Porphyrin metabolism & porphyrias

HMIM224

What are porphyrins? • Porphyrins are cyclic compounds that bind metal ions (usually Fe2+ or

Fe3+) • Porphyrin + Metal = Metaloporphyrin• Most prevalent metalloporphyrin in humans is heme (metal here is iron ion)

Heme consists of:• One ferrous ion (Fe2+) in the centre• Protoporphyrin IX (a tetrapyrrole ring)

Heme is the prosthetic group of hemoglobin, myoglobin, cytochromes, catalase, tryptophan pyrrolase

So, heme + globin protein = hemoglobin

Structure of porphyrins

• Porphyrins are cyclic molecules formed by 4 pyrrole (tetrapyrrole) rings linked by methenyl bridges.

• Different porphyrins vary in the nature of side chains that are attached to each of the 4 pyrrole rings Protoporphyrin IX contains vinyl, methyl & propionate

Distribution of side chains - Side chains can be ordered around tetrapyrrole nucleus in 4 different ways designated I, II, III & IV series. Only type III porphyrins are physiologically important in humans - Protoporphyrin IX is a member of type III series

• Porphyrinogens are porphyrin precursors intermediate between porphobilinogen & protoporphyrin Porphobilinogen Porphyrinogens Protoporphyrins

Methenyl bridge

Pyrrole ringSide

Chains

Structure of Porphyrin

Determine the type of porphyrin

Derived from protoporphyrin IX

Binds metals: Heme- Fe2+ (ferrous)Hemin- Fe3+ (ferric)

Extended conjugation across ring systemZinc protoporphyrin (ZnPP)- Zn2+

Pattern of side chains defines isomer

Structure and Properties of Iron Protoporphyrin IX

propionate

methyl

vinyl

pyrrole ring

Biosynthesis of Heme

Uroporphyrinogen I Coproporphyrinogen I

Overview of Heme Synthesis

Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are

the liver and the erythroid progenitor cells of the bone marrow.

Succinyl CoA + Glycine

-aminolevulinic acid

-aminolevulinic acid

Porphobilinogen Uroporphyrinogen III Coproporphyrinogen III

Coproporphyrinogen III

Protoporphyrinogen IX

Protoporphyrin IX

Heme

ALA synthase

cytoplasm

mitochondrial matrix

Biosynthesis of heme

Site of biosynthesis: Liver & Bone Marrow (erythryoid producing cells)

Steps:

Step 1: Formation of -amino levulinic acid (ALA): in mitochondria

(Rate Controlling Step)

Glycine + Succinyl CoA (nonessential amino acid) (intermediate of citric acid cycle)

Enzyme: ALA Synthase Coenzyme: Pyridoxal Phosphate (PLP)

-Amino levulinic acid (ALA)

Clinical importance of first step:

When heme (end product) is produced in excessive amounts, heme is converted to hemin. Hemin decreases action of ALA synthase in liver. (end product inhibition).The reverse occurs when heme biosynthesis is reduced.

Drugs as grisofulvin (antifungal), hydantoin & phenobarbital (anticonvulsant) increase ALA synthase activity:as these drugs are metabolized by cytochrome p450 in liver resulting in more consumption of heme (component of cytochrome). Accordingly, heme concentration is reduced resulting in stimulation of action of ALA synthase.

Biosynthesis of heme (cont.)

ALA

Synthesis of ALAS 1

Decreased heme

Synthesis of CYP 450 is increased

Large Amount of Drug intake

Metabolised by CYP 450

Cytochrome P450 Monooxygenase System:

• Cytochrome P450s (CYPs) are actually a superfamily of related, heme-containing monooxygenase enzymes that participate in abroad variety of reactions.

This system performs different functions in two separate locations in cells.

• The over-all reaction catalyzed by a cytochrome P450 enzyme is:

R-H + O2+ NADPH + H+→R-OH + H2O NADP+

where R may be a steroid, drug, or other chemical.

• The name P450 reflects the absorbance at 450 nm by the protein.

Role of cytochrome P450 in detoxification of drugs & toxic compounds:

• It may itself activate or inactivate a drug

• It can make a toxic compound more soluble, thus facilitating its excretion in the urine or feces.

Biosynthesis of heme (cont.)

Step 2: Formation of porphobilinogen:

2 molecules of -Amino levulinic acid (ALA) condense to formporphobilinogen by the enzyme ALA dehydratase.

Clinical importance:

ALA dehydratase enzyme is inhibited by heavy metals as lead that results in anemia. (lead poisoning). In this case: ALA in blood is elevated (lab investigation)

Biosynthesis of heme (cont.)

First & second steps

of heme synthesis

Biosynthesis of heme (cont.)

Further steps: (in mitochondria)

Formation of protoporphyrin IX

Then, ferrous ions (Fe2+) are introduced into protoporphyrin IX, either:

simultaneously or: enhanced by ferrochelatase

Clinical importance: Ferrochelatase enzyme is inhibited by lead

Biosynthesis of heme (cont.)

Biosynthesis of heme (cont.)

Furthersteps of heme

synthesis

Glyine + Succinyl CoA

Enzyme: ALA Synthase STEP 1 PLP

-Amino levulinic acid (ALA)

Enzyme: ALA dehydratase. STEP 2

porphobilinogen

FURTHER STEPS

Protoporphyrin IX

Ferrous ion (Fe2+ ) introduction of iron Enzyme: ferrochelatase

heme

Summary of biosynthesis of heme

Uroporphyrinogen I Coproporphyrinogen I

Overview of Heme Synthesis

Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are

the liver and the erythroid progenitor cells of the bone marrow.

Succinyl CoA + Glycine

-aminolevulinic acid

-aminolevulinic acid

Porphobilinogen Uroporphyrinogen III Coproporphyrinogen III

Coproporphyrinogen III

Protoporphyrinogen IX

Protoporphyrin IX

Heme

ALA synthase

cytoplasm

mitochondrial matrix

uroporphyrinogen decarboxylase

Porphyrias

Porphyria are rare inherited defects in heme synthesis.

An inherited defect in an enzyme of heme synthesis results in accumulation of one or more of porphyrin precursors depending on location of block of the heme synthesis pathway.

These precursors increase in blood & appear in urine of patients.

Porphyria means purple colour caused by pigment-like porphyrins in urine of patients. (Diagnosed by lab investigation) Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoietic porphyria, which is recessive

Clinical manifestations of porphyrias:

Two types of porphyrias: erythropoietic (bone marrow) & hepatic Hepatic porphyrias are: acute & chronic porrphyrias

Generally, individuals with an enzyme defect prior to the synthesis of the tetrapyrroles manifest abdominal and neuropsychiatric signs

Those with 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, grow hair (hypertrichosis)

Porphyrias (cont.)

Porphyria Cutanea Tarda

• Chronic hepatic porphyria• 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

Porphyrias (cont.)

Acute Hepatic Porphyrias

e.g. Acute Intermittent Porphyria • 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.

Types of Porphyrias

Degradation of Heme

RBCs last 120 days then,

degraded by Reticulo-endothelial System (RES)

) in liver & spleen(

Hemoglobin (released from RBCs)

Heme + globin (reused)

Biliverdin

Bilirubin Liver

Bile

Intestine

feces

Degradation of Heme

Degradation of Heme