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					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:

       1. Approximately 10% of the CO2 remains dissolved in the plasma and it transported to the
          lungs in that form.
       2. 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.
       3. 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.
http://www.rootsweb.ancestry.com/~kyperry3/Blue_Fugates_Troublesome_Creek.html

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 2.5 nM in plasma.


7. Regarding types of normal hemoglobins, you list several 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 http://pubs.acs.org/doi/abs/10.1021/es9007239. We have
also reported a correlation between serum cadmium and pancreatic cancer using our immunoassay
procedures in
http://ehsehplp03.niehs.nih.gov/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1289%2Feh
p.8035.


14. Has anyone explored using viagra to treat sickle cell anemia if activation 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 heparin preparation is determined by the specific pentasaccharide
sequence that was illustrated on the relevant slide, and that it is not the weight of the preparation that
determines its potency. However, Marks (p. 866) mentions two forms in which heparin can be
administered: high molecular weight heparin and low molecular weight heparin. If it is the sequence of
sugars and not the weight that is relevant clinically, why are there high- and low-MW forms?

(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.


18. Regarding question 3 of the ER-Golgi I study guide, the correct answer is listed as 14. However,
the question asks AT ONCE. I understand 2 GlcNAc's, 9 Mannoses, and 3 Glc's are added to the
Dolichol Phosphate, however, aren't these added in step-wise fashion as the first two sugars are linear
GlcNAc's?

(David Franklin) The synthesis of the oligosaccharyl precursor is accomplished sequentially by adding
individual GluNAc, Man or Glu sugars (a total of 14 sugars) to build this complex structure. However,
the synthesis of the precursor is not to be confused with the N-linked glycosylation event. N-linked
glycosylation is a transfer of the entire 14 sugar precursor from Dolichol Phosphate to an Asn residue
in an Asn-X-(Ser/Thr) consensus sequence. As such the question is asking “In N-glycosylation of
proteins in the ER lumen, 14 sugars are added to the nascent chain at once”.


19. Regarding Question 4 of the ER-Golgi I study guide, the question prompt asks which proteins
interact with PROTEIN FOLDING, and the correct answer is Calnexin. However, I was under the
impression that the sugar binding lectin is important in proper folding; additionally, doesn't BiP
PREVENT folding of the protein during translation through the Translocon until ADP- is recycled and
the complex releases the nascent peptide?

(David Franklin) The lectins do not bind amino acids. They bind the oligosaccharyl (sugar)
modification, so that is why it is the correct answer. As for the BiP protein, it does prevent the
premature folding of the protein until after the entire protein is translocated. This is because reaching
the native lowest energetic structure may be dependent upon interactions of amino acids that are at the
beginning and the end of the primary sequence. So yes, BiP does maintain the unstructured state. In
context to this question, by preventing the premature folding, it helps to create the proper folding at the
appropriate time. True, at that point it is not directly interacting with the amino acids, and indeed all of
the proteins help the folding through transient interations. The only one that does not interact with an
amino acid is Calnextin (or Calreticulin).


20. In I cell disease, the proteins from the Trans Golgi network that are supposed to be targeted to the
Lysosomes are instead sent to the exterior of the cell. Is the damage in I-cell disease and Pseudo-Hurler
Polydystrophy because of the accumulation of proteins in the exterior of the cell or is it because of a
lack of these proteins in the lysosomes?

(David Franklin) In I-cell disease (and Pseudo-Hurler Polydsytrophy), the lysosomal proteins are
secreted, rather than transported to the lysosome. The secreted lysosomal proteins do not damage the
extracellular environment, because they are not active at that pH (they function at acidic pH 5.5).
Rather the damage occurs due to the accumulation of numerous compounds in the lysosomes. As
mentioned in lecture, these two diseases were classified as mucolipidoses (ML-2 and and ML-3,
respectively) due to their clinical features common to both mucopolysaccharidoses and the
sphingolipidoses. These are the major components that accumulate in the lysosomes (lipids and carbs),
so it is not surprising that ML-2 and ML-3 have such clinical features. Also mentioned in lecture was
the inherent difference between ML-2 and ML-3, as compared to other lysosomal storage diseases, in
that ML-2 and ML-3 are an absence of Mannose 6-Phosphate targeting of all lysosomal proteins (a
deficiency in cis-Golgi GluNAc-1-phosphotransferase activity), rather than a defect in a single
lysosomal protein. This also goes to the reason why numerous compounds accumulate in ML-2 and
ML-3.


21. What is the interaction of Taxol with microtubules?

(David Franklin) Taxol binds to the beta-tubulin subunit. Photoaffinity labeling experiments indicate
taxol interacts with the beta subunit at both it N-terminal and C-terminal ends. As mentioned, Taxol
has the effect of stabilizing the microtubules, such that depolymerization does not readily occur. Since
the functions of microtubules require dynamic flexibility of polymerization and depolymerization
(termed dynamic instability), Taxol has an adverse effect on cells. Cells become arrested in the G2/M
phase of the cell cycle with inhibition of normal cell division. At low concentrations of Taxol (10 nM),
microtubule dynamics are suppressed with a concomitant induction of apoptosis. Taxol is used to treat
many types of cancers (ovarian, breast, lung), as well as to treat Kaposi sarcomas.


22. Slide 13 of the ER-Golgi II lecture states that AP3 directs the protein to specifically go to the
lysosome, and that it uses an M6P signal. The book does not mention this, and states that the M6P
signal is specifically for lysosomal enzymes that are to be transported to the late endosomes. Can you
please clarify this?

(David Franklin) M6P ultimately directs lysosomal protein to their correct organelle (the lysosome).
Protein can go indirectly to the late endosome (most common, in AP1-containing vesicles) or directly
to the lysosome (less frequent, in AP3-containing vesicles). More commonly, M6P-containing
lysosomal protein first goes to the late endosome. The protein is released and modified by removal of
the phosphate, and the M6P receptor is recycled. In this sense, the late endosome becomes a lysosome
by addition of lysosomal proteins transported from AP1 vesicles. Additionally, endosomes can fuse
with a pre-existing lysosome. Either way we need to get lysosomal proteins to both structures. As
such, we have the same M6P signal going to either structure, albeit in vesicles with different APs.
Table 14-1 on slide 7 summarizes these situations. Also note that the early endosome was derived
from receptor mediated endocytosis in clathrin-AP2 vesicles.

The text does discuss transport to the endosome and lysosome on page 599. Targeting to the late
endosome is first discussed as transport via clathrin-AP1 vesicles using a cargo Try-X-X-ɸ targeting
sequence. The next paragraph discusses AP3 vesicles bypassing the endosome and going directly to
the lysosome. It is not until the next page (p600) that M6P is mentioned, and in this case in context to
late endosome sorting. Again, this may be because in targeting these lysosomal proteins to the late
endosome, these organelles essentially become lysosomes. It is important to note that M6P signals are
essential for targeting endosome/lysosomal proteins, and the absence is what causes I-Cell disease.


23. The enzyme difficiency in Morquio Syndrome - Type B is a lack of Beta-Galactosidase in which
the beta bond between Gal and Glc cannot be broken. This appears to be similar if not the same case
in lactose intolerance. Why then don‟t lactose intolerant patients also have Type B Morquio
Syndrome? The only reason I can think of is that Morquio implies a lack of beta-galactosidase specific
only for keritan sulfate.

(Y-T Li) There are several different beta-galactosidases in the normal tissues. The intestinal beta-
galactosidase is different from the lysosomal beta-galactosidase responsible for the cleavage of the
beta-linked galactose in keratan sulfate. Another good example is that the beta-galactosidase
(galactocerebrosidase) responsible for cleaving GalCer is different from that for the hydrolysis of the
beta linked galactose in GM1. This is the reason why there is no accumulation of GM1 in Krabbe
disease. However, in some cases, one can detect some accumulation of GM1 in Type B Morquio
patients. The linkage specificities of glycosidases are very complex and not absolute. The best
example is ³beta-hexosaminidase².

24. Regarding beta sandwich/ immunoglobulin folds, I understand it is a common super secondary
structure and it focuses a hydrophobic center and a hydrophilic outside. However, when I try to place
that image into an Ig, are there a total of two or four sandwiches in the Fc portion (the two heavy
chains produce one sandwich or is each sandwich made by one strand and then adjacent to another
sandwich made by the other heavy chain)? And in the Fab region, how many beta sandwiches are
there?

(Diane Blake) There are 4 beta sandwiches in
the Fc region. Two in each of the 2 pieces of
the heavy chain. There are 4 beta sandwiches
in each Fab. If you have trouble seeing it in the
ribbon diagram, you can also visualize it using
this diagram from your textbook (also slide 10
of the lecture handout). Each color bloc is a
beta sandwich, so the entire IgG molecule has
10 beta sandwiches.




25. How does a beta sandwich turns into a barrel shape when looking at
the CDR loops coming out in the ends of the arm.

(Diane Blake) It may look like a barrel, but it is really a beta sandwich
with solvent-exposed loops, as shown in the right. I think what might be
making it look like a barrel in slide 15 of the handout is that there are 2
beta sandwiches jammed together in that visualization.
26. I'm a little confused about the term "fix complement" (complement fixing?, fixing the
complement?) regarding Immunoglobulins. I've looked around, but can't seem to find a definition that
makes much sense to me. I'd really appreciate any input you might be able to offer.


(Diane Blake) When IgG molecules bind to a cell
surface antigen (like a coat protein on a bacteria)
the Fc portion of the IgG undergoes a
conformational change and this initiates a cascade
of events that leads to a membrane attack complex
that pokes a hole in the bilayers and kills the cell
to which the IgG bound. The series of reactions is
called the complement cascade, and you will learn
more about it in immunology at the end of this
year. An immunoglobulin is said to be able to „fix
complement; if it can initiate this cascade of
events. IgG and IgM molecules have the ability to
fix complement. I have attached a picture for you
so that you can see how this happens, but you are
not responsible for any of the information
on this picture for the upcoming exam.


27. The sickle cell anemia study guide, question 2 states that the increase of Oxygen will decrease the
frequency of sickling. I do not understand this when sickling of RBC is due to mutation of Glu -> Val,
creating hydrophobic regions on outside of Hb both beta chains.

(Diane Blake) You are correct that the Glu-Val conversion creates the hydrophobic region, but the
USMLE also wants you to understand how this may affect patient care. On slide 9 of the handout, it
states that the hydrophobic interactions that make the hemoglobin polymerize are favored when the
hemoglobin is in the deoxy state. Therefore, when hemoglobin is in the oxygenated state, it would be
less likely to polymerize. Increasing the concentrations of 2,3-bisphosphoglycerate, Fe2+, histidine and
glucose have no effect on the state of oxygenation of hemoglobin, so O2 is the best answer.


28. The coagulation study guide, question 3 states that thrombin directly converts Factor V to Va. -
but looking back at the chart, thrombin also converts VII to VIIa as well. Is this NOT one of the
answers because VII to VIIa also needs TF?

(Diane Blake) No, the chart on slide 9 and 12 indicates that Thrombin converts Factor VIII to Factor
VIIIa. As far as I know, thrombin has little effect on the conversion of Factor VII to VIIa. Don‟t let
those pesky Roman numerals mess you up.
29. On the 3rd slide of the blood coagulation cascade lecture, GPIIb/GPIIIa is in the picture. Is this
supposed to be the same as GPIIa/GPIIb (indicated in the text of the slide)? If so, is it a typo or are
they interchangable?

(Diane Blake) The picture is correct and I have corrected the typo on 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.


30. I am having difficulty in recognizing the alpha- and beta- anomeric configurations of sugars in
Haworth representation. Can you please help?

(YT Li) To determine whether a sugar is in the alpha or beta configuration, you need to look at the C-
1 hydroxyl (OH) and the C-5 hydrogen groups. If they face in the same direction (both up or both
down), the sugars are in the alpha anomeric configuration. If they are in the opposite direction (one up
and the other down), they are in the beta anomeric configuration. Please see the figure below for
specific examples. The arrows indicate the direction of the C-1 and C-5 groups in question.
31. Does binding of only GPIb promote the exposure of GPIIa/GPIIb, or is it the both of the preceding
binding events?

(Diane Blake) It is my understanding that it is primarily the GP1b binding that activates the
GPIIa/GPIIb integrin. These things are never as black and white as they are shown in the textbooks,
however, so it is possible that both events activate this integrin to varying extents. I will be talking
more about integrin activation this week in lecture.


32. Regarding the role of thrombomodulin, what decides whether it takes the fast or slow pathway
with prothrombin/thrombin?

(Diane Blake) I was a little confused about your question, so email me again if this answer does not
satisfy you. Thrombin reacts rapidly with fibrogen to make fibrin (it has a fast on-rate). Because
thrombin is an enzyme, it will release its substrate once the reaction is finished, and the thrombin will
react with more fibrinogen until the substrate is depleted. Thrombin reacts more slowly with
thrombomodulin (it has a slower on-rate) but the Thrombin-thrombomodulin complex, once formed, is
pretty stable, so as the fibrogen at the site of the clot becomes depleted, this reaction between thrombin
and thrombomodulin becomes more likely and the clotting cascade starts to shut down.

				
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