Hemoglobin Metabolism Flashcards
When hemoglobin is denatured and free from
RBCs, how is removed from the body ?
- it is excreted by the kidneys
- outside of the RBC hemoglobin has a short half life
- when released it is salvaged for its iron and amino acid components
- if those mechanisms are saturated then the rest is excreted via the kidneys
Aside from O2 and CO2 transport, what other
role does hemoglobin have in the body ?
- contributes to the acid base balance of the body by binding and releasing hydrogen ions
- transport nitric oxide
- this is a main regulator of vascular tone
What is the general structure of
hemoglobin ?
- globular protein
- composed of 2 different pairs of polypeptide chains
- 4 globin chains
- four heme groups
- one heme group is embedded in each of the four polypeptide chains
What is the structure of the
heme group ?
- One heme group
- ring of carbon, nitrogen, hydrogen atoms (called Protoporphyrin IX)
- there is a central atom of divalen ferrous iron (Fe 2+)
- can reversibly combine with one oxygen molecule
- oxidized to Ferric (Fe 3+) –> cant bind oxygen anymore
p. 92 great pictures (review)
What is Methemoglobin ?
- oxidized hemoglobin
What is the structure of each globin
chain ?
- each hemoglobin molecule has 2 identical pairs of unlike polypeptide chains
- variations in amino acid sequences give rise to different types of chains
- each one has a different greek letter designation
What is the primary structure of hemoglobin ?
- primary structure is the amino acid sequnces of the polypeptide chains
What is the secondary structure of hemoglobin ?
- refers to chain arrangements in helices and nonhelices
What is the tertiary structure of hemoglobin ?
- arrangement of the helices into a pretzel like configuration
- globin chains loop to form a cleft pocket for heme
- the chains are hydrophic in the pocket and hydrophilic on the outside rendering hemoglobin water soluble
- this allows iron to stay in the divalent ferrous form
What is the quaternary structure of hemoglobin ?
- it is a tetramer
- describes the complete hemoglobin molecule
- four heme groups attached to 4 polypeptides
- can carry up to 4 molecules of oxygen
What is the main adult hemoglobin ?
- Hemoglobin A
- composed of 2 alpha globin chains and 2 beta globin chains
What type of hemoglobin is Hemoglobin A1C ?
- it is a glycated hemoglobin A molecule
- glycation is a posttranslational modification that is non-enzymatic allowing the binding of various sugars to hemoglobin
- HgA1C
- glucose attaches to the N-terminal of valine of the beta chain
- normally have about 4-6% of circulating HgA1C
- increases in diabetes
- reflective of the sugar levels over the preceding 2-3 months
Where does Heme biosynthesis occur ?
- biosynthesis occurs in the mitochondria and cytoplasm of bone marrow erythroid precursors
- begins with the pronormoblast through the circulating polychromatophilic erythrocyte
- as the erythroid precursors lose their ribosomes and mitochondria they can no longer make hemoglobin
mechanism described on p. 95
What is the final step of heme production
and where does it occur ?
- Final step
- occurs in mitochondria
- Fe 2+ combines with protoporphyrin IX in the presence of ferrochelatase (aka heme synthetase)
What is the role of transferrin in
Heme synthesis ?
- Transferrin
- plasma protein that binds Fe 3+ and delivers it to developing erythroids
- binds to transferrin receptors on erythroid precursor cell membranes
- then endosomes bring the transferrin and iron into the erythroid precursor cytoplasm
- iron is transported from the endosome into the mitochondria
- there it gets reduced to Fe 2+
- united to protoporhyrin to make Heme
- Heme leaves mitchondria and in the cytoplasmic ribosomes is joined to the produced globins
How many copies of globin genes are present in the human genome ?
- one copy of each globin gene per chromatid , so two per diploid cell with the exception of:
- alpha and gamma globins
- these have 2 per chromatid or 4 per diploid cell
- alpha and gamma globins
- globins are only produced in immature erythroid precursors
- although more alpha globin mRNA is produced there is less efficient translation of the mRNA so there are equal amounts of alpha and beta globin chains produced
How do the hemoglobin molecules assemble ?
- non-alpha chains have a charge difference that determines their affinity to bind alpha chains
- alpha chains are positively charged
- alpha chains have the highest affinity for beta globin chains because they are negatively charged
- next is gamma globin
- then delta globin chain
- two heterodimers combine to form a tetramer = hemoglobin
What is the major hemoglobin in the body ?
- Hemoglobin A
- 2 alpha and 2 beta chains
- present from 6 months to adulthood
- Hemoglobin A2 (second hemoglobin in adults)
- 2 alpha and 2 delta chains
- production of delta chains is low (< 3.5% in body)
What is the composition of
Hemoglobin F ?
- usually comprises only 1-2% of adult hemoglobins
- present in an uneven distribution in the RBCs
- composed of 2 alpha and 2 gamma chains
- these RBCs are called F/A cells
How are hemoglobins tested for ?
- the different amino acids on hemoglobins have different net charges
- electrophoresis or HPLC are used for fractionation and presumptive identification
- molecular genetic testing globin gene DNA provides definitive identification of variant hemoglobins
Which hemoglobin genes are on
chromosome 16 ?
- zeta (first 3 months of embryonic development)
- alpha
Which hemoglobin genes are on
chromosome 11 ?
- epsilon (first 3 months of embryonic development)
- gamma
- delta
- beta
What hemoglobin predominates
from 4 months of embryonic development
through the first 6 months of life ?
- Hemoglobin F
- composed of 2 alpha and 2 gamma chains
When does the adult hemoglobin pattern
start to emerge?
- emerges by 6 months of age
- alpha2, beta2 (HgA) major form
- alpha2, delta2 (HgA2) minor form
- alpha2, gamma2 (HgF) extremely small amounts if any
What are the three hemoglobins
present in early embryonic development
(products of yolk sac erythroblasts) ?
- Gower 1 (zeta2, epsilon2)
- Gower 2 (alpha2, epsilon 2)
- Portland (zeta2, gamma 2)
What is the main regulatory step in
Heme production ?
- key rate limiting step:
- initial reaction of glycine with succinyl CoA
- this forms Aminolevulinic acid (ALA)
- this is catalyzed by ALA synthetase
- Heme acts as a negative feedback molecule
- if lots of heme (also maybe portoporphyrin) then the pathway is blocked
- Heme and portoporphyrin also likely inhibit
- Ferrochelatase enzyme
Where in the body is hypoxia detected ?
- detected by the peritubular cells of the kidney
- the kidney responds by increasing EPO
- this increases erythrocyte production and release into the peripheral tissues
- in addition it also accelerates hemoglobin production
How does hemoglobin concentration
vary based on altitude ?
- individuals living at higher altitudes have higher hemoglobin concentration levels
What regulates oxygen affinity for hemoglobin ?
- the parital pressure of oxygen determines the affinity for hemoglobin
- high partial pressure = high affinity
- the curve is sigmoidal
- low hemoglobin affinity at low oxygen tension
- the hemoglobin molecules work together
- whne there is no oxygen bound all have a low affinity for O2
- but as soon as one binds it changes it’s conformation and the others bind to oxygen even more quickly
What happens to oxygen affinity if the
O2 dissociation curve shifts to the left ?
- there is higher affinity for Oxygen with higher oxygen saturation
- a shift to the right of the curve lowers the oxygen affinity (greater release of O2)
What causes a left shift of the O2 saturation curve
of Hemoglobin ?
- Higher affinity for oxygen
- causes of left shift
- 2,3 BPG
- increased pH levels (increase H+ ions)
- lower partial pressure of CO2
- lower temperature
- Hemoglobin F
- other variants of hemoglobin that have a higher affinity for O2
- Myoglobin (predominantly in muscle)
What causes a right shift of
the O2 dissociation curve ?
- Right shift has lower O2 affinity (causes release of O2)
- Causes of right shift
- increased 2,3 BPG (release O2 molecules)
- lowered pH (increased H+ ions)
- increased pCO2
- increased temperature
- hemoglobin variants with decreased O2 affinity
- hypoxic conditions
- high altitude, pulmonary insufficiency
- CHF and severe anemia
What hemoglobins are incapable of O2 transport ?
(dyshemoglobins)
- methemoglobin
- sulfhemoglobin
- carboxyhemoglobin
most of these are acquired from drug and toxin exposure. A small percentage of methemoglobin is inherited
How is Methemoglobin formed ?
- reversible oxidation of heme iron to the ferric state (Fe 3+)
- normally a small amount of methemoglobin is continuously formed
- limited by NADH cytochrom b5 reductase to ~1% of total hemoglobin
- normally a small amount of methemoglobin is continuously formed
- can t carry O2 because ferric iron cant bind it
At what levels are symptoms of
Methemoglobinemia felt and what are the symptoms ?
- essentially get decreased delivery of O2 to tissues
- < 25% no symptoms generally
- ~30% symptoms start to manifest
- cyanosis
- symptoms of hypoxia: dyspnea, headache, vertigo, change in mental status
- > 50%
- leads to coma and death
What can cause the acquired form or toxic Methemoglobinemia ?
- exogenous oxidants
- nitrites
- primaquine
- dapson
- benzocaine
- the oxidant overwhelms the reduction system
What is the treatment for Methemoglobinemia
caused by an exogenous oxidant ?
- IV Methylene blue
- reduces ferric iron to ferrous iron
- goes through NADPH-Methemoglobin reductase
- in life threatening cases
- exchange transfusion
How is Methemoglobin detected in the lab?
- spectral absorption such as a CO-oximeter
- absorbs at 630 nm
- blood takes on a chocolate brown color and does not revert back to normal red after exposure to oxygen
How is Sulfahemoglobin formed ?
- formed by irreversible oxidation of hemoglobin by drugs
- or exposure to sulfur chemicals in the industrial or environmental settings
- formed by addition of a sulfur molecule to the pyrole ring
- takes on a greenish color
- has a similar peak to methemoglobin on spectrophotometry
How are Sulfahemoglobin and Methoemoglobin differentiated ?
- since they have similar peaks on spectral absorption
- use cyanide
- sulfahemoglobin spectral curves does NOT shift with the addition of cyanide
- methemoglobin does
What does Carboxyhemoglobin form from ?
- combination of Carbon monoxide and iron
- CO has 240 times the affinity for hemoglobin compared to oxygen
- it shifts the oxygene dissociation curve to the left
- less release of O2 to tissues
- carbon monoxide has been termed the silent killer
What group of people tend to have
high carboxyhemoglobin levels ?
- smokers can have levels up to 15% (due to cigarettes)
- normal is <2%
- they may compensate by increasing their RBC amounts (polycythemia)
How is carboxyhemoglobin detected ?
- detected by spectral absorption instruments (540 nm)
- makes the blood and sometimes skin look cherry red
- CO poisoning is diagnosed when:
- CO is > 3% in nonsmokers
- CO > 10% in smokers
How is carboxyhemoglobin treated ?
- administration of 100% oxygen