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Term 1 Unified Exam 3

Heme is the prosthetic group for ______

hemoglobin, myoglobin, the cytochromes, catalase, tryptophan pyrrolase

Only _____ porphyrins,which contain an asymmetric substitution on_____ (see Figure21.2), are physiologically important in humans.

Type III , ring D

Most important Nitrogen containing compounds synthesized from amino acids

porphyrins, neurotransmitters, hormones, melanin, creatine

neurotransmitters synthesized from amino acids

catecholamines, serotonin, melatonin

Hormones synthesized from amino acids

thyroid hormones are synthesized from tyrosine

melanin is synthesized from


Porphyrins are made of _____ rings


The color in porphyrins comes from

conjugated double bonds

T or F:
Porphyrinogens and porphyrins are colored compounds

False, porphyrinogens the precursors to porphyrins, are colorless

Protoporphyrin IX is found in


protoporphyrin IX, has ____ side chains on the tetrapyrrole ring

4 Methyl, 2 vinyl, 2 propionic

First step in heme biosynthesis

condensation of glycine and succinyl CoA to form δ-aminolevulinate

first step of heme biosynthesis is catalyzed by

ALA synthase

Coenzyme of ALA synthase


ALA synthase is present in


What is the rate limiting reaction of heme biosynthesis?

condensation of glycine and succinyl CoA to form δaminolevulinate catalyzed by ALA synthase

_____ dimerizes 2 molecules of ALA to produce the pyrrole ring compound porphobilinogen

ALA dehydratase

Control point of heme biosynthesis

ALA synthase

_____catalyzes the head to tail condensation of 4 molecules of porphobilinogen to produce the linear tetrapyrrole intermediate, hydroxymethylbilane

porphobilinogen deaminase (PGB deaminase, uroporphyrinogen I synthase)

uroporphyrinogen III synthase does what?

catalyzes conversion of hydroxymethylbilane to urophorphyrinogen III

______ decarboxylates the acetate substituents of uroporphyrinogen in the cytosol

uroporphyrinogen decarboxylase

acetate substituents of uroporhyrinogen III are converted to _____ substituents in coproporphyrinogen IIII


coproporphyrinogen III is transported to

the cytosol

Coproporphyrinogen oxidase does what?

decarboxylates two propionate residues, yielding vinyl substituents on the 2 of the pyrrole rings in colorless protoporphyrinogen IX

where is protoporphyrinogen IX converted to protoporphyrin IX?

The mitochondrion

What does protoporphyrinogen IX oxidase do?

converts protoporphyrinogen IX to Protoporphyrin IX

what does protoporphyrinogen IX oxidase require?

molecular oxygen

What is responsible for the characteristic red color of heme?

completely conjugated ring system

what is the final reaction of heme biosynthesis?

insertion of iron atom into the ring system of protoporphyrin IX, generating heme b

what catalyzes the last reaction of heme biosynthesis?


ferrochelatase and ALA dehydratase are highly sensitive to inhibition by

heavy metal poisoning

Increase in ALA in the circulation in the absence of an increase in porphobilinogen is characteristic of

lead poisoning

where does heme biosynthesis take place?

erythroid cells (85%) and hepatocytes

In hepatocytes, the rate limiting step of heme biosynthesis is

ALA synthase

In hepatocytes, heme accumulation leads to what?

Fe3+ oxidation product of heme (hemin),

what does hemin do to ALA synthase?

inhibits transport of ALA synthase from the cytosol to the mitochondria and represses synthesis of the enzyme

_____ represses synthesis of ALA synthase


What has a regulatory role in heme biosynthesis in erythroid cells?


Drugs that are accompanied by an induction in the system that cause an increase in the synthesis of heme

griseofulvin, hydantoins, phenobarbital

porphyrias occur due to abnormalities in

the pathway of biosynthesis of heme

All of the porphyrias are inherited _____ except congenital Erythropoietic porphyria which is _____

All of the porphyrias are inherited autosomal dominant except for Congenital Erythropoietic Porphyria which is autosomal recessive

describe porphyrin photosensitivity

Porphyrins absorb light at 400nm excited react with O2 free radicals which damage lysosomes and other organelle skin damage and scarring

Clinical features of porphyria can be precipitated by

drugs that induce the cytochrome P450 system

What does heme oxygenase do?

opens up heme ring n the ER

What does heme oxygenase produce?

linear tetrapyrrole biliverdin, ferric iron (Fe3+), CO

What does biliverdin reductase do?

it reduces a seond bridging methylene between rings III, IV of heme, producing bilirubin

Describe the color change from biliverdin to bilirubin

biliverdin is blue green, bilirubin is yello-red. This is because bilirubin is less conjugated

T or F:
Bilirubin requires albumin to be transferred to the liver


What does UDP glucuronyl transferase do to bilirubin in hepatocytes?

adds 2 glucuronic acid molecules to bilirubin, makes it more water soluble bilirubin diglucuronide, leads to excretion with bile as bile pigments

In the intestine, bilirubin glucuronides are deconjugated by bacterial beta-glucuronidases and degraded to

colorless urobilinogens

what happens to urobilinogens in the intestines?

they are oxidized to stercobilin and give the brown color to feces

Bilirubin and its catabolic products are collectively known as the

bile pigments

The initial reaction and the last three steps in the formation of porphyrins occur in the __1___, where as the intermediate steps of the biosynthetic pathway occur in the __2___

1. mitochondria, 2. Cytosol

All carbon and nitrogen atoms of a porphyrin molecule come from _____ and _____

glycine and succinyl CoA

In erythroid cells, heme synthesis is under the control of _____

erythropoietin and the availability of intracellular ion

Individuals with an enzyme defect leading to the accumulation of tetrapyrrole intermediates show _____


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