ANSWERS TO YOUR BLOCK 2 EMAIL QUESTIONS
1. In lecture, you mentioned that hemoglobin transports 15-20% of CO2 produced by respiring tissues. I understand that carbonic anhydrase produces bicarbonate from CO2, and the bicarbonate forms a Hb-H-CO3 carbon adduct. Is this the manner in which hemoglobin transports CO2?
Le Chatelier's principle makes sense in that little to no CO2 in the lungs calls for bicarbonate-->CO2 conversion. However, since bicarbonate is now covalently linked to hemoglobin, what removes it?
Finally, since hemoglobin transports 15-20% of CO2, and another portion is dissolved CO2, is the remainder transported as dissolved bicarbonate in the blood?
(Diane Blake) When CO2 enters the plasma from the tissues, it can have one of 3 fates:
- Approximately 10% of the CO2 remains dissolved in the plasma and it transported to the lungs in that form.
- The rest diffuses into the red blood cells. Once inside the red blood cell, approximately 23% forms a readily reversible, non covalent adduct with the 4 NH2 groups at the amino termini of the 4 subunits of hemoglobin. This adduct is called Carbaminohemoglobin. When the CO2 concentration becomes reduced (in the lungs) this carbaminohemoglobin is transformed back into CO2 and hemoglobin.
- The rest of the carbon dioxide (~70%) is converted to bicarbonate in a reaction that is catalyzed ~5000-fold by the red blood cell enzyme carbonic anhydrase. This enzyme catalyzes the reaction shown below:
CO2 + HOH <===> H2CO3 <===> H+ + HCO3-
As HCO3- begins to build up in the RBCs, it activates the HCO3- - Cl- antiporter, which transports some of the HCO3- back into the plasma, while increasing the intracellular Cl- concentration. The bicarbonate travels back to the blood both in the plasma and within the RBC. When the CO2 concentration is reduced (in the lung) the reaction is reversed and the HCO3- is converted back to CO2 and HOH.
2. In the coagulation lecture slide 3, you state "GPIb binding stimulates intercellular signals that activate the platelet and expose GPIIa/GPIIb (integrin αIIbβ3), which binds to vWF and fibrinogen." yet the figure on the slide shows a GPIIb/GPIIIa protein complex on the cell surface. Which proteins are initiated by the initial binding of GPIa/GPIIa to collagen and GPIb to vWF?
(Diane Blake) The picture is correct and I have corrected the rest of the slide for next year’s class. It is the GPIIb/GPIIIa complex that is activated by the binding of GPIb to von Willenbrand factor. The GPIIb/GPIIIa complex binds to van Willenbrand factor and fibrinogen. See pages 857-858 in the Marks textbook for more information.
3. Does Fe3+ bind O2 too tighly, or not at all? I am finding conflicting information out there in the world wide web.
(Diane Blake) The Fe3+ in the heme center binds to O2 MORE tightly than does Fe2+. So, the hemoglobin becomes oxygenated, but cannot release the O2 to the tissues.
4. I understand that in Hb the 6 Fe bonds are four to porphyrin ring, one to O2 and one to His of B chain, but what are they in Mb? Thank you for your help and patience.
(Diane Blake) The bonding pattern to the Fe center is the same in myoglobin and hemoglobin.
5. Regarding Methemoglobinemias, on one of your slides you mentioned an enzyme in RBC (NADH-cytochrome b5 reductase) reduces met-Hb to Hb. This means the enzyme reduces the Fe from 3+ to 2+, which means the enzyme allows Oxygen to dissociate from the Fe, since 2+ state binds to Oxygen less tightly than the 3+ state.
I want to ask if a mutation in the enzyme would lead to Methemoglobinemia, since Fe would not be reduced to the 2+ state, and remain in the 3+ state, so oxygen can't be delivered to the tissues. I guess my question is, does the regular activity of the enzyme cause the disease, or a mutation of the enzyme cause the disease?
(Diane Blake) Do mutations of the enzyme NADH-cytochrome b5 reductase cause methomoglobinemia? Yes, there is a genetic form of methemoglobinemia where patients have lower than normal levels of NADH-cytochrome b5 reductase. The most famous example of this is a family in Kentucky, the Fulgates, who had a recessive mutation in the gene.
Can the regular activity of the NADH-cytochrome b5 reductase enzyme cause the disease? Yes, infants are born with relatively low levels of NADH-cytochrome b5 reductase, so they are more susceptible to environmental toxicants that oxidize the Fe2+ in hemoglobin to Fe3+in the first few months of their lives. Because their levels of this enzyme are low, they have more trouble re-reducing the iron if it becomes oxidized. This is called acquired methemoglobinemia, and it most commonly occurs in infants that are fed formula made from water that contains high levels of nitrates (the nitrates can become high because of run-off from agricultural activities). The EPA has set the drinking water limit for nitrates at 100 ppm specifically to prevent acquired methemoglobinemia in infants. .The treatment is to give the patient methylene blue, which reduces the heme iron, and to remove the toxicant from the patient’s diet.
6. It seems from your slide that TFPI inhibits Xa and/or XaVIIaTF compex and therefore directly inhibits both the intrinsic and extrinsic pathways of BCC. I am interpreting this correctly?
(Diane Blake) Tissue factor pathway inhibitor (or TFPI) is a single-chain polypeptide which can reversibly inhibit Factor Xa (Xa), which is part of the Common Pathway of blood coagulation.
The Xa-TFPI complex can also inhibit the FVIIa-TF complex. Because the VIIa-TF complex is part of the extrinsic pathway, I would say that TFPI inhibits the Common Pathway and the Extrinsic Pathway.
TFPI contributes significantly to the inhibition of Xa in vivo, despite being present at concentrations of only 2.5nM in plasma.
7. Regarding types of normal hemoglobins, you listseveral types of hemoglobins listed, (A1/A2/F/E). Is there specific ones of these we should familiarize ourselves with? Or would it be a good idea to know the function and exact subunit compositions of each type?
(Diane Blake) The most important hemoglobins to know about as a medical student are Hb A1, A2 and F (fetal hemoglobin). Remember that Hb F does not bind 2,3 BPG as tightly as Hb A1, so that the fetus is spared during hypoxia in the mother. Hb F also can be induced in patients with Sickle Cell disease to reduce the tendency of the HB F to aggregate.
8. From the graph on slide 17 in the Sickle Cell lecture, the one depicting expression of human globin genes during development, what should we take away from this graph? I didn't have a chance to write down what you said in class because I didn't have the slide. I seem to recall that there was something significant about hemoglobin related diseases and smaller children/infants still making the transition from fetal hemoglobin to adult hemoglobin but it is escaping me.
(Diane Blake) The major take home lesson from slide #17 is the time at which the fetus switches from fetal to adult hemoglobin types. The switch to the adult alpha chain takes place very early in gestation (before 6 weeks) while the fetus still has the gamma (found in Hb F) for up to 18 weeks after gestation. For the physician, this means that hemoglobinopathies that effect the alpha chain are evident at birth, but those that effect the beta chain can take 3-6 months to fully manifest themselves.
9. Could you please clarify a few specfics about Hb for me? Do both the Alpha and Beta (and delta etc) chains have hydrophobic regions that enable them to form a tetramer? Are there two or four binding pockets for O2 to bind to the 4 Fe2+ molecules in Hb? Is the Histidine binding to the Fe also involved in the salt bridge of the hydrophobic region between subunits and or the BGP binding pocket? Is the BGP binding pocket in the hydrophobic region that "connects" side chains, or is it at some other point in the peptides? Over all I'm having trouble sorting out where these different binding events are taking place in relation to one another. Also, how is Pyruvate Kinase related to NADH levels?
(Diane Blake) Do both the Alpha and Beta (and delta etc) chains have hydrophobic regions that enable them to form a tetramer? Yes.
Are there two or four binding pockets for O2 to bind to the 4 Fe2+ molecules in Hb? Four - each subunit has a binding site for O2
Is the Histidine binding to the Fe also involved in the salt bridge of the hydrophobic region between subunits and or the BGP binding pocket? No, those are different histidines. His 8 in the F helix of the alpha and beta subunit binds to iron, a histidine at the carboxyl terminal of the beta chain binds to an asparagine elsewhere in the beta SU. The histidines that bind to 2,3-bisphosphoglycerate are elsewhere in the molecule.
Is the BGP binding pocket in the hydrophobic region that "connects" side chains, or is it at some other point in the peptides? 2,3-bisphosphoglycerate binds in a central cavity between the 2 beta subunits. It stabilizes the T state of the molecular (more ion pairs) and thus decreases the affinity for oxygen.
How is Pyruvate Kinase related to NADH levels? We have not covered this material yet; it is part of the glycolysis lectures. Pyruvate kinase catalyzes the conversion of phosphoenolpyruvate to pyruvate and ATP. In the absence of this enzyme, ATP levels in the cells are reduced, and PEP levels rise, which ultimately causes a build-up of intermediates on the glycolytic pathway that reduce NADH production. Since the enzyme that re-reduces the Fe in the heme (NADH-cytochrome b5reductase)uses NADH as a substrate, a deficiency of NADH can lead to reduced activity of that enzyme in the red cell.
10. In the Immunoglobin study guide regarding the number of Ig Folds in the Fab fragment, I was under the impression that there were two sets of folds: a 3+4 associated with the heavy and light variable chains, and a 4+5 association between the heavy and light constant chains. Does the answer of "four" simply mean that there are 2 Fab regions on each Ig molecule and therefore, 2 each of
the above listed folds or did I miss something?
(Diane Blake) The answer to that question is 4 because they are 2 IgG folds in the heavy chain and 2 in the light chain of the Fab fragment. Both the 4+3 and the 5+4 strand folds are considered immunoglobulin folds because they assume the same 'sandwich' structure, with the 2 hydrophobic faces together. I have attached a jpg file that shows the 4 immunoglobulin folds in a Fab fragment.
11. For question 2 of the sickle cell study guide, does the increased concentration of O2 decrease the likelihood of sickling by preventing the hydrophobic aggregations caused by the valine residues or does it act via another method?
(Diane Blake) The sickle cell hemoglobin tends to aggregate more readily when the HbS is in the deoxy state. It appears that the presence of O2 causes a conformational change in the protein that makes the hydrophobic patch from the valine mutation a little less accessible to the phenylalanine that it binds to on another Hb molecule. That is why oxygen (E) is the correct answer. None of the other choices should have any effect on the sickling process.
12. In reviewing your immunoglobulin lecture, I wonder under what circumstances you would want to raise antibodies to a drug? Therapeutically or preventively or for clearance in the case of an overdose? Could you give an example?
(Diane Blake) A lot of the tests to determine therapeutic drug levels in serum are immunoassays. It is much faster, easier and cheaper to run an ELISA assay for a specific drug than to perform GC or GC/MS on serum samples. For example, I think that digoxin is almost always assayed using immunoassay procedures.
13. You mentioned abs raised to heavy metals; is this for chelation and clearance or other aims? How are targeted heavy metals degraded and expelled? I can understand Ab-mediated pathogen degradation, but am less clear how heavy metals are neutralized and/or removed from system.
(Diane Blake) In my own laboratory, we make antibodies to heavy metals in order to measure their levels in environmental water samples and serum. We recently validated two of our antibody-based sensors for uranium at a contaminated uranium mine tailings site in Colorado. You can read more about it in your spare time (that was a joke) at We have also reported a correlation between serum cadmium and pancreatic cancer using our immunoassay procedures in
14. Has anyone explored using viagra to treat sickle cell anemia ifactivation of guanylyl cyclase increases the production of HbF.
(Diane Blake) I did some sleuthing on the web in response to your question about OH-urea and SCD. I think the jury is still out about the precise molecular mechanisms whereby hydroxyurea reduces the incidences of crises in SCD. Although it looks like nitric oxide may be involved, the use of Viagra to increase levels will probably not work. As a clinical trial test, the use of Viagra for pulmonary hypertension in patients with sickle cell disease was recently canceled. There is a guy, S. Bruce King at Wake Forest that is working on other HNO donors for use in SCD. There is also some newer evidence that OH-urea might modify red cell adhesion to the capillary wall. This is also well above what you need to know for the exam.
15. I know this wasn't your lecture (it was Dr. Kahn's) but my question is about JAK-STAT and Hif1a/b. Are these two pathways related in the context of hypoxia and Epo? Or are these two different ways to enduce Epo expression?
(David Franklin) Hif1 a/b were the transcription factors that were activated in kidney cells in response to hypoxia to promote erythropoietin expression. Under nonhypoxic conditions VHL lead to the degradation of Hif1a, thereby blocking erythropoietin expression. Hypoxia blocks this degradation pathway, so Hif1a and b can interact to promote transcription of erythropoietin. This is all shown on slide 9.
Epo is then sent through the blood to the RBC precursor cells in the bone marrow. Epo binds it receptors to activate a signal into the cells, activating Jak/Stat and promoting proliferation of more RBCs. This helps to compensate for the hypoxia and is summarized in slides 10-12.
The overall interaction of what goes on the kidneys (Epo transcription through Hif1 TFs) and the bone marrow (erythropoiesis through Jak/Stat signaling) in response to hypoxia is summarized in slide 8.
16. You told us that the potency of a heparinpreparation is determined by the specific pentasaccharide sequence thatwas illustrated on the relevant slide, and that it is not the weight ofthe preparation that determines its potency. However, Marks (p. 866) mentions two forms inwhich heparin can be administered: high molecular weight heparin and lowmolecular weight heparin. If it is the sequence of sugars and not theweight that is relevant clinically, why are there high- and low-MWforms?
(Y-T Li) The composition of a heparin preparation is extremely complex. This is the reason why any heparin preparation can be fractionated into high molecular weight heparin and low
molecular weight heparin. It should be pointed out that the composition of a high molecular weight heparin preparation is by no mean homogeneous. The same is also true for a low molecular weight heparin preparation. Furthermore, the structure of heparin is very heterogeneous. The anticoagulant potency of a heparin preparation is largely proportional to the content of the specific anticoagulant pentasaccharide sequence. Heparin dose/potency is expressed in international units. An international unit of heparin is defined as that amount required to prolong the coagulation of 1 ml of whole blood for 3 minutes. Thus, heparin doses should be expressed in units not the weight of the preparation.
17. I was reviewing the lectures today, specifically the Mucopolysaccharidosis' and wanted to know if we should be aware of ALL the enzymes related with the MPS' or just the ones boxed in class (i.e. L-Iduronate sulfatase, L-Iduronidase, and N-Acetylgalactosamine-4-sulfatase). The remaining enzymes were those dealing with the different subclasses of Sanfillipo and Morquio Syndrome. I understand them quantitatively, as each subtype refers to an enzymatic deficiency for the next additional cleavage (either glycosidic or sulfate), however, wanted to know whether we will be tested on our memorization of the names of the enzymes.
(Y-T Li) Students will not be tested on their memorization of the name of every enzyme associated with the degradation of different GAGs. Structures of GAGs can be obtained from a textbook. I hope after my lecture, when students see the structure of a GAG, they can understand/recognize its sugar composition and linkages and subsequently use the structure to reason its enzymatic degradation. Since dermatan sulfate, heparan sulfate, and keratan sulfate are associated with different mucopolysaccharidoses (MPS), it is a good idea for students to be able to recognize (not memorize) the structures of these three GAGs and use them to understand/reason the biochemical basis of Hurle/Scheie, Hunter, Morquio, Maroteaux-Lamy, and Sanfilippo syndrome. Although it is not necessary to remember the three enzymes associated with the three types of Sanfilippo syndrome it is a good idea to know that Sanfilippo is associated with the impaired catabolism of the alpha-linked N-sulfated-glucosamine in heparan sulfate. Finally, I would like to point out that Hurler/Scheie (MPS I) and Hunter (MPS II) are the two best-known MPS.