilizing
the T, lower-affinity state of Hb
 lower affinity means Hb will be releasing O2 to the mm tissues
during work
o resting mm:
 opposite of above;
 less H+ means less binding to Hb and less stabilization of T
state Hb
 shifts to the R state, which is the high affinity state for O2
 now, Hb will hold onto the O2 (since it’s not needed in resting
mm) and transport to other tissues that need it
o This can apply to the lungs too
 lungs have high pH (low [H+])
 Hb will be in R state bc H+ not present to stabilize the T (low
affinity) state of Hb
 R state has high affinity for O2
 Hb will bind O2 in lungs for transport to tissues
o Process described above= Bohr effect (illustrated below)- at any given
pH (7.2, 7.4, 7.6) we see a higher affinity for O2 as the pH increases

Define the term “allosteric modulator” and explain how the following
physiological allosteric modulators affect hemoglobin oxygen binding
affinity: protons (the Bohr Effect), CO2 binding directly to hemoglobin, 2,3bisphosphoglycerate (2,3-BPG).
o allosteric modulator: a molecule that binds to a protein, causing a
conformational change in the protein and affecting the binding of a
subsequent ligand
o see above for Bohr effect
o CO2:
 indirectly increases the Bohr effect
 some CO2 attaches to amino termini of Hb
 this produces an H+ which can then bind Hb and stabilize the T
state, decreasing its affinity for O2
 makes sense- if high [CO2] where Hb is present- then want to
take up the CO2 and release O2 to that tissue, so will want to
decrease Hb affinity for O2
o 2,3-BPG
 can bind in pocket between beta subunits of Hb and stabilize
the T state
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 binds to 2 Lys, 4 His, 2-N-termini
when O2 binds in R state, gap between subunits closes and
BPG can no longer bind
helps create sigmoidal/cooperative binding of Hb
Hb requires very little pO2 to reach ½ binding sites occupied
when BPG absent
Decrease in pO2 from lungs to tissues:
 People who live at sea level:
o higher initial pO2 in lungs
o decreases about 38% from lungs to tissues
 People who live at high altitudes:
o lower pO2 in lungs
o decrease about 37% from lungs to tissues
 People who live at sea level and then visit high
altitudes:
o lower pO2 in lungs
o decrease only about 30% from lungs to tissues
o this is 8% less of a decrease than normal
 correlates to a 20% decrease from normal
amount delivered (8/~40= .2)
 why?
 8% of the O2 that is normally
delivered to the tissues is not
being delivered to the tissues
because there is less to start off
with.
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How does the person cope with changes in O2 delivery to
tissues due to the decrease in pO2 at higher altitudes?
 upregulate transcription of BPG
 increases [BPG] in blood
 decreases Hb affinity for O2 in tissues
 now able to unload more O2 in tissues
Explain the structural modification of hemoglobin that gives rise to the
sickle cell variant (HbS), and explain how this modification affects
hemoglobin structure, oxygen binding ability, and ultimately red blood cell
function and morphology.
o Glu6 Val6
o Glu is neg charged AA; would normally cause Hb molecules to repel
each other
o when Val is subs. in place of Glu, no longer repel
o as a result, Hb molecules become clumped
o get hydrophobic interaction between HbS molecules
o sickled, flat/crescent-shaped RBCs
o aggregated O2- poor O2 transporter
o can clog capillaries
Explain the physiological significance of fetal hemoglobin (HbF) having a
higher oxygen affinity than maternal hemoglobin (HbA). Explain the
structural basis for this difference.
Fetal Hb and BPG
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Fetal Hb- gamma chains instead of Beta chains
fetus must extract O2 from mother’s blood
o therefore- must have greater affinity for O2
synthesizes Hb with gamma chains, which have much LOWER affinity
for BPG than beta

o AA His143 in Beta and Ser143 in gamma
o Positive charge to no charge
o remember- BPG binds because of +++ in binding pocket of T
state
o if no +++, won’t bind as readily
LOWER affinity for BPG, HIGHER affinity for O2
Shows HbF higher affinity for O2 than HbA