Hemoglobin Flashcards
_____is a specialized protein designed to transport oxygen
(O2) from the lungs, a region of high O2 concentration, to peripheral
tissues where oxygen tension is low.
Metabolism in the peripheral
tissues generates _______that are transported back to the
lungs, in part, by hemoglobin
Hemoglobin
CO2 and H+
O2 has _____solubility in plasma (the non-cellular part of blood).
As a consequence, >98% of the O2 that reaches tissues is carried in
___________
very low
red blood cells (RBCs) bound to Hemoglobin.
RBCs contain ______which catalyzes the rapid reversible hydration of CO2 to carbonic acid (H2CO3).
H2CO3 then rapidly and spontaneously
dissociates to bicarbonate (HCO3-) and a H+.
carbonic
anhydrase
_____and, _____especially
are soluble in plasma and RBC cytosol and most of the CO2 made in
tissues returns to the lungs as those species.
About _____of the CO2 made is carried bound to Hb
CO2
HCO3-
14%
What is the structure of hemoglobin?
Hemoglobin is a heterotetrameric protein having the subunit composition (αβ)2. The α and β subunits have similar sequences and tertiary structures.
Both subunits in hemoglobin are evolutionarily related to____, a monomeric protein abundant in muscle that is
designed to store O2.
myoglobin
In hemoglobin what binds O2?
contain a heme prosthethic group that is responsible for binding O2.
What is the form of Fe that can bind O2?
Fe2+ is the ferrous form of iron that is capable of binding O2;
What is the form of Fe that cannot bind O2 and what is it called?
Fe3+ is the ferric form of iron that CANNOT bind O2 and is present in an INACTIVE form of hemoglobin called methemoglobin (metHb)
Describe hemoglobin’s binding curve
what about myoglobin?
Hemoglobin shows sigmoidal cooperative binding of oxygen that is a direct result of its more complex subunit structure.
Myoglobin gives a normal binding curve which is hyperbolic in shape.
the partial pressure of oxygen yielding 50%
saturation of binding is termed ______ this value is analogous to Km for the binding of substrates to enzymes.
the P50;
Why is cooperative binding so important in hemoglobin?
The cooperative binding of oxygen by hemoglobin is critical for its
efficiency in loading oxygen in the lungs and unloading oxygen in the peripheral tissues.
the______
of Hb allows it to release a
much larger fraction of its O2
load at the pO2 levels found in
the blood of working and
even resting muscle
cooperativity
Hemoglobin exhibits sequential cooperativity for oxygen binding. Thus, the binding of oxygen to one subunit induces a ______ that is partially transmitted to adjacent subunits.
conformational change
The transmission of the partial conformational change induces an
increased affinity for oxygen by these adjacent subunits.
R=
T=
R=relaxed=high affinity;
T=taut=low affinity
Why is high amounts of CO a problem for O2 transport?
Carbon monoxide (CO) has ~250-fold higher affinity
for Hb than does O2. When bound to the heme group
of one subunit, it causes all four subunits to “lock” in
the R conformation, thereby limiting oxygen release
in peripheral tissues
How Does O2 Binding Change the Conformation of a Hb Subunit?
Without O2 bound, the heme Fe2+ is pulled away from the plane of the porphyrin ring by a His residue of the Hb polypeptide chain
When O2 binds, it pulls the Fe2+ back into the plane of the ring and that moves the His residue and its whole section of the polypeptide chain. That in turn causes the Hb subunits to shift relative to one another to an arrangement that favors the R-conformation.
An ______ or____ is a molecule that can bind to a protein and induce aconformational change that alters the affinity for substrate (or ligand such as oxygen) at some other site.
allosteric regulator
effector
(H+), CO2, and 2,3-diphosphoglycerate (DPG) ALL can bind to Hb and
reduce its
affinity for O2
–causes LEFT shift
If O2 is high, then the equilibrium is driven to the____ and H+ and
CO2 and DPG will dissociate from Hb as O2 binds
right
How does CO2 and H+ served the body well in regards to O2 transport?
in lungs, O2 is high and we want Hb to bind O2 and also to release any CO2 and H+ it is carrying. CO2 is expired directly and H+ is needed to convertplasma bicarbonate (HCO3-) back to CO2.In the blood of tissues, CO2 and H+ are high and we want HbO2 to release its O2 load
so catabolism can continue. CO2 and H+ help that happen.
Arterial blood typically is
pH
pO2 =
Hg; pCO2=
; HCO3- =
pH 7.35 – 7.45;
pO2 = 80-100 mm
Hg; pCO2= 35-45 mm Hg
; HCO3- =22-26 mM
Increased [H+] (decreased in pH)
=
Decreased O2 affinity
What are H+ and CO2 are heterotropic negative allosteric effectors that decrease the affinity of Hb for O2.
They are heterotropic because they are not O2; negative because they decrease affinity for O2; and
allosteric because they bind to a site other than the O2 site(s) affected.
Oxygen is a positive homotropic allosteric effector of____ binding (the basis of oxygen bindingcooperativity).
Oxygen is a negative allosteric effector of ________
O2
H+ and CO2 binding
The reciprocal
relationship between O2 and H+ binding is termed the
Bohr effect or isohydric shift.
Changes in H+ binding result from a shift in the pKa of specific residues (mostly histidines) due to microenvironment effects triggered by
conformational changes in the hemoglobin molecule.
Special regulatory mechanisms exist in____ to control the concentration of 2,3-DPG in order to fine tune the affinity of hemoglobin for
O2 in response to changes in metabolism and environment.
RBCs
is a
negative allosteric effector of O2 binding.
\_\_\_\_\_charges on 2,3-DPG allow it to bind to positively charged groups (blue) of Hb β-chains.
2,3-DPG binding stabilizes the ____ of
deoxyhemoglobin
Negative
T state
without any 2,3-DPG, Hb would be
much more like_____ and nearly useless for
delivering O2 from lungs to tissues.
2,3-DPG stabilizes the ____, making it easier for Hb to
release O2
myoglobin
T-state
2,3-DPG levels_____ at high
altitudes. Because there is less O2 at high altitudes,
tissues tend to become somewhat____.
increase
hypoxic
Why do we increase DPG in high altitude?
By increasing the concentration of 2,3-DPG, RBC adapt
to hypoxia by making it easier for O2 to dissociate
from Hb.
Causes of tissue hypoxia to increase DPG?
how long does this take?
anemia and smoking also cause an increase in 2,3-DPG.
hours to days
Each chromosome___ has two α-globin genes
(i.e., a person has 4 total, each is active and codes ~1/4 of expressed α-globin subunit)
16
Each chromosome___ has a single β-globin gene
11
Developmental forms of hemoglobin differ not only in subunit composition, but also in their _____
binding affinity for O2
The higher affinity of fetal Hb (HbF; α2γ2) means the fetusʼ circulation can
draw O2 from maternal blood at the pO2 present in placenta
Normal adult red blood cells
___ HbA (α2β2)
___ HbA2 (α2δ2)
___ HbF (α2γ2)
~95%
~3%
~2%
Sickle cell anemia is a ______ disease that is the most common hemoglobinopathy.
homozygous recessive
SS anemia is caused by a point mutation in the adult ___ gene that causes substitution of _____ for _____at amino acid 6.
β-globin
valine (Val) for glutamic acid (Glu)
SS anemia: patients RBCs containing mainly hemoglobin S
(HbS), which is comprised of
two normal adult α-globin subunits and
two sickle adult β-globin subunits.
Describe pathophysiology for SS anemia
The amino acid at position 6 is on the surface of the β-chain and it should be a hydrophilic amino acid (Glu is).
Valine is hydrophobic and its presence creates a sticky patch on deoxyHb that leads to polymerization of Hb tetramers into long chains. Those intracellular fibers cause the sickle cell shape and reduced deformability of the RBCs
We see: vaso-occlusive manifestations of sickle cell disease.
These structural changes also lead to a shorter erythrocyte half-life and chronic hemolytic anemia.
DT Val for Glu point mutation
The basis for the sickling phenomenon
A critical contact is made by the β6 VAL of one molecule and a
hydrophobic acceptor pocket of the β subunit of another molecule
formed by Leu88 and Phe85 surrounded by hydrophilic residues. The
presence of a charged amino acid (Glu) in HbA prevents this interaction.
The rate and
extent of polymer formation in a circulating SS RBCs depends primarily
on three independent variables:
- degree of deoxygenation (which can be affected by subtle changes in
pH, ionic strength and temperature) [Deoxygenated HbS forms insoluble
polymers] - intracellular hemoglobin concentration
- relative amount of HbF present
_____ inhibits polymerization owing to a
GLU residue at position 87 of the gamma chain, which prevents a critical
lateral contact in the sickle cell fiber. _____ decreases with post-partum
age but varies from 1-30% of total Hb in sickle cell individuals.
HbF
HbF
are a heterogeneous group of disorders caused by inherited mutations that decrease the synthesis of adult hemoglobin HbA (α2β2).
Thalassemia syndromes
β-Thalassemias; Caused by mutations that
diminish the synthesis of β-globin chains.
____ mutations, absent β-globin chain synthesis
____mutations, reduced (but detectable) β-globin chain synthesis
βo
β+
Unpaired α-chains precipitate in RBC precursors, resulting in apoptosis
Deficit in HbA
= two β-thalassemia alleles (β+/β+ , β+/βo, or βo/βo) have a severe, transfusion-dependent anemia
β-thalassemia major
= heterozygote (β+/β or βo/β) have a mild asymptomatic microcytic anemia
β-thalassemia minor
Caused by mutations that result in reduced or absent synthesis of α-globin chains
Unpaired β-chains are more soluble than upaired α-chains, thus effects less severe than in β-thalassemias
α-Thalassemias
