1. Porphyrin metabolism & porphyrias
HMIM224

2. Objectives Understand the structure of metalloprphyrins
Apply that the role of heme is dictated by the environment created by the three dimensional structure relation to protein
Identify the rate limiting step and the iso-enzymes sites of production
Describe the site of effect of certain drugs on heme biosynthesis and its clinical importance
Identify how blocking in one of the enzyme involved in heme biosynthesis will affect the mode of presentation of the disease
Identify the most common type of porphyrias & its cause
Identify the heme degradation product

3. What are porphyrins ?
Porphyrins are cyclic compounds that bind metal ions (usually Fe2+ or Fe3+)
Porphyrin + Metal = Metalloporphyrin
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)
The role of heme is dictated by the enviroment created by the three dimensional structure of protein
Heme is the prosthetic group of hemoglobin, myoglobin, cytochromes,
catalase, tryptophan pyrrolase
So, heme + globin protein = hemoglobin

4. 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

5. Structure of Porphyrin
Pyrrole
ring
Methenyl
bridge
Side
Chains
Structure of Porphyrin
Determine the type of porphyrin

6. Structure and Properties of Iron Protoporphyrin IX
propionate
methyl
vinyl
pyrrole ring
Derived from protoporphyrin IX
Pattern of side chains defines isomer
Binds metals: Heme- Fe2+ (ferrous)
Hemin- Fe3+ (ferric)
Zinc protoporphyrin (ZnPP)- Zn2+
Extended conjugation across ring system

7. Overview of Heme Synthesis
Succinyl CoA + Glycine
Protoporphyrin IX
ALA synthase
Protoporphyrinogen IX
-aminolevulinic acid
Coproporphyrinogen III
mitochondrial matrix
cytoplasm
-aminolevulinic acid
Uroporphyrinogen III
Coproporphyrinogen III
Porphobilinogen
Uroporphyrinogen I
Coproporphyrinogen I
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.

8. First & second steps of heme synthesis
Biosynthesis of heme (cont.)
First & second steps of heme synthesis

9. Biosynthesis of heme Steps:
Site of biosynthesis: Liver & Bone Marrow (erythryoid producing cells)
There are 2 isozymes ALA S1 (liver) & ALAS2 (erythroid)
Steps:
Step 1: Formation of d-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)
d-Amino levulinic acid (ALA)

10. 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.
These modifications is two-fold:
First, it may itself activate or inactivate a drug or
Second, make a toxic compound more soluble, thus facilitating its excretion in the urine or feces.

11. Biosynthesis of heme (cont.)
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.

12. First & second steps of heme synthesis
Biosynthesis of heme (cont.)
First & second steps of heme synthesis

13. Biosynthesis of heme (cont.)
Step 2: Formation of porphobilinogen:
2 molecules of d-Amino levulinic acid (ALA) condense to form
porphobilinogen 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)

14. Biosynthesis of heme (cont.)
Further
steps of
heme synthesis

15. 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

16. Summary of biosynthesis of heme
Glycine Succinyl CoA
Enzyme: ALA Synthase STEP 1
PLP
d-Amino levulinic acid (ALA)
Enzyme: ALA dehydratase STEP 2
porphobilinogen
FURTHER STEPS
Protoporphyrin IX
Ferrous ion (Fe2+ ) introduction of iron
Enzyme: ferrochelatase
heme

17. 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

18. Porphyrias (cont.) 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)

19. 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

20. Porphyrias (cont.)

21. 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.

22. Types of Porphyrias

23. Degradation of Heme

25. CO has a vasodilator effect while bilirubin has an antioxidant effect
RBCs last 120 days, degraded by reticuloendothelial (RE) system [liver and spleen].
Most heme from RBCs (85%) - rest from turnover of cytochromes, p450s, immature erythrocytes.
Microsomal heme oxygenase hydroxylates methenyl bridge carbon and oxidizes Fe2+ to Fe3+. Second reaction open ring and release methenyl carbon as CO.
Serum albumin carries bilirubin in circulation, ligandin in hepatocytes.
CO has a vasodilator effect while bilirubin has an antioxidant effect
Bilirubin & its derivative is collectively known as bile
pigments

26. 26

27. Metabolised by CYP 450