NBME - 6/9 - RBC Metabolism and Heme Synthesis

Question 1

What do Basophils do

Answer 1

Allergic reactions, degranulation releases histamine, heparin, leukotrienes, etc.

Question 2

Monocytes?

Answer 2

Circulating precursors of tissue macrophages

Question 3

Most common granulocyte in the body?

Answer 3

Neutrophils

Question 4

Life span for RBCs, WBCs, and platelets

Answer 4

RBCs = 120 days

Platelets = 8-10 days

Neutrophils/WBCs = 6 hours

Question 5

Discuss the reaction involving Basophils for something like a Bee sting

Answer 5

Bee sting, venom into blood, we make an antibody which binds to the basophils and waits for round two. Bee sting again causes antigen to bind to FC portion of the basophil antibody, causing agglutination and activativation, leading to the release of histamine to vasodilate and increase fluid excretions, which presents as constricted airways and struggle to breathe

Question 6

Function of Erythropoietin. When is it released?

Answer 6

Stimulates erythrocyte maturation. Released from the kidney at low O2 conditions and acts on cytokine receptors of erythroid progenitor cells, stimulating their proliferation and maturation.

Question 7

Problem with Kidney failure for the blood

Answer 7

Kidney failure means no EPO, so there is nothing to mature the RBCs, causing an anemia

Question 8

Erythroid progenitor cells (CFU-E) undergoes how many rounds of division in the bone marrow? When does Hemoglobin synthesis begin?

Answer 8

4 divisions. Hemoglobin synthesis begins in cells that have undergone the first division

Question 9

True or False: When the new RBC is released from the bone marrow, there is no nuclear material in it

Answer 9

False. We lose the nucleus prior, but when this cell gets released into the blood from the bone marrow, it still has ribosomal material and RNA. This cell, called a reticulocyte, will mature in the blood quickly and turn into a normal RBC. Important note: Heavy hematopoiesis means lots of cells being churned out quickly. The reticulocytes will be released earlier than usual, and will have their ribosomes and RNA material a little longer than usual. These high reticulocyte counts are indicative of high hematopoietic activity.

Question 10

Oxidative stress in the RBC is very high. How do we manipulate glycolysis in the RBC to handle this stress? Read this long Flash card, its wordy in order to make sense. Not really that bad.

Answer 10

It’s all about playing hot potato with the oxidation. We start with some oxidizing dangerous agent in the blood. Gutathione peroxidase reduces that molecule and in the process has to pass on the oxidative stress to something else, in this case, reduced glutathione, which generates oxidized glutathione.

But oxidized glutathione doesn’t like this, it wants to be reduced again. It passes that nonsense onto NADPH with help from glutathione reductase, turning NADPH into NADP+ (so now oxidized glutathione is reduced glutathione, and NADPH is pissed because it’s oxidized to NADP+).

This is where glycolysis comes in to play with the Hexose monophosphate shunt, which takes the Glucose-6-P we made from glucose and gives it the oxidative stress, thus turning our angry NADP+ to NADPH. It does this with G6PDH.

Our angry oxidized glucose-6-P gets recycled to Fructose 6-P to continue on with Glycolysis after going through the HMP shunt.

Summary:

Question 11

Most common enzyme deficiency in humans

Answer 11

G6PDH deficiency

Question 12

Another important shunt used by RBS is the one that reduces Hemoglobin back to Fe2+. Recall that Met-Hgb is the inactive form, the oxidized form. Sometimes in the RBC, the good Fe2+ hemoglobin gets turned back into Fe3+ Met-Hgb. Outline the mechanism for reducing Met-Hgb back to what it is supposed to be

Answer 12

Fe3+ Hgb gets reduced to Fe2+ by Reduced Cytochrome b5, which then turns to Oxidized cytochrome b5.

NADH restores the oxidized cytochrome b5 back to reduced cytochrome b5 by taking on the stress and turning in to NAD+.

Glycolysis restores NAD+ back to NADH.

Question 13

What causes methemoglobinemia?

Answer 13

An issue with Cytochrome b5 reductase, which is needed to allow NADH to reduce the now oxidized cytochrome b5 back to its reduced cytochome b5 form. Because we can’t do that, the shunt shuts down and Fe3+ stays oxidized as met-hgb in the blood. So we can’t carry oxygen!

Question 14

Outline the Rapoport Luebering shunt and why it is important.

Answer 14

We use this shunt to generate 2,3 BPG. An increase in 2,3-BPG at low O2 conditions adjusts our O2 saturation curve for better performance (right shift of the curve) Mutase turns 1,3 BPG in the glycolysis cycle to 2,3 BPG, which can be put back into the glycolysis cycle by phosphatase as the next glycolysis step, 3 phosphoglycerate.

Question 15

What does a rightward shift in the Hgb curve really mean? It’s important to understand this

Answer 15

A rightward shift indicates that the hemoglobin under study has a DECREASED AFFINITY for oxygen. This makes it more difficult for hemoglobin to bind to oxygen (requiring a higher partial pressure of oxygen to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release oxygen bound to it, so we can unload more O2 to tissues. The effect of this rightward shift of the curve INCREASES PARTIAL PRESSURE of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock

Question 16

Relate BPG to fetal hemoglobin

Answer 16

In Fetal hemoglobin, a histidine is replaced by serine, not as many charges for BPG to bind to, so it doesn’t really have much of an effect. The fascinating thing, is that 2,3 BPG mutase exists at the fetal-maternal junction to allow the maternal blood to give up its O2 more easily to the fetal hemoglobin.

Question 17

Pyruvate kinase deficiency leads to what and why

Answer 17

The 2nd to last step of glycolysis is turning PEP to Pyruvate, turning ADP to ATP in the process via a pyruvate kinase. Without this pyruvate kinase, the cell doesn’t generate ATP, and low energy RBCs means hemolytic anemia.

Question 18

The erythrocyte has a cytoskeleton, which becomes affected in diseases like Hereditary Spherocytosis, an autosomal dominant disorder of RBCs. Discuss the major protein involved in the cytoskeleton of RBCs and what it attaches to

Answer 18

The major protein, spectrin, is linked to the plasma membrane through one of two ways:

1. Interaction with Ankyrin and band 3 (chloride-bicarbonate antiporter) or
2. With actin, band 4.1, and glycophorin

These combine to make a mesh-like framework

Question 19

Discuss formation of the heme precursor Porphyrinogen

Answer 19

Basic building block is a pyrrole, a 5 carbon ring with a Nitrogen between two of the carbons. 4 Parroles come together putting their Nitrogens towards a center area and linking together with 1 carbon in between each parrole, making a porphyrinogen, which is colorless because it only has unconjugated double bonds, not conjugated.

Question 20

Coming off the outside carbons of this big tetrapyrrole (Porphyrinogen) are Acetates, Methyls, Vinyls, and propionates. How do we distinguish different types of porphyrinogens?

Answer 20

Based on these outside molecules.

Uro-porphyrinogen = A’s and P’s

Copro-porphyrinogen = M’s and P’s

Final stage: Proto-porphyrinogen = M’s, V’s, and P’s

Question 21

Porphyrinogens become porphyrin how?

Answer 21

Spontaneous oxidation! Conjugated double bonds, and now we are red because we are conjugated! So the red comes from porphyrin, not the iron, interesting.

Question 22

How do we make heme?

Answer 22

Need to take our coolest porphyrinogen, Proto-porphyrinogen (it has all three extensions so we like it more), mature it spontaneously to protoporphyrin (protoporphyrin IX is a particular isotype we care more about for whatever reason), and then shove an Fe 2+in the middle to make Heme.

Question 23

If there is no globin around when we make Heme, like when we’ve already made a ton of hemoglobin, what happens to the heme?

Answer 23

It turns to hematin/Hemin, an Fe 3+ form of heme that inhibits further creation of heme.

Question 24

How does Hemin/Hematin stop synthesis of new Heme? Discuss where it inserts into the cycle to stop things.

Answer 24

We get the carbons to make heme from succinyl CoA and glycine coming together with ALAS to make ALA. This begins the long process. Hematin directly inhibits ALAS and also indirectly inhibits ALAS by inhibiting its transport into the mitochondrion.

Question 25

ALAS requires ____

Answer 25

Pyridoxal phosphate.

Question 26

How in the hell do we get from this ALA to the pyrolle thing we started with before? Don’t we just have pyrolles floating around we can use?

Answer 26

Nope, but we’re so close. 2 ALAs come together with ALA Dehydratase to make our Porphobilinogen, a pyrrole.

Question 27

ALAD (ALA Dehydratase) is helped and hurt by metals. Which ones do what?

Answer 27

Zinc is needed to activate this guy, and Pb inactivates it, which is why Pb poisoning leads to our microcytic anemia!

Question 28

Don’t bother memorizing all of the intermediate tetraporphyrin stuff, there are carbons flying around, breaks and reactivation etc. What is porphyria?

Answer 28

Porphyria - Inherited or acquired disorders resulting from decreased activity of specific enzymes in the heme biosynthetic pathway. Characterized by excessive accumulation and excretion of porphyrins and/or their precursors.

Question 29

Discuss Acute Intermittent Porphyria (symptoms, mechanism, inheritance)

Answer 29

Autosomal Dominant

Half-normal levels of PBG deaminase, causing an increase in ALA and PBG in the urine since we can’t go forward.

This one is not related to light since it cannot yet absorb light

Presents as acute abdominal pain, tachycardia, and psychiatric symptoms like agitation.

This is the most common acute-type porphyria

Question 30

Discuss Porphyria Cutanea Tarda (symptoms, mechanism)

Answer 30

Most common porphyria in general.

Due to low uroporphyrinogen decarboxylase, so we start peeing out the intermediate uroporphyrin III.

There is no neuro involvement with this one, just the skin and liver.

Chronic blistering lesions on sun-exposed skin, and various factors can precipitate the attacks, such as chemical exposure, drugs, and smoking.

Question 31

Congenital Erythropoietic Porphyria, the ONLY Autosomal recessive and one of the only marrow tissue porphyrias, leads to build up of what in the urine and why?

Answer 31

Coproporphyrinogen I and uroporphyrinogen I which both rise due to inactivation of URO III cosynthase

Question 32

Hereditary Coproporphyrinogen Porphyria (HCP) leads to what buildup?

Answer 32

Coproporphyrinogen III builds up. This is a strongly light sensitive one.

Question 33

Variegate Porphyria (VP) leads to what building up

Answer 33

Protoporphyrinogen IX builds up

Question 34

Why do light sensitive porphyrias cause tissue damage?

Answer 34

Porphyrinogens in excess means we can oxidize to random porphyrins. Light excites the porphyrin and turns O2 from its ground state to its singlet oxygen species which damages tissue. We want these porphyrins to fluoresce, since that means they are dropping back to their low energy safer state.

Question 35

What enzyme turns our final protoporphyrin IX into heme and what happens if we don’t have this?

Answer 35

Erythropoietic Proto-porphyria (EPP) - Protoporphyrin IX buildup (dang, so close). AD disease of the marrow that occurs due to damage of the terrochelatase enzyme.

Question 36

Hemin vs Hematin and what we can use hematin for

Answer 36

Hemin is an iron-containing porphyrin. More specifically, it is protoporphyrin IX containing a iron ion with a chloride ligand. It is sometimes distinguished from hematin which has a hydroxide ligand in place of the chloride. This is used in treatment of Acute Porphyria

It works by negative feedback mechanism to block the first RDS in heme synthesis.