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Biochemistry > Heme Synthesis > Flashcards

Heme Synthesis Flashcards

1
Q

What are porphyrins?

A

They are cyclic compounds that bind metal ions. Such as heme.

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2
Q

What are the components of heme

A

A ferrous ion in the center, a protophyrin IX ring.

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3
Q

Heme acts as a prosthetic group to ?

A

Hemoglobin (heme + globus proteins) , myoglobin, cytochrome, tryptophan pyrrolase

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4
Q

What are cytochromes? What are the 3 kinds and how can we distinguish them?

A

They are electron transporting heme proteins. They have 3 different forms : Heme A(has an isoprenoid moiety, for the noncovalent interactions with cytochrome a) , Heme B (is the iron-protoporphyrin IX found in Hb), Heme C (the pyrrol ring has covalent interactions with proteins thanks to Cys residues).
We can distinguish them by differing absorption spectrum.

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5
Q

What are porphyrins?

A

They are cyclic molecules, made of 4 pyrrol rings which are linked by methenyl bridges

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6
Q

What are the side chains present in protoporphyrin IX . What type of side chain organisation does it have?

A

Vinyl, methyl, propripnate. It has a type III organisation

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7
Q

What are porphyrinogens

A

They are a porphyrin precursor.

Porphobilinogen–porphyrinogens–protoporphyrins

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8
Q

What determines the type of porphyrin

A

The position of the side chains. In type III the acetate and proprionate are exchanged in position on a single moiety

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9
Q

What does iron protoporphyrin IX derive from? What does it bind to?

A

Its a derivative of protoporphyrin IX, it binds metals like Fe2+ (heme-ferrous), Fe3+ (hemin-ferric), Zinc protoporphyrin Zn2+

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10
Q

Where does heme synthesis occur?

A

Occurs in the mitochondrial matrix and cytoplasm of all cells.

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11
Q

list the overall pathway of heme synthesis

A 
succinyl-coA and glycine. ALA SYNTHASE. aminolevulinic acid. porphobilinogen.
uroporphyrinogen III.
coproporphyrinogen III
protoporphyrinogen IX.
protoporphyrin IX.
Heme
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12
Q

what is the rate limiting step of heme synthesis.

A

first step. Glycine and Succinyl coA. condense together. Enzyme aminolevulinate synthase.
it is inhibited by heme and hemin.
there is the release of coA and co2

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13
Q

What is inhibited by heme and hemin in the liver?

A

they inhibit ALA synthase synthesis, rate of translocation from cytosol to mitochondria, and the activity of ALA synthase

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14
Q

the activity of ALA synthase depends on?

A

it responds to the detoxification needs,

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15
Q

what kind of amino acid aminolevulinic ? Where is it formed?

A

its a non essential amino acids. In the mitochondria

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16
Q

what does heme do in reticulocytes?

A

it stimulates the synthesis of globins and the enzymes of its own biosynthetic pathway

17
Q

what kind of amino acid is glycine?

A

non essential amino acid

18
Q

what is the coenzyme needed for the rate limiting step?

A

pyridoxal phosphate, is needed for the decarboxylation of glycine

19
Q

what regulates the level of ALA synthase?

A

iron and protohemin

20
Q

what is hemin?

A

when there is excess heme, it is converted to hemin. This decreases the action of ALA synthase. It is an example of end product inhibition

21
Q

what is the function of grisofulvin, antifungal or anticonvulsant drugs?

A

they increase ALA synthase activity. This is because they are metabolized in the liver by cytochrome p450 and consume heme.

22
Q

what is the second step of heme synthesis.

What enzyme catalyses it?

A

the formation of porphobilinogen from the condensation of 2 molecules of ALA. It is catalyzed by porphobilinogen synthase/ ALA dehydratase. It is dependent on Zn.

23
Q

ALA dehydratase is inhibited by?

A

Pb, resulting in anemia or lead poisoning. When this occurs ALA in blood is elevated

24
Q

how is protoporphyrin IX formed?

A

porphobilinogen is changed to hydroxymethylbilane, which forms uroporphyrinogen III, whose side chains are modified to form protoporphyrin IX

25
Q

what are steps 3,4,5 of heme synthesis?

A

tetrapyrrole formation.
conversion to protoporphyrin III.
protoheme synthesis

26
Q

how is protoporphyrin IX turned into heme?

A 
ferrous ions (Fe2+) are introduced into protoporphyrin IX by
ferrochelatase
27
Q

how is hemoglobin formed?

A

protoheme + globin

28
Q

what is the consequence of protohemin formation?

A

formation of super oxides

29
Q

when is protohemin formed from protoheme?

A

when the O2 added is negatively charged. SO its Fe3+

30
Q

refer to slide 25 for regulation of porphyrin synthesis?

A

do itttt

31
Q

what is porphyria?

A

they are rare inherited defects in heme synthesis. An inherited defect in an enzyme of heme synthesis results in accumulation of one
or more of the porphyrin precursors depending on where the blocking is in the heme
synthesis pathway.
These precursors increase in blood and appear in urine of affected individuals.
Porphyria means purple colour caused by the pigment-like porphyrins in the urine.
(Diagnosed by lab investigation)
Most porphyrias show a prevalent autosomal dominant pattern, except
congenital eythropoietic porphyria, which is recessive

32
Q

what are the 2 kinds of porphyria?

A

erythropoietic and hepatic (hepatic can be acute or chronic)

33
Q

know that

A

Generally, individuals with an enzyme defect prior to the synthesis of the
tetrapyrroles manifest abdominal and neuropsychiatric signs
Those with defects after the synthesis of tetrapyrrole intermediates show
photosensitivity with formation of reactive oxygen species (ROS)
that damage membranes by oxidation resulting in the following effects:
- Skin blisters, itches (pruritis)
- Skin may darken, hair growth (hypertrichosis)

34
Q

go to slide 28

A

pls

35
Q

what is the most common type of porphyria and what is it caused by?

A

Chronic hepatic porphyria (porphyria cutanea tarda)
• The most common type of porphyria
• a deficiency in uroporphyrinogen decarboxylase
• Clinical expression of the enzyme deficiency is influenced by
various factors, such as exposure to sunlight, the presence of
hepatitis B or C
• Clinical onset is during the fourth or fifth decade of life.
• Porphyrin accumulation leads to cutaneous symptoms and
urine that is red to brown in natural light and pink to red in
fluorescent light

36
Q

discuss acute hepatic porphyrias?

A

Porphyrias leading to accumulation of ALA and porphobilinogen cause
abdominal pain and neuropsychiatric disturbances, ranging from
anxiety to delirium.
• Symptoms of the acute hepatic porphyrias are often precipitated by
administration of drugs such as barbiturates and ethanol.

37
Q

what is the treatment of porphyrias?

A

hematin (hemin hydroxide)

Biochemistry (21 decks)

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