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Biochemistry Exam 3 > L22- Heme > Flashcards

L22- Heme Flashcards

1
Q

Describe the structure of porphin rings.

A

All porphin rings are a tetrapyrrole connected with methene bridges.

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2
Q

What is the most abundant porphyrin in mammals?

A

Protoporphyrin.

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3
Q

In which tissue is most of the heme in mammals synthesized?

A

Pre-erythroid cells and hepatocytes.

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4
Q

In which proteins is most of the heme in mammals found?

A

Approximately 85% in hemoglobin and 15% in cytochromes (mainly liver mitochondria and smooth ER).

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5
Q

What are the two initial substrates of heme synthesis?

A

Glycine and succinyl CoA.

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6
Q

What is the major site of regulation in heme synthesis?

A

At the delta-aminolevulinaste synthase (ALAS) reaction, which is the first reaction of this pathway.

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7
Q

What are two inhibitors of delta-aminolevulinaste synthase (ALAS)?

A

Heme and glucose.

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8
Q

Heme synthesis occurs in both the mitochondrion and the cytoplasm. Which heme synthesis reactions occur in the mitochondrion?

A

ALA-synthase reaction, protoporphyrinogen reaction, and chelation of iron to protoporphyrin.

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9
Q

What is the slowest (i.e. regulated) step in heme synthesis?

A

The first step: the delta-aminolevulinaste synthase (ALAS) reaction.

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10
Q

There are two distinct genes for the enzyme delta-aminolevulinaste synthase: ALAS-1 and ALAS-2. What is the difference in the selective expression of these two genes?

A

ALAS-1 is expressed in all tissues whereas ALAS-2 is expressed specifically in erythropoeitic cells.

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11
Q

How many molecules of delta-aminolevulinic acid (delta-ALA) must condense to form porphobilinogen?

A

Two.

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12
Q

True or False. All cells synthesize heme.

A

True.

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13
Q

Which heme synthesis metabolites are water soluble?

A

Delta-aminolevulinic acid (delta-ALA) and porphobilinogen.

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14
Q

Which enzyme condenses two delta-aminolevulinic acid molecules to form porphobilinogen?

A

ALA-dehydratase.

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15
Q

Which enzyme in heme synthesis is particularly sensitive to lead poisoning?

A

ALA-dehydratase.

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16
Q

What is the cofactor for ALA-dehydratase?

A

Zinc.

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17
Q

What is the first tetrapyrrole formed in heme synthesis?

A

Uroporphyrinogen Type III.

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18
Q

Which enzymes catalyze the condensation of four porphobilinogen molecules to form uroporphyrinogen Type III?

A

A synthase (also called porphobilinogen deaminase) and a cosynthase.

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19
Q

What is the function of the cosynthase that helps to condense four porphobilinogens into uroporphyrinogen Type III?

A

It assures proper orientation of the side groups so as to yield a Type III porphyrin.

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20
Q

Deficiency of the cosynthase that helps to condense four porphobilinogens into uroporphyrinogen Type III results in the formation of what product?

A

Uroporphyrinogen Type I.

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21
Q

What is the significance of the uroporphyrinogen decarboxylase reaction?

A

It makes the porphyrin ring more hydrophobic so that the product, coproporphyrinogen Type III can re-enter mitochondria.

22
Q

True or False. The coproporphyrinogen decarboxylase reaction involves both decarboxylation and oxidation.

A

True. The product of this reaction is protoporphyrinogen Type III.

23
Q

What is the color of protoporphyrin?

A

Red-brown.

24
Q

What is the color of urophorphyrinogen and coproporphyrinogen?

A

Both are colorless.

25
Q

What is a classic symptom of accumulation of tetrapyrrole intermediates?

A

Photosensitivity and skin damage.

26
Q

Fe2+ vs. Fe3+: which one is incorporated into heme?

A

Fe2+.

27
Q

Which reaction is catalyzed by protoporphyrin ferrochelatase?

A

Chelation of iron to protoporphyrin Type III to form heme.

28
Q

What is the main regulator of delta-aminolevulinate synthase in non-erythroid cells?

A

Heme, which inhibits the enzyme.

29
Q

What is the main regulator of delta-aminolevulinate synthase in erythroid cells?

A

Iron, which stimulates the enzyme.

30
Q

True or False. Ferrochelatase activity decreases in response to an increase in iron.

A

False. Ferrochelatase activity increases in response to increased iron.

31
Q

Porphyrias are all associated with increased activity of which enzyme?

A

Delta-aminolevulinate synthase because production of heme would be decreased.

32
Q

Acute intermittent porphyria results from decreased activity of which enzyme?

A

Uroporphyrinogen synthase (also known as PBG deaminase).

33
Q

What are some symptoms of acute intermittent porphyria?

A

Neurological signs and abdominal pain are common. Note there is no photosensitivity!

34
Q

Porphyria cutanea tarda results from decreased activity of which enzyme?

A

Uroporphyrinogen decarboxylase.

35
Q

What are some symptoms of porphyria cutanea tarda?

A

Live cirrhosis and skin photosensitivity.

36
Q

In which organ does the conjugation of bilirubin occur?

A

The liver.

37
Q

Which enzyme breaks open the ring of heme to form biliverdin?

A

Heme oxygenase.

38
Q

What is the color of biliverdin?

A

Green.

39
Q

Which protein acts as the major iron transporter in blood?

A

Serum transferrin.

40
Q

Which proteins acts as a binding protein for iron storage?

A

Ferritin.

41
Q

Which protein acts as the major transporter of bilirubin?

A

Albumin.

42
Q

What is the color of bilirubin?

A

Yellow-brown.

43
Q

Where is conjugated bilirubin stored?

A

In the gallbladder.

44
Q

What can cause an increase in free bilirubin?

A

Liver disease, hemolysis, and UDP-glucuronyltransferase deficiency.

45
Q

What can cause an increase in conjugated bilirubin?

A

Occlusion of the bile duct.

46
Q

UV-light causes ________ of bilirubin to make it more water soluble.

A

Isomerization.

47
Q

UDP-glucuronyltransferase is responsible for what reaction?

A

Conjugation of bilirubin to two molecules of UDP-glucuronic acid.

48
Q

Bilirubin is excreted via the kidneys into the urine as what metabolite?

A

Urobilinogen.

49
Q

Bilirubin is excreted into the feces as what metabolite?

A

Stercobilinogen.

50
Q

Abnormal accumulation of bilirubin in serum and tissues can result in what condition?

A

Jaundice.

Biochemistry Exam 3 (6 decks)

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