hemoglobin module 6

8/9/2019 Hemoglobin Module 6

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Hemoglobin


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Hemoglobin

Human hemoglobin A, present in adults, consists of four

subunits:

two -subunits and two -subunits.

The - and -subunits are homologous and have similar

three-dimensional structures.

The capacity of hemoglobin to bind oxygen depends on

the presence of a bound prosthetic group (heme).

The heme group is responsible for the distinctive red

color of blood.


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The heme group consists of a protoporphyrin,

and a central iron atom.

Protoporphyrin is made up of four pyrrole rings

linked by methene bridges to form a tetrapyrrole

ring.

Four methyl groups, two vinyl groups, and two

propionate side chains are attached.


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GLOBIN(protein part or apoprotein):

It is a simple protein (histone) which is characterized by

its high content of histidine and lysine.

It is composed of four polypeptide chains 2 and 2

chains.

The -chain contains 141 amino acids and -chain

contains 146 amino acids.

Each -polypeptide chain is folded into 8 right handed -

helices termed A-H starting from NH2-terminal,

-subunit is folded into 7-helices.

The ratio of haem to globin is 4:1. So each haem moietyis linked to one peptide chain.


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The myoglobin-hemoglobin family of

proteins has produced a way in which Fe++

can be bounded to the proteins so as to

produce an O2 binding site.


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deoxyHbdeoxyMb

Myoglobin and Hemoglobin Structure

F

F

E

E

oxyMb (MbO2)

O2


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The iron atom lies in the center of the protoporphyrin,

bonded to the four pyrrole nitrogen atoms.

Under normal conditions, the iron is in the ferrous (Fe2+)

oxidation state.

The iron ion can form two additional bonds, one on each

side of the heme plane. These binding sites are called

the fifth and sixth coordination sites.

In hemoglobin, the fifth coordination site is occupied by

the imidazole ring of a histidine residue from the protein.

In deoxyhemoglobin, the sixth coordination site remains

unoccupied.


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Types of normal haemoglobin

I. Adult haemoglobin: There are 2 types HbA1 and HbA2.

a) Majoradult haemoglobin: Hb A1 (2 2)

- Contains 2 alpha chains and 2 beta chains.

This haemoglobin A1 constitutes 95-97% of the totalhaemoglobin.

b) Minoradult haemoglobin: Hb A2 (2 2)

- HbA2 forms about 2-4% of total haemoglobin.

- Contains 2 -chains and 2 -chains.

- In the -chains there is more than one aminoacid

different than those in -chain e.g. arginine residue at

the position 16 instead of glycine which is normally

present in beta chain


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II. Glycosylated haemoglobin (Hb A1c):

- It is modified form of haemoglobin similar to

haemoglobin A1 but it contains glucose linked to aminogroup present on lysyl residues and at the NH2 - terminal

ends. The reaction is non enzymatic and its rate

depends on the concentration of glucose .It is present in

normal value 5% of the total haemoglobin.

- This percentage is increased in prediabetic and diabetic

patients up to 8-14%. This glycohaemoglobin gives an

idea about the blood glucose level during the last three

months and is useful in the assessment of diabetic

control


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III. Fetal haemoglobin = HbF (2 2):

It is present normally in newborn and early fetal life and

at age of 7 months 90 % of fetal haemoglobin isreplaced by adult haemoglobin ( HbA1)

- It consists of 2 alpha chains and 2 gamma chains.

- In gamma chain there is more than one aminoacid

different from those in -chain e.g. His21 residue isser21

- HbF has a great affinity for O2 under physiological

conditions, because -chains do not bind 2,3 BPG well.

BPG is responsible for lowering the O2 affinity of Hb andallowing Hb to release O2 at the typical PO2 of tissues.


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Hemoglobin Genes and Gene Products


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Haem biosynthesis

Haem is the iron protoporphyrin, synthesized mostly in

the bone marrow( 85%) for incorporation into

haemoglobin and in liverfor synthesis ofcytochromes.

The initial and last three enzymatic steps are catalyzedby enzymes that are present in mitochondria whereas

the intermediate steps are taking place in cytoplasm.


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Heme synthesis begins with condensation of glycine &

succinyl-CoA, with decarboxylation, to form H-

aminolevulinic acid (ALA). Pyridoxal phosphate (PLP)

serves as coenzyme forH-Aminolevulinate Synthase (ALA

Synthase

ALA synthase

OOC CH2 CH2 C S-CoA

O

+ OOC CH2 NH3+

OOC CH2 CH2 C

O

CH2 NH3+

CO2CoA-SH

H+succinyl-CoA glycine

H-aminolevulinate (ALA)

H-AminolevulinicAcid Synthase

1

1


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2 ALA dehydratase

3

4

5

6

Uroporphyrinogen-I

Synthase + III cosynthase

Uroporphyrinogen-IIIdecarboxylase

Coproporphyrinogen-III

oxidase

Protooporphyrinogen

-III oxidase


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Heme synthase7


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Summary of heme synthesis


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Regulation of Haem Biosynthesis

Regulation of transcription or post-translationalprocessing ofenzymes of the heme synthesispathways differs between erythrocyte forming cells &other tissues.

In erythrocyte-forming cells there is steadyproduction of pathway enzymes, limited only by ironavailability.

In other tissues expression of pathway enzymes is

more variable & subject to feedback inhibition byheme.


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Allosteric Regulation

ALA synthase enzyme controls the rate-limiting stepof Haem synthesis.

Haem and also haematin act as a repressor of thesynthesis of ALA synthase and act as feed backinhibitor at this step.

The block in haem biosynthesis in pantothenic acidor vitamin B6 deficiency occurs at very early step inhaem synthesis (ALA synthase).

ALA dehydratase is sulfhydryl enzyme and is very

sensitive to inhibition by heavy metals as mercuryor lead.


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Porphyrias

Porphyrias are genetic diseases in which activity of one

of the enzymes involved in heme synthesis is decreased

(e.g., PBG Synthase, Porphobilinogen Deaminase, etc).

Symptoms vary depending on the enzyme

the severity of the deficiency

whether heme synthesis is affected primarily in liveror in developing erythrocytes


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Sickle Cell Disease

Hemoglobin S (HbS) 50% Hb present.

Homozygotic HbSS (sickle cell anemia) - HbS = 100% Hb present,Giving Sickle cell disease

HbSA disease - Double heterozygote for HbS and HbA, with

intermediate clinical severity. It is called Sickle cell trait


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Basic abnormality - glutamic acid is replaced by

valine at the sixth position of the F-globin chain.

2 normal E-globin and 2 abnormal F-globin

chains forms HbS.

HbS carries O2 normally but begins to form

semisolid aggregate structures once O2 is

unloaded to the tissues. These HbS aggregates

distort RBCs and cause them to lose their

normal elasticity.

Molecularand cellularchanges of

hemoglobin S


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Thalassaemias:

The name is derived from the Greek word

thalassa,which means sea.Greek identified this

disease present around Mediterranean sea.

They are hereditary hemolytic diseases in whichthe synthesis of either - or - globin chain is

defective.

This decreased rate of synthesis of the globin

chains is due to mutation affecting the regulatory

gene rather than the structural gene.


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A) - thalassaemias: there are decreased or

absent synthesis of -chains of haemoglobin

with compensatory increase in the synthesis ofother chains.

a.Homozygous -chain thalassaemia

(thalassaemia major):

Incompatible with life, and present as hydrops

foetalis usually die in utero , due to complete

absence of -chains which are required for

synthesis of HbF.

b.Heterozygous -chain thalassaemia

thalassaemia minor,(trait):


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B) - thalassaemias: Synthesis of of -chains is

decreased or absent whereas synthesis of -chains

is normal and will combine with -chains givingexcess of HbA2 (22) or it may combine with -

chains producing excess of HbF (22).

The abnormal haemoglobin do not function as normal

haemoglobin

- Homozygous (Thalassaemia-major = Cooley's

anaemia = Mediterranian sea anaemia):

There is complete absence of -globin chain andthere is marked increase of HbF.

- Heterozygous thalassaemia (Thalassaemia-

minor):

There is slow rate of synthesis of -globin chain.


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Heterozygous thalassaemia (Thalassaemia-

minor):

There is slow rate of synthesis of -globin chain.


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Methemoglobinemias:

This is oxidized Hb,

The Fe2+ normally present in heme being

replaced by Fe3+,the ability to react as an O2

carrier is lost. The normal erythrocyte contains small amount of

met Hb, formed by spontaneous oxidation of Hb.

Met Hb is normally reconverted to Hb by

reducing systems in the RBC, the mostimportant of which is NADH-methemoglobin

reductase.


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Congenital methemoglobinemias

A. Hemoglobin M (Hb-M):

It is a congenital condition due to mutation inglobin biosynthesis in which distal or proximal

histidine is replaced by tyrosine.

B. Deficiency of NADH cytochrome b5methemoglobin reductase system

Acquired (toxic) methemoglobinemea

Usually arises following the ingestion of large

amounts of drugs e.g. phenacetin or thesulphonamides, excess ofnitrites or certain

oxidizing agents present in the diet.


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Glucose Metabolism in RBCs

1- Glycolysis2- Hexose mono-phosphate shunt

(HMPS)


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Glycolysis in RBCs

Glucose

2 NAD

2 NADH+H

2 ATP

2 Lactate

Glucose

1,3bisphosphoglycerate

2,3bisphosphoglycerate


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Importance of glycolysis in red cells:

a) Energy production: it is the only pathway thatsupplies the red cells with ATP.

haemolytic anaemia may occur due to an inherited

deficiency of glycolytic enzymes mainly

pyruvate kinase deficiency.b) Reduction of methaemoglobin: glycolysis

provides NADH for reduction of met Hb by

NADH-Cyto.b5 reductase

c) In red cells 1,3 bisphosphoglycerate is

converted to 2,3 bisphosphoglycerate which

binds to oxy Hb and helps release of O2 to

tissues.


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Importance of HMPS in Red cells:

Red cells are liable for oxidative damage byH2O2 due to their role in O2 transport.

In RBCs, H2O2 can cause both oxidation of iron

in haemoglobin (to form methaemoglobin) andlipid peroxidation (increases the cell membrane

fragility).

The major role of HMS in red cells is the

production of NADPH, which protect these cellsfrom oxidative damage by reduction of

glutathionethat helps removal of H2O2.


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Non- Oxidative PhaseOxidative Phase

Reversible

Non-Regulatory

Irreversible

Regulatory

6 moles ofPentose-P

5 moles of G-6-P

6 moles of G-6-P

6 moles ofPentose-P

12 NADP

12 NADPH +12 H

6 H2O

6 CO2


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Role of NADPH+H in reduction of glutathione

G-S-S-G 2-GSH

2 GSH

+H2O2

G-S-S-G

+ 2 H2O

Glutathione

Reductase

Glutathioneperoxidase

NADPH+H NADP

hemoglobin module 6

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