15. hemoglobin

HEMOGLOBIN

HEMOGLOBIN

is a hemoprotein only found in the cytoplasm of erythrocytes (ery)

normal concentration of Hb in the blood: adult males 13.5 – 16.5 g/dL adult females 12 – 15 g/dL

HEMOGLOBIN

Blood can carry very little oxygen in solution.

Hemoglobin is required to carry oxygen around.

Hemoglobin is found in red blood cells

HEMOGLOBIN

In fact if the body had to depend upon dissolved oxygen in the plasma to supply oxygen to the cells –

The heart would have to pump 140 liters per minute - instead of 4 liters per minute.

HEMOGLOBIN Each red cell has 640

million molecules of Hb

Hemoglobin is 97% saturated when it leaves the lungs

Under resting conditions is it about 75% saturated when it returns.

HEMOGLOBIN

Hemoglobin is made from two similar proteins that "stick together".

Both proteins must be present for the hemoglobin to pick up and release oxygen normally.

One of the component proteins is called alpha, the other is beta.

HEMOGLOBIN Blood cells are made up of two components.

The hemoglobin is in solution inside the cell.

The cell is surrounded by a membrane that holds in the hemoglobin.

STRUCTURE OF HEMOGLOBIN

• Hb is a spherical molecule consisting of 4 peptide subunits (globins) = quartenary structure

• Hb of adults (Hb A) is a tetramer consisting of 2 - and 2 β-globins → each globin contains 1 heme group with a central Fe2+ ion (ferrous ion)

HEME STRUCTUREHEME IS A METALOPORPHYRINE(CYCLIC TETRAPYRROLE)

Heme contains: conjugated system of

double bonds → red colour

4 nitrogen (N) atoms

1 iron cation (Fe2+)→ bound in the middle of tetrapyrrole skelet by coordination covalent bonds

methine bridge pyrrole ring

STRUCTURE OF HAEM

PROPERTIES OF IRON IN HEME

6 bonds:• 4x pyrrole ring

(A,B,C,D)• 1x link to a protein• 1x link to an oxygen

IRON AND HEMOGLOBIN

The mineral, iron, plays an important role in the body’s delivery and use of oxygen to and by working muscles.

It binds oxygen to hemoglobin, which then travels in the bloodstream to locations throughout the body.

IRON AND HEMOGLOBIN Generally, the more

oxygen there is being delivered, the greater the body’s ability to perform work.

For this reason, iron receives much attention for its role in supporting aerobic exercise, and it has been postulated that a lack of iron in the body can reduce aerobic capacity and impair endurance performance.

IRON AND HEMOGLOBIN

Iron deficient red blood cells

Low number or cells Note the hollow and

blanched appearance of the red blood cells.

IN WHICH COMPOUNDS CAN WE FIND A HEME GROUP?

Hemoproteins• Hemoglobin (Hb)• Myoglobin (Mb)• Cytochromes • Catalases

(decomposition of 2 H2O2

to 2 H2O and O2)

• Peroxidases

MYOGLOBIN (MB)

• is a single-chain globular protein of 153 AA, containing 1 heme group

• transports O2 in skeletal and heart muscle

• is found in cytosol within cells

• is a marker of myocard damage

FUNCTION OF HEMOGLOBIN

• Hb is a buffer (Hb/Hb-H+) in the erythrocytes• Hb is a carrier of O2 and CO2

Binding of O2 is a cooperative. Hb binds O2 weakly at low oxygen pressures and tightly at high pressures. The binding of the first O2 to Hb enhances the binding futher O2 molecules → allosteric effect → S-shaped (sigmoidal) saturation curve of Hb

PROCESS OF O2 BINDING TO HB

Hb can exist in 2 different forms: T-form and R-form.T-form (T = „tense“) has a much lower oxygen affinity than the R-form. The subunits of Hb are held together by electrostatic interactions. The binding of the first O2 molecule to subunit of the T-form leads to a local conformational change that weakens the association between the subunits → R-form („relaxed“) of Hb.

Increasing of oxygen partial pressure causes the conversion of T-form to R-form. T R Hb + ↑pO2 HbO2


Hemoglobin is a remarkable molecular machine that uses motion and small structural changes to regulate its action.

Oxygen binding at the four heme sites in hemoglobin does not happen simultaneously.

Once the first heme binds oxygen, it introduces small changes in the structure of the corresponding protein chain.


These changes nudge the neighboring chains into a different shape, making them bind oxygen more easily.

Thus, it is difficult to add the first oxygen molecule, but binding the second, third and fourth oxygen molecules gets progressively easier and easier.

This provides a great advantage in hemoglobin function.


When blood is in the lungs, where oxygen is plentiful, oxygen easily binds to the first subunit and then quickly fills up the remaining ones.

Then, as blood circulates through the body, the oxygen level drops while that of carbon dioxide increases.


In this environment, hemoglobin releases its bound oxygen. As soon as the first oxygen molecule drops off, the protein starts changing its shape.

This prompts the remaining three oxygens to be quickly released.

In this way, hemoglobin picks up the largest possible load of oxygen in the lungs, and delivers all of it where and when needed.

HEMOGLOBIN

The heme group of one subunit, shown in the little circular window, is kept in one place so that you can see how the protein moves around it when oxygen binds.

As it binds to the iron atom in the center of the heme, it pulls a histidine amino acid upwards on the bottom side of the heme.

This shifts the position of an entire alpha helix, This motion is propagated throughout the protein chain and on to the other chains, ultimately causing the large rocking motion of the two subunits

AGENTS THAT INFLUENCE OXYGEN BINDING ● 2,3-bisphosphoglycerate (2,3-BPG) only binds to

deoxyHb (β-chains) → deoxyHb is thus stabilized ● H+ ions (lower pH) – binding of H+ by Hb lowers its

affinity for O2 → Bohr effect ● CO2 – high CO2 levels in the plasma also result in a right

shift of saturation curve = Bohr effect

OXY & DEOXYHAEMOGLOBIN

HB-OXYGEN DISSOCIATION CURVE


The normal position of curve depends on

Concentration of 2,3-DPG H+ ion concentration (pH) CO2 in red blood cells Structure of Hb


Right shift (easy oxygen delivery)

High 2,3-DPG High H+

High CO2

HbS

Left shift (give up oxygen less readily) Low 2,3-DPG HbF

Hb AHb A Hb AHb A22 Hb FHb F

structurestructure aa2222 aa22dd22 aa2222

Normal %Normal % 96-98 %96-98 % 1.5-3.2 %1.5-3.2 % 0.5-0.8 %0.5-0.8 %

ADULT HAEMOBLOBIN

TYPES OF HEMOGLOBIN Adult Hb (Hb A) = 2 α and 2 β subunitsHbA1 is the major form of Hb in adults and in children over 7 months.HbA2 (2 α, 2 δ) is a minor form of Hb in adults. It forms only 2 – 3% of a total Hb A.

Fetal Hb (Hb F) = 2 α and 2 γ subunits- in fetus and newborn infants Hb F binds O2 at lower tension than Hb A → Hb F has a higher affinity to O2

After birth, Hb F is replaced by Hb A during the first few months of life.

Hb S – in β-globin chain Glu is replaced by Val = an abnormal Hb typical for sickle cell anemia

DERIVATIVES OF HEMOGLOBIN Oxyhemoglobin (oxyHb) = Hb with O2

Deoxyhemoglobin (deoxyHb) = Hb without O2

Methemoglobin (metHb) contains Fe3+ instead of Fe2+ in heme groups

Carbonylhemoglobin (HbCO) – CO binds to Fe2+ in heme in case of CO poisoning or smoking. CO has 200x higher affinity to Fe2+ than O2.

Carbaminohemoglobin (HbCO2) - CO2 is non-covalently bound to globin chain of Hb. HbCO2 transports CO2 in blood (about 23%).

Glycohemoglobin (HbA1c) is formed spontaneously by nonenzymatic reaction with Glc. People with DM have more HbA1c than normal (› 7%). Measurement of blood HbA1c is useful to get info about long-term control of glycemia.

Mutations in hemoglobin (hemoglobinopathies:

1- Sickle cell anemia (Hb S disease):It is a genetic disorder of blood caused by mutation in β-globin chain resulting in the formation of Hb S. The mutation occurs in 6th position of β-chain where glutamic acid is replaced by valine (non polar). Valine residues aggregate together by hydrophobic interactions leading to precipitation of Hb within RBCs. RBCs assume sickle-shaped leading to fragility of their walls and high rate of hemolysis.

Such sickled cells frequently block flow of blood in narrow capillaries and block blood supply to tissue (tissue anoxia) causing pain and cell death.Note: The lifetime of erythrocyte in sickle cell is less than 20 days, compared to 120 days for normal RBCs.Patients may be : - Heterozygotes (Hb AS): mutation occurs only in one β-globin chain. These patients have sickle cell trait with no clinical symptoms and can have normal life span.Or: Homozygotes (Hb SS): mutation occurs in both β-globin chain with apparent anemia and its symptoms

2- Hb C disease: Like HbS, Hb C is a mutant Hb in which glutamic acid in 6th position of β-chain is replaced by lysine. RBCs will be large oblong and hexagonal.The heterozygous form (HbAC) is asymptomatic.The homozygous form (Hb CC) causes anemia, tissue anoxia and severe pain.

3- Thalassemia: A group of genetic diseases in which a defect occur in the rate of synthesis of one or more of Hb chains, but the chains are structurally normal. This due to defect or absence of one or more of genes responsible for synthesis of α or β chains leading to premature death of RBCs.

Types: β -thalassemia: When synthesis of β chains is decreased or absent. There are two copies of the gene responsible for synthesis of β chains. Individuals with β globin gene defects have either :-β -thalassemia minor (β –thalassemia trait) : when the synthesis of only one β –globin gene is defective or absent. Those individuals make some β chains and usually not need specific treatment.

-β -thalassemia major ( Cooley anemia): if both genes are defective.-Babies will be severely anemic during the first or second year of life and so require regular blood transfusion. Bone marrow replacement is more safe treatment.

-α-thalassemia: in which synthesis of α globin chain is defective or absent. There are four copies of gene responsible for synthesis of α globin chains so patients may have:i - Silent carrier of α-thalassemia with no symptoms: if one gene is defective

ii- α-thalassemia trait: if two genes are defective. Minor anemia present

iii- Hb H disease - if three genes are defective. Moderate anemia present.

iv. Hydrops fetalis – Lacks all 4 genes. Fetus may survive till birth then dies.

15. hemoglobin

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