V.S.RAVIKIRAN, MSc.

STRUCTURE OF HEMOGLOBIN

V.S.RAVIKIRAN, MSc., Department of Biochemistry,

ASRAM Medical college, Eluru-534005.AP, India.vsravikiran2013@gmail.c

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STRUCTURE OF HEMOGLOBIN

STRUCTURE OF HEMOGLOBIN

• Normal level of Hemoglobin (Hb) in blood in males is 14–16 g/dL and in females, 13–15 g/dL. Hb is globular in shape.

• The adult Hb (HbA) has 2 alpha chains and 2 beta chains.

• Molecular weight of HbA is 67,000 Daltons (66,684 to be exact).

STRUCTURE OF HEMOGLOBIN

STRUCTURE OF HEMOGLOBIN

• Hb F (fetal Hb) is made up of 2 alpha and 2 gamma chains.

• Hb A2 has 2 alpha and 2 delta chains. • Normal adult blood contains 97% HbA, about

2% HbA2 and about 1% HbF.

STRUCTURE OF HEME

• Alpha chain gene is on chromosome 16 while the beta, gamma and delta chains are on chromosome 11.

• Each alpha chain has 141 amino acids. The beta, gamma and delta chains have 146 amino acids.

STRUCTURE OF HEME

• There are 36 histidine residues in Hb molecule; these are important in buffering action.

• The 58th residue in alpha chain is called distal histidine, because it is far away from the iron atom. The 87th residue in alpha chain is called proximal histidine, because it lies nearer to the iron atom

Attachment of Heme with Globin Chain

There are 4 heme residues per Hb molecule, one for each subunit in Hb.

The iron atom of heme occupies the central position of the porphyrin ring.

The reduced state is called ferrous (Fe++) and the oxidized state is ferric (Fe+++).

The ferrous iron has 6 valencies and ferric has 5 valencies.

In hemoglobin, iron remains in the ferrous state.

Linkage of heme with globin. Pink circle represents the globin chain. Blue rectangle represents the protoporphyrin ring.

STRUCTURE OF HEME

• Hemoglobin is a conjugated protein having heme as the prosthetic group and the protein, globin.

• It is a tetrameric protein with 4 subunits, each subunit having a prosthetic heme group and the globin polypeptide.

STRUCTURE OF HEME

• The polypeptide chains are usually two alpha and two beta chains.

• Hemoglobin has a molecular weight of about 67,000 Daltons.

• Each gram of Hb contains 3.4 mg of iron.

Heme is present in:

• a. Hemoglobin• b. Myoglobin• c. Cytochromes• d. Peroxidase• e. Catalase• f. Tryptophan pyrrolase• g. Nitric oxide synthase.

Heme

• Heme is produced by the combination of iron with a porphyrin ring.

• Chlorophyll, the photosynthetic green pigment in plants is magnesium-porphyrin complex.

Structure of Heme

• Heme is a derivative of the porphyrin. • Porphyrins are cyclic compounds formed by

fusion of 4 pyrrole rings linked by methenyl (=CH-) bridges

Pyrrole ring

Structure of Heme• Since an atom of iron is

present, heme is a Ferroprotoporphyrin.

• The pyrrole rings are named as I, II, III, IV and the bridges as alpha, beta, gamma and delta.

• The possible areas of substitution are denoted as 1 to 8.

Porphyrin ring

Structure of Heme• When the substituent

groups have a symmetrical arrangement (1, 3, 5, 7 and 2, 4, 6, 8) they are called the I series.

• The III series have an asymmetrical distribution of substituent groups (1, 3, 5, 8, and 2, 4, 6, 7).

Structure of Heme

• Type III is the most predominant in biological systems. (It is also called series 9,)

a. Propionyl (–CH2–CH2–COOH) group.b. Acetyl (–CH2–COOH) group.c. Methyl (–CH3) groupd. Vinyl (–CH=CH2) group.Complete structure of heme is shown in Figure.

Structure of heme

Structure of Heme

BIOSYNTHESIS OF HEME

• Heme can be synthesized by almost all the tissues in the body.

• Heme is synthesized in the normoblasts, but not in the matured erythrocytes.

• The pathway is partly cytoplasmic and partly mitochondrial.

Step 1: ALA Synthesis

Condensation of succinyl CoA and glycinein the presence of pyridoxal phosphate to form delta amino levulinic acid (ALA).

Step 2: Formation of PBG

ALA dehydratase

- lead+ Zn

In the cytoplasm, two molecules of ALA are condensed to form porphobilinogen (PBG).

(monopyrrole)

Step 3: Formation of UPG

Condensation of 4 molecules of the PBG, results in the formation of the first porphyrin of the pathway, namely uroporphyrinogen (UPG). The pyrrole rings are joined

together by methylene bridges (-CH2-).

PBG-deaminase

uroporphyrinogen III synthase

Step 3: Formation of UPG

Condensation occurs in a headto-tail manner, so that a linear tetrapyrrole is produced; this is named as hydroxy methyl bilane (HMB).

Step 4: Synthesis of CPG

uroporphyrinogen decarboxylase

The UPG-III is next converted to coproporphyrinogen (CPG-III) by decarboxylation. The acetate groups (CH2–COOH) are

decarboxylated to methyl (CH3) groups

Step 5: Synthesis of PPG• Further metabolism takes place in the mitochondria

Two propionic acid side chains are oxidatively decarboxylated to vinyl groups

coproporphyrinogen oxidase

Step 6: Generation of PPThe protoporphyrinogen-III is oxidized to protoporphyrin-III (PPIII).

The methylene bridges (–CH2) are oxidized to methenyl bridges (–CH=) and colored porphyrins are formed. Protoporphyrin-9 is

thus formed.

protoporphyrinogen oxidase

Step 7: Generation of Heme• The last step is the attachment of ferrous iron to the

protoporphyrin in mitochondria. Iron atom is coordinately linked with 5 nitrogen atoms (4 nitrogen of pyrrole rings of protoporphyrin and 1st nitrogen atom of a histidine residue of globin). The remaining valency of iron atom is satisfied with water or oxygen atom.

Heme synthase or ferrochelatase

(- lead)

iron atom is co- nately linked withnitrogen atoms

Summary of heme biosynthesis. The numbers denote the enzymes. Part of synthesis is in mitochondria, and the rest in

cytoplasm.

Regulation of Heme SynthesisALA synthase is regulated by repression

mechanism. Heme inhibits the synthesis of ALA synthase by acting as a co-repressor.

Repression by heme on the enzymes responsible for heme synthesis

Regulation of Heme Synthesis

ALA synthase is also allosterically inhibited by hematin.

The compartmentalization of the enzymes of heme synthesis. The rate-limiting enzyme is in the mitochondria.

Drugs like barbiturates induce heme synthesis and require the heme containing cytochrome P450 for their metabolism.

Regulation of Heme SynthesisThe steps catalyzed by ferrochelatase and ALAdehydratase

are inhibited by lead.

INH (Isonicotinic acid hydrazide) that decreases the availability of pyridoxal phosphate may also affect heme synthesis.

High cellular concentration of glucose prevents induction of ALA synthase. This is the basis of administration of glucose to relieve the acute attack of porphyrias.

Disorders of Heme Synthesis

• Porphyrias are a group of inborn errors of metabolism associated with the biosynthesis of heme. (Greek ‘porphyria’ means purple).

• These are characterized by increased production and excretion of porphyrins and or their precursors (ALA + PBG).

• Most of the porphyrias are inherited as autosomal dominant traits.

Porphyrias may be broadly grouped into 3 types:

• a. Hepatic porphyrias• b. Erythropoietic porphyrias• c. Porphyrias with both erythropoietic and

hepatic abnormalities.

This classification is based on the major site, where the enzyme deficiency is manifested. The clinical manifestations vary. Porphyrias in general, are not associated with anemia.

Enzyme deficiencies in

porphyrias

Features of important types of porphyrias

Porphyria cutanea tarda

Porphyria cutanea tarda. (A-B) Sun-exposed hands of a PCT patient showing areas of atrophy and scarring. (C) Urine from a symptomatic PCT patient and a healthy control in daylight (left) and under ultraviolet light (right). The PCT urine has an orange-red color in daylight that fluoresces red under ultraviolet light.

Urine may appear purple during an attack or after standing in the light

Congenital Erythropoietic Porphyria

When UV light is reflected on to teeth a red fluorescence is seen; this is called

erythrodontia.

Erythropoietic protoporphyria. (A) An EPP patient after sun exposure. Note the reddish and swollen appearance of her face and (B) scarring and thickening of the skin on the dorsum of her hand because of multiple sun/light exposures.

Repeated ulceration and scarring may cause mutilation of nose, ear and cartilage. This may mimic leprosy.

Acquired Porphyrias• A characteristic difference from congenital porphyrias is that

there is associated anemia in the acquired variety.

Acquired Porphyrias

Causes of acquired porphyrias (cont.)

Urinary excretion of porphyrias

THE END

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