Biochemistry of blood. Respiratory function of erythrocytes. Pathobiochemistry of blood.

Blood performs three major functions: transport through the body of

oxygen and carbon dioxide food molecules (glucose, lipids, amino acids) ions (e.g., Na+, Ca2+, HCO3−) wastes (e.g., urea) hormones

defense of the body against infections and other foreign materials. All the WBCs participate in these defenses.

Homeostatic functions- heat- water- salt balance- Acid – base balance

Red Blood Cells (erythrocytes)

The most numerous type in the blood.

•Features:•The erythrocytes doesn’t contain nucleus, chromatine•The erythrocytes doesn’t contain mytochondrias, thus АТP producing due to the anaerobic glycolisis till to the lactate (90%).•The glycolisis has features. During it the 2,3 BPG will be produced, not 1,3 BPG. This compound need for joining О2 to hemoglobin: low concentration of 2,3 BPG will increase the affinity hemoglobin (Нв) to О2.• The PPP is the main path for producing of reductive equivalents NADPН2 for taking part in glycolisis

Red blood cells are responsible for the

transport of oxygen and carbon dioxide. In adult humans the

hemoglobin (Hb) molecule consists of four polypeptides: two alpha (α) chains of

141 amino acids and two beta (β) chains of 146

amino acids Each of these is attached the

prosthetic group heme. There is one atom of iron at

the center of each heme. One molecule of oxygen can

bind to each heme. The reaction is reversible.

The iron atom may either be in the Fe2+ or Fe3+ state, but ferrihemoglobin (methemoglobin) (Fe3+) cannot bind oxygen. In binding, oxygen temporarily oxidizes Fe to (Fe3+), so iron must exist in the +2 oxidation state in order to bind oxygen. The body reactivates hemoglobin found in the inactive (Fe3+) state by reducing the iron center.

Carbon Dioxide Transport Carbon dioxide (CO2) combines with water forming

carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ions:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3− 95% of the CO2 generated in the tissues is carried in

the red blood cells: It probably enters (and leaves) the cell by diffusing

through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is the D antigen that is the most important factor in the Rh system of blood groups.)

Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen).

The rest is converted — following the equation above — by the enzyme carbonic anhydrase into bicarbonate ions that diffuse back out into the plasma

and hydrogen ions (H+) that bind to the protein portion of

the hemoglobin (thus having no effect on pH).

Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma.

When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.

H+, CO2, BPG

Hb Equilibrium

b

b a

a

R (high affinity)

O2

T(low affinity)

The ability of hemoglobin to release oxygen, is affected by pH, CO2 and by the differences in the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in the tissues is considerably lower (more acidic) than in the lungs. Protons are generated from the reaction between carbon dioxide and water to form bicarbonate: CO2 + H20 -----------------> HCO3- + H+This increased acidity serves a two fold purpose. - First, protons are lower the affinity of hemoglobin for oxygen,

allowing easier release into the tissues. As all four oxygens are released, hemoglobin binds to two protons. This helps to maintain equilibrium towards the right side of the equation. This is known as the Bohr effect, and is vital in the removal of carbon dioxide as waste because CO2 is insoluble in the bloodstream. The bicarbonate ion is much more soluble, and can thereby be transported back to the lungs after being bound to hemoglobin.

- If hemoglobin couldn’t absorb the excess protons, the equilibrium would shift to the left, and carbon dioxide couldn’t be removed

In the lungs, this effect works in the reverse direction. In the presence of the high oxygen concentration in the lungs, lead to the proton affinity decreasing. As protons are shed, the reaction is driven to the left, and CO2 forms as an insoluble gas to be expelled from the lungs. The proton poor hemoglobin now has a greater affinity for oxygen, and the cycle continues.

 

Bohr Effect (pH)

100 mm O2

20 mm O2

CO2 effect

20 mm CO2

80 mm CO2

Effect of BPGBPGEffect

pO2 (mm Hg)

0 20 40 60 80 100 120 140 160

0

20

40

60

80

100

pO2 vs p50=8pO2 vs p50=26

Hb alone

Hb + BPG

BPG is the main player in Hb cooperativity.

High altitude increases BPG, pushing curve further to right

Cooperativity

Oxygen binding to one subunit of Hb, increases the affinity of the other subunits for additional oxygens. In other words, the first one is the hardest, the rest are easy.

Anaemia Anaemia is a shortage of RBCs and/or the

amount of haemoglobin in them.

Myoglobin and Hemoglobin

Mb is monomer, Hb is a tetramer (ex. a2b2). Hb subunits are structurally similar to Mb, with 8

a-helical regions, no b-strands and no interior water.

Both contain one heme prosthetic group per chain.

Both Mb and Hb contain proximal and distal histidines.

Affinity of Mb for oxygen is high, affinity of Hb for oxygen is lower and more variable.

Sickle cell hemoglobin (HbS)

G lu-

G lu-

H b A 1

G lu-

H b S (h e te ro z y g o u s)S ic k le ce ll tra it

H b S (h o m o zy g o u s)

S ic k le c e ll d isea se

Polymerization of HbS

Association shown in previous figure is repeated over and over to produce large, rod-like aggregates that bind oxygen poorly and distort shape of erythrocytes.\\

Sickle cell trait is usually asymptomatic, but strenuous exercise at altitude could elicit sickling and destruction of erythrocytes. This lowers serum Hb and hematocrit, while raising Hb breakdown products such as bilirubin, which can accumulate to form gallstones.

Thalassemias

Rare, since gene is duplicated (four genes per diploid chromosome set).

Usually more severe than thalassemia because there is no substitute for gene in adults. Almost all thalassemias are deletions

In thalassemia intermedia (o/o) - appearance of HbH (4) In thalassemia major (o/oo), Hb Bart’s (4) is predominant (usually lethal). BPG is ineffective in HbH & Hb Bart’s.

Thalassemias

More common, since gene is present in only one copy per chromosome.

Less severe than thalassemia, since chain can effectively substitute in adults.

The chain can also persist into adulthood (HPFH).

In thalassemia major (0/0) excess chains do not form soluble homotetramers.