Lecture 43 & 44: Porphyrins and Related Pathologies

Question 1

What is the basic structure of a porphyrin?

Answer 1

Cyclic molecule formed from:

1) four pyrrole rings
2) linked by methenyl bridges
3) each with 2 asymmetrical side chains
4) metal in the center (i.e. it “chelates”)

Question 2

Which porphyrin has Fe2+ in its central pocket?

Answer 2

heme

Question 3

What is cobalamine?

Answer 3

porphyrin which has Cobalt in its central pocket (a.k.a. Vitamin b12)

Question 4

chlorophyll?

Answer 4

porphyrin w/ Mg in the central pocket

Question 5

Name the physio roles of hemes

How are these heme groups attached to these various larger proteins

Answer 5

1. O₂ transport: hemoglobin and myoglobin
2. electron transport chain
3. catalase (H₂O₂—>H₂O)
4. drug metabolism via CYP450s: also: metab fat soluble cmpds, formation of cholesterol, steroids, and arachadonic acid metabolites

They are not covalently attached!

Question 6

Why are CYPs medically important? How many different types are there?

Which one is most important?

Why did they evolve?

Answer 6

There are ~50 differe cytochrome P450s

They are medically important because they are involved in many different drug interactions (drugs either induce or inhibit their actions)

CYP3A4 is particularly important

CYPs evolved to protect us from plant-based alkaloids

Question 7

How does CN kill you?

Answer 7

cyanide irreversibly inhibits cytochrome a3 in the electron transport chain

Question 8

What is the committed step of heme synth path?

Answer 8

The very first step:

succinyl CoA + glycine—ALA synthase—>ALA + CO₂ + CoA-SH

ALA = δ-aminolevulinic acid

**Irreverisible: no going back no matter how much product builds up

Question 9

Tell me the pathway for heme synthesis

Answer 9

1. Make ALA: glycine + succinyl-CoA—ALA synthase—>ALA + CO₂ + CoA-SH
2. Stick 2 ALAs together to make 1 pyrrole ring: ALA—ALA DH—>PBG (porphobilinogen) + 2H₂O
3. Stick 4 PBGs into 1 linear chain: 4 PBG—hydroxymethylbilane synthase—>hydroxymethylbilane + 4NH₄
4. Close the ring: hydroxymethylbilane—uroporphyrinogen synthase—>Uroporphyrinogen I + H₂O
5. Isomerize it to get an asymmetrical compound: Uroporphyrinogen I—Uroporphyrinogen cosynthase—>Uroporphyrinogen III
6. conjugate its double-bonds—successive decarboxylation and oxidation rxns—protoporphyrin IX
   
   1. (this glows purple and is commone precursor to chlorophyll, cobalamine and heme)
7. Add Iron: protoporphyrin IX—ferrochelatase—>heme

Question 10

How is heme synthesis regulated?

Answer 10

negative feedback from products: heme and hemin (breakdown product from heme) to ALA synthase

also glucose is an allosteric regulator of ALA synthase

all decrease transcription of ALA synthase

Question 11

What is(are) the rate-limiting step(s) of heme synthesis?

Answer 11

two rate-limiting steps:

1. Formation of the pyrrole
   
   1. succinly-CoA + glycine—ALA synth.->ALA
2. Condensation of 4 pyrroles into one molecule
   
   1. PBG—hydroxymethylbilane synthase—>hydroxymethylbilane

Question 12

How are different forms of heme made (i.e. those destined for electron transport chain, hemoglobin, CYPs, etc)?

Answer 12

there are liver- and marrow-specific isoforms of the catalytic enzymes involved in the heme synth path

Question 13

What causes acute intermittent porphyria?

Pathogenesis?

What are the sx?

Tx?

Dx?

Who had it?

Answer 13

Example of an acute porphyria

Acute intermittent porphyria is caused by a deficiency in hepatic hydroxymethylbilane synthase (PBG—>hydroxymethylbilane)

Pathogenesis:

1. Acute intermittent porphyria flares up when there is an increased demand for heme to make more CYPs (e.g. with EtOH consumption or arsenic poisoning)
2. The resultant suddenly wicked low levels of heme shuts off all inhibition on ALA formation (which recall is irreversible)
3. This causes massive amounts of ALA and PBG to accumulate upstream of the broken hydroxymethylbilane synthase enzyme
4. Massively incr hepatic stores of ALA cause abdominal pain
5. Incr plasma [ALA] antagonizes GABA (an inhib neurotrans) causing psychosis

Tx: Give IV hemin or glucose to pt; have them avoid inducers of CYP formation: antibiotics, EtOH, aresenic, hormones of pregnancy even

Dx: PBG in urine (purple!); PCR family members to find others with mutation

Famous cases: Van Gogh, King George III, Dracula (the blood would have helped the psychosis!)

Question 14

What causes porphyria cutanea tarda?

Pathogenesis?

What are the sx?

Tx?

Dx?

Answer 14

porphyria cutanea tarda is an example of a non-acute porphyria

caused by reduced uroporphyrinogen decarboxylase activity

Pathogenesis

1. High levels of porphyrins build-up in the urine, skin (causing the photosensitive skin rash) and liver (causing liver damage)
2. ALA levels are not out of control because mutation is downstream of the two rate-limiting steps—>therefore no psychosis

Sx: blistering rash on sunlight-exposed skin; urine left out for 2 hours turns reddish

Tx: routine phlebotomy

Dx: check urine for uroporphyrins

Question 15

Lead poisoning

Pathogenesis?

What are the sx?

Tx?

Dx?

Answer 15

Pathogenesis: Pb inhibits 2 steps in the heme synth path and can get into both liver and blood pathways:

1. ALA—ALA DH—>PBG (this step causes AIA-like presentation e.g. the psych sx)
2. Pb gets into marrow where it is chelated like Fe by ferrochelatase

Sx: All sx of AIP (abdominal pain, confusion, neuropsychiatric sx) plus anemia (see hematopoietic precursor cells in peripheral blood)

Tx = chelation therapy

Dx

Question 16

Fe-def anemia

Pathogenesis?

What are the sx?

Tx?

Answer 16

Sx: fatigue, anemia w/ thin-walled, pale RBCs

Pathogenesis: Since Fe is needed for many vital processes, DNA has IREs (iron-response elements) that won’t allow for the translation of enzymes like erythroid isoform of ALA synthase until there is adequate iron. That allow iron stores to be conserved for more vital f(x)

Tx: Give iron

Question 17

Where’s heme degraded?

Answer 17

Primarily in the liver and spleen

Question 18

What is the heme degradation pathway?

Answer 18

IN MACROPHAGES (i.e. in spleen)

1. Linearized and turned GREEN: Heme + NADPH + H⁺ + O₂—heme oxygenase—>biliverdin NADP⁺
2. Reduced and turned yellow-red: biliverdin + NADPH + H⁺—biliverdin reductase—>bilirubin + NADP⁺
3. bilirubin carried by serum albumin to LIVER
4. bilirubin + 2UDP-glucuronic acid—bilirubin glucuronyltransferase + 2UDP—>bilirubin diglucuronide
5. bilirubin diglucuronide is secreted into the INTESTINES bile
6. bilirubin diglucuronide—intestinal bacteria—>urobilinogen
7. Most of this is oxidized to stercobilin (gives poop its brown color)
   
   1. some is however reabsorbed by gut and converted in KIDNEY to urobilin (gives urine its yellow color)

Question 19

What causes jaundice and when does it become a real problem?

Answer 19

excessive bilirubin in the blood; usually itself just a sign of some other problem but can cause pathology itself when it crosses the blood-brain barrier causing encephalopathy are HIGH doses and eventually coma

Question 20

How do you tell where in the heme degradation pathway there is a problem?

Answer 20

Use the van den Bergh rxn:

1. one sample in water (direct)—shows level of conjugated only
2. one sample in methanol (total)—shows both conjugated and unconjugated

Question 21

Interpret the following van den Bergh results

increased total bilirubin

high unconjugated bilirubin

Answer 21

excessive production of heme (as in hemolytic jaundice: red cell lysis as in sickle crisis)

Question 22

Interpret the following van den Bergh results

increased total bilirubin

increased conjugated

Answer 22

obstructive jaundice (e.g. due to gallstones)

Question 23

Interpret the following van den Bergh results

both unconjugated and conjugated bilirubin are elevated

Answer 23

Hepatocellular jaundice

hepatocytes are damaged likely to do hepatitis; a little less clear dx

To r/o other DDx:

—get LFT: expect to see increased AST/ALT

—get Hep titers: expect poz

Question 24

What conjugated vs. unconjugated bilirubin levels do you expect to see in an infant?

Why?

Tx?

Answer 24

high unconjugated

have low glucuronyltransferase activity right after birth

blue fluroescent light turns this into water-soluble metabolites