Molecular Diseases and Mutant Haemoglobins: Comprehensive Study Guide

Common Haemoglobin Variant Forms:
  - $HbA$: Adult form, structure $ext{α}_2 ext{β}_2$.
  - $HbF$: Foetal form, structure $ext{α}_2 ext{γ}_2$.
  - $HbA_2$: Minor adult form, structure $ext{α}_2 ext{δ}_2$.
  - $HbS$: Sickle-cell anaemia form, structure $ext{α}_2 ext{β}^S_2$.

Locations of Globin Genes:
  - $ ext{α}$ globin genes: Located on human chromosome $16$ with $2$ gene-copies per chromosome.
  - $ ext{β}$ globin genes: Located on human chromosome $11$ with $1$ gene-copy per chromosome.
  - Other genes code for types such as $ext{γ}$ (gamma) and $ext{δ}$ (delta) globin.

Oxygen Binding Properties of Hb:
  - $P_{50}$ value for adult hemoglobin ($HbA$): approximately $26.8$ Torr, indicating the partial pressure of oxygen at which hemoglobin is 50 ext{%} saturated.
  - In deoxyhemoglobin, iron is in the $+2$ oxidation state with a coordination number of $5$ (high spin, paramagnetic). During oxygenation, coordination number increases to $6$ (low spin, diamagnetic).
  - Myoglobin (Mb) has a $P_{50}$ of $1$ Torr, showing it has a higher affinity for oxygen than adult hemoglobin.

Oxygen Binding Properties of Foetal Hb:
  - Composition: $HbF$ has a structure of $ext{α}<em>2 ext{γ}_2$.   - It binds oxygen with a greater affinity (lower $P</em>{50}$ of $19$ Torr) than adult hemoglobin, allowing efficient oxygen uptake from maternal blood across the placenta.

How Mutant Hb Variants Arise:
  - Mutant hemoglobins originate from changes in the DNA base sequence (mutations), leading to altered globin amino-acid sequences, which ultimately modify protein structure and function. These mutations can cause inherited diseases.

Haemoglobinopathies and Thalassaemias:
  - Haemoglobinopathies: Qualitative defects where normal amounts of defective globin subunits are produced, typically due to amino acid substitutions.
  - Thalassaemias: Quantitative defects characterized by abnormal or insufficient amounts of otherwise normal globin subunits, resulting from gene mutations.

Causes and Consequences of Hb M and Hb S:
  - Hb M (Methhemoglobinemia): Caused by a substitution of proximal Histidine residue by Tyrosine, leading to ineffective oxygen binding due to oxidation of iron from $+2$ (ferrous) to $+3$ (ferric).
  - Hb S (Sickle-cell disease): Results from the mutation of glutamate to valine in the $\beta$ subunit, changing a polar residue to hydrophobic, leading to aggregation of HbS molecules into fibers, causing sickling of red blood cells and related complications such as pain and organ damage.

Hb S Diagnosis:
  - Diagnosis of Hb S can be achieved through protein analysis or DNA tests that identify the specific mutation responsible for sickle-cell anemia.

α Thalassaemia and β Thalassaemia:
  - α Thalassaemia: Resulting from insufficient production of $ext{α}$ globin chains, leading to a range of clinical consequences that can be mild to severe.
  - β Thalassaemia: Involves reduced production of $ext{β}$ globin chains, which can result in severe anemia and complications such as bone deformities and splenomegaly.
  - Epidemiology: Both conditions are prevalent in Mediterranean regions, certain parts of Africa, and South East Asia with clinical symptoms varying from mild to severe depending on the specific mutation.