Hemoglobin formation Flashcards
What are the functions of heme?
Transport of oxygen (hemoglobin, myoglobin)
Electron transport (respiratory cytochromes)
Oxidation-reduction reactions (cytochrome P450 enzymes)
What are the sites of heme synthesis?
Major sites of synthesis:
bone marrow–> hemoglobin (6-7g hemoglobin are synthesized
each day to replace heme loss through normal turnover of RBC)
** liver** –>cytochrome P450 enzymes (drug detoxificaton)
heme is required for other important cellular
proteins and is synthesized in virtually all cells, except
_________ which lack mitochondria.
mature erythrocytes
________are cyclic tetrapyrroles capable of chelating to
various metals to form essential prosthetic groups for
various biological molecules.
Porphyrins
_____is predominantly a planar molecule composed of a
porphyrin derrivative + a single ferrous ion (Fe2+ = reduced
form of iron)
Heme
Heme consists of a ferrous (Fe2+) chelate of ________ which is rapidly
autooxidized to ______
protoporphyrin IX
ferriprotoporphyrin IX
“hemin”; contains ferric Fe3+ iron
The substituent groups provide important
sites for binding of porphyrins to their
respective apoproteins.
ferriprotoporphyrin IX
7 major steps in heme synthesis:
1st & last 3 steps
occur in the
______.
Intermediate steps
occur in_____
mitochondrion
cytosol
All carbon & nitrogen
atoms of porphyrin
molecules are provided
by
succinyl CoA
and glycine.
_______ catalyzes the committed
step of heme biosynthesis: the condensation of glycine
(nonessential amino acid) and succinyl-CoA (intermediate of the
TCA cycle), with decarboxylation, to yield 5-aminolevulinate
(ALA)
5-aminolevulinate synthase (ALAS)
5-aminolevulinate synthase (ALAS) catalyzes the committed
step of heme biosynthesis:
the condensation of glycine
and succinyl-CoA with decarboxylation, to yield :
5-aminolevulinate
(ALA)
ALAS is localized to the ________ but is encoded by a
nuclear gene family; therefore, the nascent protein must be imported into the
mitochondrion.
inner mitochondrial membrane
ALAS is a __________ dependent enzyme. Condensation with
succinyl-CoA takes place while the amino group of glycine is in Schiff base
linkage to the PLP aldehyde. CoA and the glycine carboxyl are lost following
the condensation.
pyridoxal phosphate (PLP)
There are 2 forms of ALAS:
ALAS1 is in the
ALAS2 is in the
1 is liver
2 is reticulocytes
Heme biosynthesis in erythroid cells is _________ by feedback repression of ALAS2 by heme.
NOT regulated
How do erythrocytes ensure that heme and globin are synthesize in the correct ratio?
in reticulocytes (immature RBCs), **heme stimulates synthesis** of globin and ensures that heme & globin are synthesized in the correct ratio for assembly into hemoglobin. (ALAS2 enZ)
Drugs that cause a marked
elevation in ALAS1 activity, such as
_____, do not affect ALAS2
phenobarbital
In ALAS1 in liver; Feedback inhibition by_____
or_____ regulates heme biosynthesis in the liver:
heme
hemin
Heme (hemin) exerts multiple regulatory effects on hepatic hemebiosynthesis by
inhibiting ALAS1 synthesis at what three levels?
both transcriptional and
translational levels, as well
as its mitochondrial import.
~100 different drugs or metabolites can increase
ALAS1 activity… HOW?
Many drugs are metabolized
by cytochrome P450s in the
liver; many drugs increase the
synthesis of cytochrome P450
enzymes, thereby increasing
the demand for heme.
_______ is a cytosolic
enzyme catalyzing the condensation of two
molecules of ALA to form one molecule of
porphobilinogen (PBG)à the first pathway
intermediate that includes a pyrrole ring.
ALA dehydratase (ALAD)
ALA dehydratase (ALAD) is a cytosolic
enzyme catalyzing the condensation of two
_____ to form one molecule of
_______–> the first pathway
intermediate that includes a pyrrole ring.
molecules of ALA
porphobilinogen (PBG)
ALAD The enzyme requires _____ which is
complexed to an active site cysteine.
Zn2+
Lead and other heavy metals can displace
the Zn2+ and eliminate catalytic activity.
Thus, lead poisoning (increase ALA in urine)
can lead to :
clinical manifestations that mimic
acute porphyrias
Inhibition of ALA dehydratase (aka, porphobilinogen synthase) by
lead (Pb2+) results in __________, as impaired heme
synthesis leads to _______of transcription of the ALAS gene.
elevated blood ALA
de-repression
High_____ is thought to cause some of the neurological effects of
lead poisoning, although Pb2+ may directly affect the nervous
system.
ALA
ALA is toxic to the brain, perhaps due to:
• Similar ALA & neurotransmitter GABA (γ-aminobutyric acid) structures. • ALA autoxidation generates reactive oxygen species (ROS).
________ catalyzes the head-to-tail condensation of 4 porphobilinogen molecules to form a linear________ (hydroxymethylbilane). Each condensation results in the liberation of one ammonium ion.
Porphobilinogen deaminase (PBGD)
tetrapyrrole
The tetrapyrrole can spontaneously cyclize to form
uroporphyrinogen I (nonenzymatic) which contains a symmetric
arrangement of side chains and IS NOT in the normal pathway for
heme biosynthesis. However, PBGD is tightly associated with a
second enzyme ________
uroporphyrinogen III cosynthase (UROS)
______has no enzymatic activity alone but serves to direct the
stereochemistry of the condensation reaction to yield the
uroporphyrinogen III isomer which IS on the pathway for heme
biosynthesis
UROS
__________ catalyzes the decarboxylation of acetate side chains to methyl groups on uroporphyrinogens III to yield the respective coprophorphyrinogen III products
Uroporphyrinogen decarboxylase (UROD)
Uroporphyrinogen decarboxylase (UROD) catalyzes the decarboxylation of acetate side chains to methyl groups on uroporphyrinogens III to yield the respective
coprophorphyrinogen III products
Coproporphyrinogen III is transported into the intermembrane space where it is converted to protoporphyrinogen IX by
coproporphyrinogen III oxidase (CPO)
CPO is located __________ of the mitochondrion,
implying that its product or protoporphyrin IX must cross the inner mitochondrial membrane because heme is formed within
the inner membrane.
in the intermembrane space
Another mitochondiral oxidase,
_________
converts protoporphyrinogen IX to
protoporphyrin IX.
protoporphyrinogen IX oxidase (PPO)
converts protoporphyrinogen IX to
protoporphyrin IX
protoporphyrinogen IX oxidase (PPO)
Insertion of Fe2+ into _________ to generate HEME occurs spontaneously but at a slow rate. This rate is enhanced by_______.
protoporphyrin IX
ferrochelatase
This enzyme is inhibited by lead (lead poisoning; increase protoporphyrin in urine) and is alsoinhibited during iron deficiency (anemia).
ferrochelatase.
In the absence of Fe2+, ferrochetalase can insert Zn2+ into the protoporphyrin ring to yield
a brilliantly fluorescent complex
=
defect in
heme
biosynthesis
Porphyrias
Porphyrias are a group of inherited genetic or acquired (disorders resulting from deficiency in specific enzymes of the porphyrin/heme biosynthetic pathway.
Which is more common, inherited or acquired?
Inherited, rarely acquired
How are porphyrias classified?
Porphyrias are classified as either hepatic or erythroid,
reflecting the principal sites of heme biosynthesis and depending on the site of expression of the enzyme defect (different isoforms).
All porphyrias, with the exception of ________ which is autosomal recessive, are inherited as_______
congenital erythropoietic porphyria
autosomal dominant disorders.
Porphyrias are rare or common?
Affect how many in US?
Rare
200,000
most common porphyria, ________, is 1 in 10,000
the most common acute porphyria, ________, is about 1 in 20,000
most common erythropoietic porphyria,
_________, is estimated at 1 in 50,000 to 75,000.
porphyria cutanea tarda (PCT)
acute intermittent porphyria (AlP)
erythropoietic protoporphyria (EPP)
___is the most common porphyria in
childhood, and the one associated with the longest delays in diagnosis.
EPP
________ is extremely rare with prevalence estimates of 1 in 1,000,000 or less.
Congenital erythropoietic porphyria (CEP)
Accumulation of intermediates_______ from the enzyme defect (measure in urine, blood, feces) results in the clinical symptoms associated with thevarious porphyrias
upstream
Defects_____ in the biosynthetic pathway (accumulation of ALA,
prophobilinogen) result in neurologic dysfunction.
early
Accumulaiton of _______are seen in early defects to porphyrin path
ALA, prophobilinogen
Defects _____in the pathway (accumulation of cyclic tetrapyrroles, but not prophobilinogen) result in sunlight-induced cutaneous lesions: in the presence of molecular oxygen, UV irradiation of cyclic tetrapyrroles generates reactive oxygen species that can produce cellular damage
later
Porphyia can be acute or chornic, in acute: Periodic acute attacks; symptoms include
abdominal pain,
neurologic deficits, psychiatric symptoms, and reddish-colored urine.
4 examples of acute porphyria
Doss porphyria (ALA dehydratase deficiency)
Acute intermittent porphyria
Hereditary coproporyphyria
Variegate porphyria
New recognized causes of acute porphyria attacks:
Nutritional changes (e.g., hypoglycemia) , smoking, certain drugs (such as barbiturates and sulfonamide antibiotics) and steroid hormones, especially progesterone
Some women develop attacksduring the second half of the menstrual cycle, when progesterone levels are high
3 chronic porphyria diseases
Congenital erythropoietic porphyria (Guntherʼsdisease)
Erythropoietic porphyria/protoporphyria
Porphyria cutanea tarda
Dermatologic diseases that may or may not include the liver and nervous system.
Chronic porphyria
Congenital erythropoietic porphyria (Guntherʼs
disease)
Erythropoietic porphyria/protoporphyria
Porphyria cutanea tarda
chronic porphyrias
1st Example that Porphyria Syndromes can be Acquired
Acquired Porphyria Cutanea Tarda (PCT)
how did people get the aquired porphyria cutanea tarda?
ate wheat that was covered in fungicide HCB
____ and ____ are very sensitive to inhibtion by Lead
Ferrochelatase adn ALA dehydratase
In lead poisoning ____ and ____ accumulate in urine
Protoporphyin and ALA
is relatively rare except in South Africa where it is estimated that the incidence is 3/1,000.
~10,000 persons areaffected, all descended
from a single Dutchsettler who immigratedin 1680.
Variegate porphyria
Acute disease caused by deficieny in protoporphyrinogen oxidase resulting in protoporphyrinogen IX and other intermerdiates priro to block to accumulate in urine
Pts are photosensitive
Varigate Porphyria
