Pathophysiology of Heme Synthesis

Beth A. BouchardBIOC 212: Biochemistry of Human Disease

Spring 2006

HEME-CONTAINING PROTEINS

Hemoglobin

Myoglobin

Cytochromes

Catalase

Some peroxidases

STRUCTURE OF HEME

An Overview of Iron MetabolismGut

Blood

Cells

• Low pH of stomach solubilizes Fe-containing ionic compounds.• Fe transporters facilitate absorption into blood stream

• Fe3+ ions are bound and chelated by Transferrin (Tf).• Transferrin transports Fe to tissues

•Maintains solubility•Keeps Fe ions unreactive

• Transferrin endocytosis is receptor-mediated (TfR)• Endocytosis results in Fe3+ release• Fe is distributed to topologically distinct regions of the cell via Fe transporter and/or channels (?)

• Usage: Protein components (Heme)• Storage: Ferritin (Fe2+)• Toxicity

Roles of Iron in the Cell

Transferrin Receptors (TfR)

Fe(III)2-Tf Tf

Proteins: Catalysis Electron, oxygen transport Structural stabilization Sensor of Fe, ROS Formation of protein-bound radicals

Storage and Sequestration: FerritinFerritin

Toxicity: Oxidative stress

[Fe]

[Fe]

[Fe]

Iron Control of Translation

• IREs are found in the 5’-UTR or the 3’-UTR of mRNAs

• Regulate mRNA translation via IRBP

• Decreased cellular iron levels: – IRBP is free of iron and can therefore,

interact with the IREs in the 3'-UTR of the transferrin receptor (TfR) mRNA, which prevents its degradation.

– IRBP binds to the IRE in the 5’-UTR of the ferritin mRNA preventing its translation.

• Increased cellular iron levels: – IRBP binds iron and cannot interact with

the IREs in the TfR mRNA resulting in an increase in its degradation.

– IRBP cannot bind to the IRE in the ferritin mRNA allowing for its translation.

IRE

STRUCTURE OF HEME

• Ferrous iron (Fe2+)

• Protoporphyrin IX: contains 4 pyrrole rings linked together by methenyl bridges

Heme

8

8

Succinyl CoA

Glycine**

HEME SYNTHESIS

** Amino acid (building blocks of protein) synthesized in your body

Heme synthesis

HEME SYNTHESIS

HEME SYNTHESIS: Red blood cells

•85% of total heme synthesis occurs in red blood cells (RBC)

•Ceases when RBC’s mature

•Erythroid-specific ALA synthase is regulated by an IRE in the mRNA – binding of IRBP inhibits mRNA translation

Heme stimulates hemoglobin synthesis in reticulocytes

HCI = heme controlledinhibitor

Reduced initiation of translation

***

In RBCS, heme synthesis is also regulated at the level of the en-zymes ferrochelatase* and porphobilinogen deaminase**

HEME SYNTHESIS: Liver

•The liver is the main non-RBC source of heme synthesis

•Heme produced in the liver is used mainly for the synthesis of the cytochrome P450 class of enzymes that are involved in detoxification

Regulated at level of ALA synthase: Formation of 5-ALA is the rate-limiting step in heme synthesis in the liver

Formation of 5-aminolevulinate (5-ALA)

5-ALA

5-ALA is formed in the mitochondria and transported to the cytoplasm

Regulation of ALA Synthase

Level of enzyme synthesis

Enzyme synthesis, as well as its transport to the mitochondria, is inhibited by elevated levels of

heme and hemin, the Fe3+ oxidation product of heme

Enzyme synthesis is upregulated by a large number of drugs including barbiturates, steroids with a 4,5 double bond (e.g. testosterone) and some oral contraceptives: These drugs are metabolized by the microsomal cytochrome P450 mono-oxygenase system, a heme- containing protein.

Level of enzyme activity

Heme and hemin inhibit ALA synthase activity

Requires pyridoxal phosphate (Vitamin B6) as a coenzyme

Disorders of Heme Synthesis

• Acquired: Lead poisoning

• Congenital: Porphyrias

• Deficiency of heme has far-reaching effects (hemoglobin, cytochromes, etc.)

LEAD TOXICITY

Symptoms Irritibility Poor appetite Lethargy Abdominal pain (with or Sleeplessness without vomiting) Headaches Constipation

Pathophysoiology Binds to any compound with a sulfhydryl group Inhibits multiple enzyme reactions including

those involved in heme biosynthesis (PBG synthase & ferrochelatase)

One symptom of lead toxicity is increases in 5-ALA without concomitant increases in PBG

HEME SYNTHESIS (CONT.)

lead

Vitamin B6

PORPHYRIAS

A group of rare disorders caused by deficiencies of enzymes of the heme biosynthetic pathway

The majority of the porphyrias are inherited in a autosomal dominant fashion - thus, affected individuals have 50% normal levels of the enzymes, and can still synthesize some heme

Affected individuals have an accumulation of heme precursors (porphyrins), which are toxic at high concentrations

Attacks of the disease are triggered by certain drugs, chemicals, and foods, and also by exposure to sun

Treatment involves administration of hemin, which provides negative feedback for the heme biosynthetic pathway, and therefore, prevents accumulation of heme precursors

Scriver et al., The Metabolic & Molecular Basis of Inherited Disease, 8th edition, 2001.

ACUTE INTERMITTENT PORPHYRIA

Hepatic, autosomal dominant

Caused by a deficiency in porphobilinogen deaminase, which is involved in the conversion of porphobilinogen (PBG) to uroporphyrinogen III

PBG, uroprophryin, and 5-ALA accumulate in the plasmaand the urine

Patients have neuropyschiatric symptoms and abdominal pain (neurovisceral)

PORPHYRIA CUTANEA TARDA

Most common porphyria

Hepatic, autosomal dominant

Disease is caused by a deficiency in uroporphyrinogen decarboxylase, which is involved in the conversion of uroporphyrinogen III to coproporphyrinogen III

Uroporphyrinogen accumulates in urine

Patients are photosensitive (cutaneous photosensitivity)Accumulation of porphyrinogens results in their conversion to porphyrins by light

Porphyrins react with molecular oxygen to form oxygen radicals

Oxygen radicals can cause severe damage to the skin