Hemoglobin Flashcards
Hemeproteins
A group of specialized proteins that contain heme as a tightly bound prosthetic group.
Prosthetic group
coenzyme that is permanently associated with the enzyme and returned to its original form.. Example is Heme
What is Heme?
a Prosthetic group
Examples of hemeproteins
cytochromes
catalase
myoglobin
hemoglobin
Myoglobin
heme is used to reversibly bind oxygen
hemoglobin
heme is used to reversibly bind oxygen
Structure of heme
Most prevalent porphyrin in humans
Uses Fe2+ held in center by four bonds to the nitrogens
Heme iron can form two additional bonds (6 total); in myo/hemoglobin one bond with R group of a histadine and other available to bind oxygen
What can sense O2 status on hemoglobin and myoglobin?
R-group of Histidine
Where is myoglobin present?
heart and skeletal muscle where it serves as
- O2 reservoir
- O2 carrier that increases the rate of transport of O2 within the cell
Myoglobin is a single polypeptide, similar to the individual alpha and beta globin chains found in hemoglobin
What is myoglobin mainly composed of?
80% alpha helices
What is the interior of the molecule of myoglobin?
nonpolar AA
What is located on the surface of myoglobin?
polar AA
Where does the hem group sit on myoglobin
in a crevice in the molecule lined with nonpolar AA except for two histidines
histidine in myoglobin
one hisitine binds to the iron of heme, the other histidine stabilizes the binding of oxygen to the iron. Histidine can actually become charged in this environment
Where is hemoglobin found?/
exclusively in the RBC
what is function of hemoglobin
transport O2
Hemoglobin A
major adult hemoglobin composed of 4 polypeptide chains, two alpha chains, and two beta chains, held together by noncovalent interactions. This allows flexibility
What can hemoglobin do that myoglobin cannot?
interact with other subunits
How many O2 molecules can hemoglobin transport from lungs to peripheral tissue?
4
What can hemoglobin do other than transport O2?
transport CO2 and H+ from peripheral tissue back to the lungs
What are oxygen binding properties regulated by?
interactions with allosteric effectors
Quaternary structure of hemoglobin
tetramer with 2 identical dimers (alpha-beta)1 and (alpha-beta)2, and each dimer is held together primarily by hydrophobic interactions
What holds the two dimers of hemoglobin together?
weaker ionic and hydrogen bonds.
What state of hemoglobin has low affinity for O2?
T. deoxy.
What state of hemoglobin has high affinity for O2
R, some ionic and hydrogen bonds between alpha-beta dimers are broken in the oxygenated state.
How many O2 can myoglobin bind?
1
How many O2 can hemoglobin bind?
4
Degree of saturation of O2 sites
can vary on all hemoglobin and myoglobin molecules between 0%(all sites are empty) and 100% (all sites are full)
The oxygen dissociation curve for Hb is steepest where?
at the concentrations that occur in the tissues. This permits oxygen delivery to respond to small changes in PO2
As you move from left to right on the binding curve, what happens?
O2 binding happens, also O2 affinity decreases
P50
Partial pressure of O2 to acheive half saturation of the binding sites. Low P50=high affinity, High P50=low affinity
Myoglobin shape of binding curve
hyperbolic
very low P50
hemoglobin shape of binding curve
sigmoidal
allosteric effector
cooperative binding
Allosteric effects in hemoglobin
causes cooperativity
heme-heme interactions
heme-heme interactions
cooperative binding allows hemoglobin to deliver more oxygen to the tissues in response to relatively small changes in the pO2. Very small range where O2 is going to want to dissociate. You want hemoglobin to NOT bind O2 in tissues
How does the affinity increase as each O2 binds the heme group?
300X greater than the last one
Significance of the sigmoidal curve of Hemoglobin binding curve.
the steep slope of O2 dissociation curve over the range of O2 concentrations that occur in the lungs and peripheral tissues allows hemoglobin to carry and deliver O2 from sites of high to low pO2
Bohr effect (hemoglobin)
Hb has less affinity for O2 at lower pH, which shifts the oxygen dissociation curve to the right. Will release O2 at higher pO2.
What could cause the pH to lower in tissues?
organic acids produced by tissues such as lactic acid from rapidly contracting muscle.
High levels of CO2 at peripheral tissues, which result in production of carbonic acid and bicarbonate.
How does a pH gradient effect the lungs and tissues?
pH is higher in lungs and lower in peripheral tissues, increasing the efficiency of Hb as an oxygen transporter
mechanism of Bohr effect
deoxyhemoglobin has a higher affinity for protons than oxyhemoglobin, because, for example, histidine R-groups have a higher pKa when not bound to O2.’
- become protonated
- protonated version forms stronger ionic interactions
- more stable T form
- lower affinity for O2
What is the effect of 2,3-BPG on oxygen affinity
2,3-BPG only found in significant concentration in RBCs, side product of glcolysis. 2,3-BPG binds to deoxyhemoglobin, stabilizes T-form, therefore decreases affinity of hemoglobin for oxygen
Where does 2,3BPG bind?
in pocket between 2 beta-globin chains in deoxyhemoglobin, which contain positively charged amino acids that bind to the negatively charged 2,3-BPG, stabilizing the T form. When oxygen is bound, the 2,3-BPG is expelled.
unloading at capillaries, 2,3-BPG reduces affinity for O2
What does 2,3-BPG do to the dissociation curve for hemoglobin?
shifts to the right. Puts the steepest part of the curve in the range of the pO2 found in the peripheral tissues
What do 2,3 BPG levels do in chronic hypoxia?
rise.
In anemic patients with less Hb or RBC, 2,3-BPG levels are also increased to allow more efficient unloading of oxygen to peripheral tissues.
Role of 2,3 BPG in transfused blood
2,3-BPG is essential for normal function of oxygen, without it the oxygen dissociation curve shifts to the left such that it binds oxygen with such high affinity, it can not unload oxygen to peripheral tissues very efficiently. During storage of blood 2,3-BPG will be lost over time, and as such patients who receive large quantities of this blood can be compromised, as it will bind to oxygen too tightly until 2,3-BPG levels return to normal.
Binding of CO2 in hemoglobin
most CO2 is transported in the form of bicarbonate ion, but some CO2 is transported as carbamate bound to the N-terminal amino group of hemoglobin forming carbaminohemoglobin, which stabilizes T form. results in right shift of curve. CO2 dissociates from hemoglobin at the lungs and is released as breath
carbaminohemoglobin
CO2 is transported as carbamate bound to the N-terminal amino group of hemoglobin
What form does the binding of CO2 to hemoglobin stabilize?
T form
Compare Hb affinity for CO2 and O2
Hb has 200 times the affinity for CO2 than for O2, so it binds CO2 very tightly
What does binding CO2 to Hb do?
goes to R form and shifts to the left. Will not release O2 in capillaries easily.
A combo of high oxygen affinity and oxygen binding sites occupied by CO2 results in critically low oxygen unloading at peripheral tissues
Fetal Hb
2 alpha chains and 2 gamma chains
gamma chains lack some of the positively charged amino acids found in the beta-chains that interact with 2,3-BPG, therefore has a lower affinity for 2,3-BPG. This results in HbF having higher affinity for O2. Makes it possible to absorb oxygen from maternal circulation across the placenta.
HbA1C
HbA is slowly and nonenzymatically glycolated under physiological conditions forming this.
Depends on the concentration of glucose that the RBC is exposed to over its 120 day lifespan. Higher blood glucose, higher HbA1C
HbS disease
- Sickle Cell Anemia.
- Most common inherited blood disorder
- symptoms not seen until 6 mo of age
- symptoms: crises, chronic hemolytic anemia, increased suseptibility to infection
- life time of RBC is 20 days, resistant to malaria
- mutation glutamate replaced by valine
- At low oxygen tentions, Hb polymerizes forming a network of fibrous polymers that stiffen and distort the cell. they can lodge blood flow
- high altitudes, flying in nonpressurized plane, increased CO2, decreased pH, dehydration, increased cxn of 2,3-BPG can increase severity
treatment for sickle cell anemia?
hydration, analgesics, antibiotic therapy, blood transfusions
HbC disease
lysine substituted for glutamate, mild chronic hemolytic anemia, no infarctive crises, no therpy requires. prevent malaria
HbSC
when person inherits one copy of HbS and one copy HbC. symptoms and severity vary.
