Biochemsitry Wk 8

Question 1

What are globular proteins

Answer 1

Amino acid chains fold into shapes that resemble spheres are called globular proteins.
Hemoglobin: oxygen transport function
Myoglobin: oxygen storage/supply function in heart and muscle g-globulins (immunoglobulins): immune function

Question 2

Hemoglobin

Answer 2

Hemoglobin is red blood pigment, found in erythrocytes

It is a chromoprotein, containing heme as the prosthetic group & globin as the protein part

Heme containing proteins are characteristic of aerobic organisms

Normal levels

Adult male:14to 16gm

Female:13to 15gm

Question 3

Functions of Hemoglobin

Answer 3

• Delivery of O2 from lungs to the tissues
• Transport of CO2&protons from tissuesto lungs for
excretion

Question 4

Structure of Globin

Answer 4

Globin consists of 4polypeptide chains
Adult Hb is made up of 2 alpha chains and 2 beta chains
Each alpha chain contains 141AAs and beta chain contains 146AAs
The four subunits of hemoglobin are held together by non covalent interactions- hydrophobic, ionic, hydrogen bonds

Question 5

Structure of Heme

Answer 5

Heme is a Fe-porphyrin compound

Porphyrins are cyclic compounds formed by fusion of 4 pyrrole rings linked by methenyl (=CH–) bridges

Since an atom of iron is present, heme is a ferroprotoporphyrin.

Question 6

Heme structure

Answer 6

Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+). The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring. The heme Fe2+ can form two additional bonds, one on each side of the planar porphyrin ring

Question 7

HbA structure

Answer 7

The hemoglobin tetramer can be envisioned as composed of two identical dimers, (αβ)1 and (αβ)2.

Question 8

Minor haemoglobins

Question 9

Myoglobin structure and function

Answer 9

Myoglobin, a hemeprotein present in heart and skeletal muscle, functions both as an oxygen reservoir and as an oxygen carrier that increases the rate of oxygen transport within the muscle cell.
• Myoglobin consists of a single polypeptide chain that is structurally similar to the individual polypeptide chains of the tetrameric hemoglobin molecule.

Question 10

Oxygen binding to myoglobin and hemoglobin

Answer 10

The oxygen dissociation curve for Hb is steepest at the oxygen concs that occur in the tissues. This permits oxygen delivery to respond to small changes in pO2

The graph illustrates that myoglobin has higher affinity for oxygen at all pO2 value than does Hb

The partial pressure of oxygen needed to achieve half saturation of the binding sites is approx 1mm Hg for myoglobin and 26mm Hg for hemoglobin

The higher the oxygen affinity the lower the P50

Question 11

Myoglobin

Answer 11

the oxygen dissociation curve for myoglobin has a hyperbolic shape. This reflects the fact that myoglobin reversible binds a single molecule of oxygen. Thus oxygenated and deoxygenated myoglobin exist in simple equilibrium

Mb+o2 ———> Mbo2

The equilibrium can be shifted either sides if o2 is added or removed

Question 12

Hemoglobin

Answer 12

Sigmoidal shape- subunits cooperate in binding oxygen

This cooperative binding of all 4 subunits means that the binding of an o2 molecule at one heme group increases the oxygen affinity of the remaining heme groups in the same hemoglobin molecule. This effect is the heme heme interaction

It is difficult for the first o2 to bind to Hb this binding occurs at a high affinity for o2 at about 20-30mm Hg

Question 13

Allosteric effects

Answer 13

Heme heme interaction- the sigmoidal oxygen dissociation curve reflects specific structural changes that are initiated at one heme group and transmitted to other heme groups in the hemoglobin tetramer

The net affinity is that the affinity of the hemoglobin for the last oxygen is approx 300 times > than the first o2 bound

Question 14

Loading and unloading oxygen

Answer 14

The cooperative binding of O2 allows hemoglobin to deliver more O2 to the tissues in response to relatively small changes in the pO2.
In the lung, oxygen concentration is high, and hemoglobin becomes virtually saturated (or “loaded”) with O2.
In contrast, in the peripheral tissues, oxyhemoglobin releases (or “unloads”) much of its O2 for use in the oxidative metabolism of the tissues

Question 15

Bohr effect

Answer 15

The release of oxygen from hemoglobin is enhanced when the pH is lowered or when the hemoglobin is in the presence of an increased pCO2

Both result in a decreased affinity of hemoglobin and therefore a shift to the right in the oxygen dissociation curve, then stabilise the T state

This is the Bohr effect

Raising the pH or lowering the conc of CO2 results in greater affinity for oxygen, a shift to the left and stabilisation of the R state

Question 16

Source of the protons that lower pH

Answer 16

The concentration of both H+ and CO2 in the capillaries of metabolically active tissues is higher than that observed in alveolar capillaries of the lungs, where CO2 is released into the expired air. In the tissues, CO2 is converted by zinc-containing carbonic anhydrase to carbonic acid:

CO2+ H20 <——-> H2CO3

H2CO3 <——-> HCO3- ANMD H+

The H+ produced by this pair of reactions contributes to the lowering of pH. This differential pH gradient favors the unloading of O2 in the peripheral tissues and the loading of O2 in the lung.
Thus, the oxygen affinity of the hemoglobin molecule responds to small shifts in pH between the lungs and oxygen-consuming tissues, making hemoglobin a more efficient transporter of O2.

Question 17

2,3 BPG effect on oxygen affinity

Answer 17

2,3 bisphosphoglycerate is an important regulator of the binding of the oxygen to hemoglobin

It is synthesised from an intermediate of the glycolytic pathway

Binding of 2,3 bpg to deoxyhemoglobin:

This substance decreases the oxygen affinity of hemoglobin by binding to deoxyhemoglobin but not to oxyhemoglobin

The binding stabilises the taut conformation of deoxyhemoglobin. The effect of binding is 2,3 bpg can be represented by

Hbo2 + 2,3 bpg <——> Hb2,3bpg (deoxyhemoglobin)+o2

Question 18

2,3-BPG binding site

Answer 18

One molecule of 2,3-BPG binds to a pocket, formed by the two β-globin chains, in the center of the deoxyhemoglobin tetramer. This pocket contains several positively charged amino acids that form ionic bonds with the negatively charged phosphate groups of 2,3-BPG.Oxygenation of hemoglobin narrows the pocket and causes 2,3- BPG to be released.

Question 19

2,3-BPG effect on oxygen affinity

Answer 19

Shift of the oxygen dissociation curve:

Hemoglobin from which 2,3 bpg has been removed has a high affinity for oxygen
As seen in the RBC, the presence of 2,3 bpg reduces the affinity for oxygen shifting the curve to the right

This reduced affinity enables hemoglobin to release oxygen efficiently at partial pressures found in the tissues

Question 20

Response of 2,3 BPG levels to chronic hypoxia or anemia

Answer 20

The conc of 2,3 BPG in the rbc increases in response to chronic hypoxia such as observed in chronic obstructive pulmonary disease (COPD) like emphysema or at high altitudes where circulating hemoglobin may have difficult receiving sufficient oxygen

Intracellular levels of 2,3 BPG are also elevated in chronic anemia in which fewer than normal RBI’s are available to supply o2

Elevated 2,3 bpg levels lower the oxygen affinity of hemoglobin permitting greater unloading of oxygen in the capillaries of the tissues

Question 21

Role of 2,3 bpg in transfused blood

Answer 21

2,3 bpg is essential for the normal oxygen transport function of hemoglobin. However, storing blood in available media results in a decrease in 2,3 PBG

Stored blood displays an abnormally high oxygen affinity and fails to unload its bound oxygen properly in the tissues

Hemoglobin deficient in the 2,3 bpg thus acts as an oxygen trap rather than oxygen transport system

Transfused RBcs are able to restore their depleted supplies of 2,3 bpg un 6- 24 hours. However severely ill patients may be compromised if transfused with large quantities of 2,3 bpg stripped blood

Question 22

CO2 binding

Answer 22

Most of the co2 produced in metabolism is hydrated and transported as a bicarbonates ion.

However, some co2 is carried as carbamate bound to the N terminal amino groups of hemoglobin (forms carbominohemoglobin)

Hb-NH2+CO2,<———> Hb-NH-COO-+H+

The binding of co2 stabilises the T taut or deoxyhemoglobin form of hemoglobin resulting in a decrease in its affinity for oxygen and right shift in the oxygen dissociation

In the lungs, co2 dissociates from Hb AND is released in breath

Question 23

Carbon Monoxide Poisoning

Answer 23

Carbon monoxide (CO) is a colorless, odorless gas that is responsible for more than half of the annual poisoning deaths worldwide.
• CO has a 250-fold greater affinity for Hb than O2, and exposure to CO reduces the oxygen-carrying capacity of the blood.
• Symptoms of CO poisoning depend on the percent of total Hb that is bound to the gas.
• At 10% bound, symptoms are rarely observed.
• At 20% to 30%, the individual will experience a severe headache accompanied by nausea, dizziness, confusion, and visual disturbances.
• At CO Hb levels of 30% to 50%, neurological symptoms become more severe, and at levels near 50%, the individual loses consciousness and can fall into a coma. Respiratory failure may follow.
• Death normally occurs rapidly when CO Hb levels rise above 60%. CO is a component of tobacco smoke, and chain-smokers can have CO Hb levels of 15%. Thus, smoking predisposes individuals to the effects of CO poisoning.

Question 24

Sickle cell anemia

Answer 24

Sickle cell anemia, the most common of the RBC sickling diseases, is a genetic disorder caused by a single nucleotide substitution in the gene for β-globin.
• Sickle cell anemia is an autosomal- recessive disorder.
• Sickle cell anemia is characterized by lifelong episodes of pain (“crises”), chronic hemolytic anemia with associated hyperbilirubinemia , and increased susceptibility to infections, usually beginning in infancy.
• Other symptoms include acute chest syndrome, stroke, splenic and renal dysfunction, and bone changes due to marrow hyperplasia.

Question 25

Amino acid substitution in HbS β chains

Answer 25

A molecule of HbS contains two normal α-globin chains and two mutant β-globin chains (βS), in which glutamate at position six has been replaced with valine

Question 26

Sickling and tissue anoxia

Answer 26

The replacement of the charged glutamate with the nonpolar valine forms a protrusion on the β chain that fits into a complementary site on the β chain of another hemoglobin molecule in the cell.
• At low oxygen tension, deoxyhemoglobin S polymerizes inside the RBC, forming a network of insoluble fibrous polymers that stiffen and distort the cell, producing rigid, misshapen RBC.
• Such sickled cells frequently block the flow of blood in the narrow capillaries.
• This interruption in the supply of O2 leads to localized anoxia in the tissue, causing pain and eventually ischemic death of cells in the vicinity of the blockage.

Question 27

Hemoglobin c disease

Answer 27

Hb C is hemoglobin variant that has a single a.a substitution in the sixth position of the beta globin chain

In this case, a lysine is substituted for the glutamate

Patients homozygous for Hb c generally have mild, chronic haemolytic anemia

These patients do not suffer from infraction crises and no specific therapy is required

Question 28

Methemoglobins

Answer 28

Oxidation of the heme component of hemoglobin to ferric (fe3+) state forms methemolgobins, which cannot bind oxygen.

The oxidation may be causes by the action of certain drugs such as nitrates or endogenous products such as reactive oxygen intermediates

They are characterised by ‘chocolate cyanosis’ and chocolate coloured blood as result of dark coloured methemoglobin

The oxidation may also result from congenital defects, for example, a deficiency of NADH- cytochrome b5 reductase , the enzyme responsible for the conversion of methemoglobin (Fe3+) to hemoglobin (Fe2+), leads to the accumulation of methemoglobin

Question 29

Differences between myoglobin and haemoglobin