Heme - Offner Flashcards
what does a pyrrole group look like
five-membered ring with one N and two double bonds
pyrrole side chain abbreviations: M V A P
M = methyl -CH3 V = vinyl -CH=CH2 A = acetyl -CH2-COO- P = propionyl -CH2-CH2-COO-
structural formula of heme
cyclic tetrapyrrole with side chains
MV, MV, MP, PM
α-C is between the two MV groups
what is the simplest tetrapyrrole ring
porphyrin
all side groups are H
lots of double bonds (conjugated)
do porphyrins and/or porphyrinogens fluoresce under UV?
porphyrins (oxidized, conjugated)
how do porphryinogens compare to porphyrins?
more complex
different side-chains (not just H)
reduced (not conjugated with double bonds)
colorless (do not UV fluoresce)
how many steps in the heme biosynthesis pathway
8
what is your mnemonic for heme biosynthesis?
So Guys, A Proper Bili Uses Condoms, Protection Protection for Him
what are the 9 molecules in the path of heme biosynthesis?
succinyl CoA + glycine ALA PBG Urogen Coprogen Protogen Proto Heme
what does ALA stand for in heme synthesis
delta aminolevulinic acid
what does PBG stand for in heme synthesis
porphrobilinogen
what does bilane stand for in heme synthesis
hydroxymethyl bilane
what does urogen stand for in heme synthesis
uroporphyrinogen III
what does coprogen stand for in heme synthesis
coproporphyrinogen III
what does protogen stand for in heme synthesis
protoporphyrinogen IX
what does proto stand for in heme synthesis
protoporphyrin IX
what are the 8 enzymes involved in heme biosynthesis?
ALA synthase ALA dehydrase PBG deaminase (urogen I synthase) urogen III cosynthase urogen III decarboxylase coprogen III decarboxylase protogen dehydrogenase (oxidase) ferrocheletase
what are the products of the ALA synthase reaction
CO2
CoA-SH
ALA
what are the products of the ALA dehydrase reaction
2H20
PBG
what are the products of the PBG deaminase reaction?
4NH2
bilane
what are the products of the urogen III cosynthase reaction?
urogen III (cyclic)
what are the products of the urogen III decarboxylase reaction?
4CO2
coprogen III
what are the products of the coprogen III decarboxylase reaction?
2CO2
protogen IX
what are the products of the protogen dehydrogenase reaction?
6H+
proto
what are the products of the ferrocheletase reaction?
heme
when in heme biosynthesis is a linear tetrapyrrole formed?
bilane
when in heme biosynthesis is a cyclic tetrapyrrole first formed?
urogen III
what are the side-chains on bilane?
AP, AP, AP, AP
what are the side chains on urogen III?
AP, AP, AP, PA
what is the “type III problem” in heme biosynthesis
the problem of why only Urogen III: AP,AP,AP,PA is found in heme biosynthesis, and not for instance Urogen I: AP,AP,AP,AP
-solution - PBG deaminase forms only Urogen III isomer
what does succinyl CoA look like?
COOH-CH2-CH2-CO-S-CoA
what does ALA look like?
COOH-CH2-CH2-CO-CH2-NH2
how many ALA in a PBG?
2
what does PBG look like?
mono pryrrole with AP and a -CH-NH2 branch
how many PBG in bilane?
4
how many ALA in bilane?
8
what does bilane look like?
linear tetrapyrrole with AP, AP, AP, AP and a methanol on the end
when in heme biosynthesis are pyrrole rings made?
PBG
what does coprogen III look like?
cyclic tetrapryrrol with MP,MP,MP,PM
what happens in the transition from urogen III to coprogen III
loss of 4CO2
cyclic tetrapyrrol with AP,AP,AP,PA becomes
cyclic tetrapyrrol with MP,MP,MP,PM
where do the 4CO2 come from in the urogen III decarboxylase reaciton?
the A groups turn to M groups
AP,AP,AP,PA urogen
MP,MP,MP,PM coprogen
which is more hydrophobic, urogen III or coprogen III?
coprogen III
MP (1PM) groups instead of AP (1PA) groups
which is more hydrophilic, urogen III or coprogen III?
urogen III
AP (1PM) groups instead of MP (1PM) groups
what does protogen IX look like?
cyclic tetrapyrrole with MV,MV,MP,PM
what is the difference between coprogen III and protogen IX?
coprogen III is MP,MP,MP,PM
protogen IX is MV,MV,MP,PM
how does coprogen decarboxylase change coprogen III into protogen IX?
turns two propionyl groups into vinyl groups
MP,MP,MP,PM becomes
MV,MV,MP,PM
what is the difference between protogen IX and proto IX
-6H+
deprotonates α,β,γ,δ carbons
deprotonates pyrrole I & pyrrole III nitrogens
ring is now conjugated (red color)
what color is protogen IX?
no color (not conjugated)
what color is protoporphyrin IX?
red (conjugated)
what is the evolution of side-chains through heme biosynthesis?
AP (PBG) AP,AP,AP,AP (bilane) AP,AP,AP,PM (urogen III) MP,MP,MP,PM (coprogen III) MV,MV,MP,PM (protogen IX) MV,MV,MP,PM (proto IX)
how many ALA does it take to make one heme?
8
which heme biosynthesis intermediate is most hydrophobic?
proto IX
where in the cell does heme biosynthesis take place?
ALA transported out of mitochondrion
coprogen III transported back into mitochondrion
which heme biosynthesis intermediates is transported out of the mitochondria?
ALA
which heme biosynthesis intermediate is transported back into mitochondria?
coprogen III
how does heme regulate its own synthesis?
- inhibit ALA synthase
- inhibit ALA synthase transcription (when complexed with aporepressor protein)
- inhibit ALA synthase translation
- inhibit ALA synthase transport from cytoplasm to mitochondria
which is the rate limiting enzyme of the heme biosynthesis pathway?
ALA synthase
how soluble is heme in water?
poorly soluble
heme associates with apoproteins to form __
hemoproteins
4 common hemoproteins
Hb, Mb, cytochromes, P-450
where are cytochrome proteins found
inner mitochondrial membrane; endoplasmic reticulum
where is heme found to be broken down?
- senescent erythrocytes
- hepatic hemoproteins being turned over
where does heme breakdown occur?
mononuclear phagocytic system MPS
- spleen
- Kuppfer cells of liver
- tissue macrophages (small amount)
where does cleavage of heme occur?
the α carbon
between MV MV pyrroles
what enzyme catalyzes the cleavage of heme at the alpha carbon?
heme oxygenase
what is the first step in heme catabolism?
heme
2O2 + NADPH + H+ + heme oxygenase
biliverdin
CO + Fe
what is the structure of biliverdin?
linear tetrapyrrole with (II)MV,MP,PM,MV(I)
with lactam oxygens on either end
conjugated
what is the color of biliverdin?
blue green
conjugated
what is your mnemonic for heme catabolism?
Ho! BiliV BiliR BiliR Di BiliR U Suck
in heme catabolism, what is biliverdin converted into?
bilirubin
what are the 7 intermediates in heme catabolism?
heme biliverdin bilirubin bilirubin diglucuronide bilirubin urobilinogen stercobilin
what enzyme converts biliverdin into bilirubin?
biliverdin reductase
what is the difference between biliverdin and bilirubin?
the double bond at the old γ carbon has shifted left
what are the colors of biliverdin and bilirubin?
blue-green
yellow-orange
is bilirubin soluble?
no
where do the heme oxygenase and biliverdin reductase reactions take place?
blood, MPS?
phagocytic cells in spleen
kuppfer cells in liver
some tissue macrophages
how is bilirubin transported in blood?
complexed with albumin (otherwise heme is insoluble)
in the process of heme catabolism, where is bilirubin transported to for further breakdown?
hepatocytes of liver
what happens to bilirubin in the hepatocytes?
undergoes conjugation bilirubin + 2 UDP glucuronic acid + bilirubin glucuronyl transferase --> bilirubin diglucuronide
what enzyme conjugates bilirubin into bilirubin diglucuronide?
bilirubin glucuronyl transferase
what does bilirubin glucuronyl transferase do?
uses 2 UDP glucuronic acids to conjugates bilirubin into bilirubin diglucuronide
in heme catabolism, what happens to bilirubin diglucuronide?
secreted from hepatocyte into bile
what is the purpose of conjugating bilirubin in the liver?
make it soluble so it can be secreted into bile
which heme catabolism intermediates are bile pigments?
biliverdin (green-blue)
bilirubin (yellow-orange)
what is the difference between bilirubin and bilirubin diglucuronide?
2 sugars (diglucuronydyl) makes soluble
where do heme catabolites go in bile?
from liver
to small intestine
to large intestine
what happens to heme catabolites in the large intestine?
intestinal bacteria (beta glucuronidase) convert bilirubin diglucuronide to bilirubin, reduce to uroblinogen, further reduce to stercobilin
in what 3 locations does heme catabolism take place?
- MPS (mononuclear phagocytic system)
- hepatocytes
- large intestine
what intermediates and enzymes in the heme catabolism pathway take place in the MPS?
heme-biliV-biliR
heme oxygenase
biliverdin reductase
what intermediates and enzymes in the heme catabolism pathway take place in the hepatocytes?
biliR-biliRDi
bilirubin glucuronyl transferase
what intermediates and enzymes in the heme catabolism pathway take place in the large intestine?
biliRDi-biliR-urobolinogen-stercobilin
β glucuronidase
reducing bacterial enzymes
reducing bacterial enzymes
what are the enzymes involved in heme catabolism?
heme oxygenase (+O2+NADPH+H+) biliV reductase (+NADPH) biliR glucuronyl transferase (+2UDPgluc acid) β glucuronidase reducing bacterial enzymes reducing bacterial enzymes
in heme catalysis, what happens to bili R in the large intestine
reduced by bacteria to urobilinogen
in heme catalysis, what happens to urobilinogen in the large intestine
reduced by bacteria to stercobilin
or
reabsorbed into blood to be urinated
in heme catalysis, what happens to stercobilin
excreted in feces
in heme catalysis, where does urobilin come into play
urobilinogen reabsorbed in large intestine is excreted in urine along with its oxidized form, urobilin (yellow pigment)
what color is urobilin
yellow
what is the difference between urobilinogen and stercobilin
stercobilin is reduced
what is the difference between urobilinogen and urobilin
urobilin is oxidized
how much urobilinogen is processed into stercobilin to be excreted in feces, and how much urobilinogen is reabsorbed to be urinated?
90% stercobilin feces
10% reabsorbed urine
what circulatory system is responsible for reabsorbing a minor percentage of the urobilin in the large intestine?
enterohepatic system
from left to right on biliverdin / bilirubin, what are the pyrrole ring numbers?
II, III, IV, I
from left to right on biliverdin / bilirubin, what are the carbons?
β, γ, δ
no α, it was the site of cleavage
from left to right on biliverdin / bilirubin, what are the pyrrole side chains?
MV, MP, PM, MV
what does “hyperbilirubinemia” mean
high bilirubin levels in the blood
what is the van den Berg reaction
attacking bilirubin at middle methelyne carbon, separating rings II & III from IV & I
-used to measure biliR levels in blood spectrophotometrically
5 possible causes of hyperbilirubinemia
- increased bilirubin produciton
- decreased uptake into hepatocytes
- impaired conjugation
- defected secretion of conjugated biliR
- biliary obstruction (cholestasis)
what is found in the blood and urine in hemolytic jaundice
blood - more unconjugated biliR
urine - more urobilinogen
(increased heme catabolism & biliR production = hepatocytes cannot conjugate fast enough so biliR in blood, more biliR moving through hepatocytes and bile so more urobilinogen reabsorbed by enterohaptic circulation and in urine)
what is juandice
yellow skin caused by high levels of bilirubin (in MPS, blood, etc)
what is found in the blood and urine with a defect in biliR uptake into hepatocytes
blood - more unconjugated biliR
urine - less urobilinogen
what is found in the blood and urine with a defect in biliR glucuronyl transferase?
blood - more unconjugated biliR
urine - no urobilinogen
if completely defective, death
(Crigler-Najar, Gilbert’s)
what is Crigler-Najar Syndrome Type I
mutation in biliR glucuronyl transferase gene
cannot conjugate biliR
death
what is Crigler-Najar Syndrome Type II
mutation in biliR glucuronyl transferase gene
bili R conjugation reduced
what is Gilbert’s syndrome
mutation in promotor region of gene for biliR glucuronyl transferase
higher levels of unconjugated biliR in blood
-often asymptomatic
why does neonatal jaundice commonly occur?
- activity of biliR glucuronyl transferase does not reach full levels until 5-14 days after birth
- buildup of unconjugated biliR
what is the symptomatic risk of neonatal jaundice and how is it prevented?
- buildup of unconjucated biliR can be neurotoxic (kernicterus)
- treated with UV lights which convert biliR into non-toxic soluble compounds that can be excreted
how is neonatal jaundice treated?
-UV lights which convert biliR into non-toxic soluble compounds that can be excreted
in the context of heme catabolism, what is kernicterus?
neurotoxicity caused by build-up of unconjugated bilirubin from low biliR glucuronyl transferase activity in liver
in the context of heme catabolism, when are increased levels of urobilinogen in urine found?
when heme catabolism / biliR production is abnormally high
in the context of heme catabolism, when are increased levels of biliR diglucuronide found in urine?
- with Dubin-Johnson Syndrome defect in hepatocyte excretion of biliRDiG
- with biliary obstruction (cholestasis)
what is cholestasis?
“slow bile”
due to biliary obstruction
when are neither BRDG or urobilin found in urine?
- with Crigler-Najar or Gilbert Syndrome defect in biliR conjugation by biliR diglucuronyl transferase
- with defect in biliR uptake by hepatocytes
what is a possible way of treating Crigler-Najar syndrome?
injecting hepatocytes with functional biliR diglucuronyl transferase enzymes
what is Dubin-Johnson Syndrome?
defect in hepatocyte secretion of conjugated bilirubin
-high levels of BRDG in blood and dark urine
what is the name for an inherited disorder of heme synthesis
porphyria
what are porphyrias
disorders of heme synthesis (mutations in biosynthetic enzymes)
are porphyrias most commonly homozygous or heterozygous?
heterozygous - expressing ~50% of functional enzyme
what are the consequences of porphyrias?
- failure to synthesize sufficient heme to downregulate ALA sythase
- inetermediates accumulate and backup causing tissue damage
what are the common symptoms of porphyrias
- cutaneous lesions
- neurologic dysfunction
- liver disease
cutaneous lesions, neurologic dysfunction, and liver disease together are suggestive of
porphyria
- defect in heme biosythesis enzyme
- failure to synthesize sufficient heme to downregulate ALA sythase
- inetermediates accumulate and backup causing tissue damage
what is the most common of the hepatic porphyrias?
PCT porphyria cutanea tarda
PCT porphyria cutanea tarda
- defect in urogen III carboxylase gene (20%) or spontaneous defect (80%)
- cutaneous lesions, neurologic dysfunction, and liver disease are symptoms common of porphyrias, due to inability of heme production to downregulate ALA synthase and build up of heme intermediates
is PCT porphyria cutanea tarda genetic or spontaneous?
20% genetic
80% spontaneous
- how does spontanous PCT porphyria cutanea tarda occur?
- how is it treated?
- often Hepatitis C virus or hemochromatosis and their treatments cause hepatic iron overload and reactive oxygen species that can inhibit urogen III decarboxylase
- treated with phlebotomy to reduce Fe concentrations
what are the defects leading to unconjugated and conjugated hyperbilirubinemias?
unconjugated -hemolytic jaundice -impaired biliR uptake -Crigler-Najar & Gilbert's (bad BRGT) conjugated -Dubin-Johnson (bad BRDG excretion) -biliary obstruction (cholestasis)
what are the defects leading to unconjugated hyperbilirubinemias?
unconjugated
- hemolytic jaundice
- impaired biliR uptake
- Crigler-Najar & Gilbert’s (bad BRGT)
what are the defects leading to conjugated hyperbilirubinemias?
conjugated
- Dubin-Johnson (bad BRDG excretion)
- biliary obstruction (cholestasis)
acute intermittent porphyria AIP: enzyme affected intermediate builtup acute or chronic light sensitive?
PBG deaminase
PBG
acute
not light sensitive
porphyria cutanea tarda PCT: enzyme affected intermediate builtup acute or chronic light sensitive?
urogen III decarboxylase
urogen III
chronic
light sensitive
heredetary coproporphyria HC: enzyme affected intermediate builtup acute or chronic light sensitive?
coprogen III decarboxylase
coprogen III
acute
light sensitive
variegate porphyria VP: enzyme affected intermediate builtup acute or chronic light sensitive?
protogen III dehydrogenase
protogen III
acute
light sensitive
which porphyria is not light sensitive?
AIP
PBG built up
which porphyria is chronic?
PCT porphyria cutanea tarda
urogen III built up
what are the 4 porphyrias you have to know, and what do you have to know about them?
AIP (PBG, not light sensitive)
PCT (urogen III, chronic)
HC (coprogen III)
VP (protogen IX)
what are the primary sites of heme synthesis?
- liver
- bone marrow
