HEMOPROTEINS 1

# SPECIAL TOPICS IN BIOCHEMISTRY BIC 351: HEMOPROTEINS

This study guide covers key concepts related to hemoproteins, heme structure, synthesis, disorders, and their functions.

HEMOPROTEINS

A hemoprotein is defined as a complex consisting of:

• Heme: The iron-containing structure that facilitates its biological functions.

• Protein: The polypeptide component that interacts with heme.

Examples of Hemoproteins

• Hemoglobin

• Myoglobin

• Cytochromes

• Catalase

• Peroxidases

HEME

Heme is recognized as the most significant porphyrin-containing compound, classified as a derivative of porphyrin.

Porphyrins

Porphyrins are cyclic compounds characterized by the fusion of four pyrrole rings linked by methenyl (=CH-) bridges.

Metal Ion Interaction

Metal ions can coordinate with nitrogen atoms from the pyrrole rings, facilitating the formation of complexes. Heme, being a ferroprotoporphyrin, contains an iron atom integrated within its structure.

Pyrrole Ring Structure

Pyrrole rings are designated as I, II, III, and IV, with their connecting bridges referred to as alpha, beta, gamma, and delta respectively.

STRUCTURE OF HEME

The structure of heme involves various substituent groups:

• M: Methyl

• V: Vinyl

• P: Propionyl

Natural Occurrence of Porphyrins

Natural porphyrins have substituent groups replacing the eight hydrogen atoms on the porphyrin nucleus.

• Type I (Symmetric): Symmetrically arranged substituents (positions 1, 3, 5, 7 and 2, 4, 6, 8).

• Type II (Asymmetric): Asymmetrical substituents arrangement (positions 1, 3, 5, 8 and 2, 4, 6, 7).

Examples of Porphyrins

• Uroporphyrin I and III: Found in urine and body fluids.

• Coproporphyrin I and III: Initially isolated from feces but also present in urine.

IMPORTANCE OF TYPE III PORPHYRINS

Type III Porphyrin is prominent in biological systems and is labeled as series 9, a designation made by Hans Fischer, the notable figure in porphyrin chemistry.

Hans Fischer's Contribution

In 1920, Hans Fischer successfully synthesized heme in a laboratory setting, which earned him the Nobel Prize in 1930.

BIOSYNTHESIS OF HEME

Heme biosynthesis occurs in various tissues, primarily within the liver and erythrocytic cells of the bone marrow, particularly normoblasts (though not in mature erythrocytes).

Pathway Components

Heme synthesis is outlined in specific pathways:

1. Step 1: ALA Synthesis

• Initiated by the condensation of succinyl CoA and glycine in the presence of pyridoxal phosphate to create delta amino levulinic acid (ALA).

• The enzyme responsible is ALA synthase, located in the mitochondria and serves as a rate-limiting enzyme.

Reaction for ALA Synthesis

$ext{Succinyl CoA} + ext{Glycine} <br>ightarrow ext{Delta ALA}$

Reaction Steps in the Cytosol

2. Step 2: Formation of PBG

• Two ALA molecules condense to form porphobilinogen (PBG) by removal of water, facilitated by the zinc-containing enzyme ALA dehydratase.

• Notably, ALA dehydratase is inhibited by lead.

2. Step 3: UPG Formation

• Condensation of four molecules of PBG results in the first porphyrin, uroporphyrinogen (UPG).

• The pyrrole rings in UPG are linked by methenyl bridges derived from glycine's alpha carbon.

• The predominance of series III isomers occurs during fusion, with the necessary enzymes being PBG-deaminase and Uroprophyrinogen-III-co-synthase.

2. Step 4: Coproporphyrinogen Synthesis

• UPG-III is converted to coproporphyrinogen (CPG-III) by the elimination of four CO2 molecules, with decarboxylation yielding methyl groups from acetate groups.

2. Step 5: Protoporphyrinogen Synthesis

• Metabolism in mitochondria further oxidizes CPG to protoporphyrinogen (PPG-III).

2. Step 6: Generation of Protoporphyrin

• This step involves oxidation of protoporphyrinogen-III by protoporphyrin-III oxidase, where methylene bridges are converted into methenyl bridges, resulting in colored porphyrins.

2. Step 7: Heme Generation

• The final stage comprises attaching ferrous iron to protoporphyrin facilitated by the enzyme heme synthase (ferrochelatase).

Iron and Hematin

1. The oxidative form of iron, ferric (Fe+++), leads to the formation of hematin, which cannot carry oxygen.

2. Heme appears red while hematin exhibits a dark brown coloration.

REGULATION OF HEME SYNTHESIS

Heme synthesis is regulated mainly through the enzyme ALA synthase, which is repressed by heme as a co-repressor and is also allosterically inhibited by hematin.

Compartmentalization of Enzymes

The regulation aspect is facilitated by enzyme compartmentalization, with mitochondrial reactions including Steps 1, 5, 6, and 7 and cytosolic reactions involving Steps 2, 3, and 4.

Drug Interaction

Barbiturates can induce heme synthesis, necessitating cytochrome P450 for drug metabolism. Approximately two-thirds of synthesized heme is allocated for cytochrome P450 production.

Inhibition by Lead

The actions of ferrochelatase and ALA dehydratase are notably inhibited by lead exposure.

DISORDERS OF HEME SYNTHESIS

Porphyrias represent inherited metabolic errors that disrupt heme biosynthesis, characterized by excess porphyrins and/or their precursors (ALA and PBG).

Classification of Porphyrias

Porphyrias are grouped into:

1. Hepatic Porphyrias

2. Erythropoietic Porphyrias

3. Mixed Porphyrias

Specific Disorders

• Acute Intermittent Porphyria: Caused by uroporphyrinogen I synthase deficiency, resulting in increased excretion of porphobilinogen and 8-aminolevulinate; not photosensitive.

• Congenital Erythropoietic Porphyria (CEP): Known as Günther disease, an autosomal recessive disorder with severe photosensitivity and hematologic anomalies; deficient in uroporphyrinogen III co-synthase.

• Porphyria Cutanea Tarda: Chronic condition linked to uroporphyrinogen decarboxylase deficiency, commonly associated with liver damage or alcohol; presents as cutaneous photosensitivity.

Management Techniques

• For Acute Intermittent Porphyria, hematin administration is the main treatment to inhibit ALA synthase.

• CEP management involved light protection, blood transfusions, and potential gene therapy interventions.

Hemoglobin Synthesis

Hemoglobin's components integrate globin protein chains, four protoporphyrin IX molecules, and four ferric iron atoms, with 2,3-diphosphoglycerate sometimes present in the center of the hemoglobin unit.

Composition of Globins

Globin chains consist of various amino acid sequences, designated as follows:

• A (Alpha): 141 Amino Acids

• B (Beta): 146 Amino Acids

• Y (Gamma): 146 Amino Acids

• D (Delta): 146 Amino Acids

HEMOGLOBINOPATHIES

Hemoglobinopathies are inherited disorders arising from mutations affecting hemoglobin globin chains, observable in approximately 7% of the global population, including conditions like sickle cell disease and thalassemia.

Sickle Cell Disease

Characterized by quantitative abnormalities in globin chain production leading to compromised hemoglobin functionality. Sickle cell trait is heterozygous whereas sickle cell disease is homozygous.

Thalassemia Types

• Alpha-Thalassemia: Characterized by the deletion of alpha-globin genes, varying in severity based on the number of genes missing.

• Beta-Thalassemia: Significant clinical consequences may show only after 3-6 months post-birth, defined by the absence of beta-globin chains.

Anemia Factors

Conditions causing reduced iron availability will lead to anemia, a prevalent cause of hemoglobin disorders, and iron metabolism disturbances merit close attention throughout diagnosis and treatment.

DIAGNOSIS OF PORPHYRIAS

Diagnosis involves the use of UV fluorescence to detect porphyrins, while urine screening for porphyrin precursors is executed using Ehrlich's reagent. Porphyrins emit strong red fluorescence under UV light.

FUNCTIONS OF HEME

Heme plays multifaceted roles as a cofactor in biological processes, particularly in:

• Oxygen transport

• Oxygen storage

• Electron transfer and cellular respiration

• Signal transduction

Heme is crucial in the functioning of hemoglobin and myoglobin, contributing to oxygen distribution within the body.