Haemoglobin - an example for globular protein
Haemoglobin is found in red blood cells and is involved in the transport of oxygen from lungs to tissues. It is a tetramer containing four polypeptide chains – 2α chains and 2 β chains. Each of these chains contains a prosthetic group called heme. Heme is a protoporphyrin ring complexed with Fe2+. This Fe 2+ ion can form six bonds, four with the nitrogen atoms of the porphyrin ring, one with a histidine of hemoglobin and the other with oxygen. Thus every haemoglobin can carry four O2 molecules.
Haemoglobin is an alpha helical protein, meaning it does not contain β sheets as its secondary structural elements. The haemoglobin tetramer structure could be considered as a dimerized dimer of (αβ)1 and (αβ)2. The α and β chain in each dimer are held together strongly by hydrophobic interactions. The interactions between (αβ)1 and (αβ)2 are comparatively weaker hydrogen bonds and ionic interactions. This allows the dimers to move with respect to each other forming two different conformational states: a relaxed ‘R’ conformation and a taut ‘T’ conformation. The binding and release of oxygen switches the hemoglobin between these two states.
Haemoglobinopathies are a set of diseases caused by synthesis of structurally abnormal haemoglobins, insufficient amount of haemoglobins or both. Sickle cell anemia, thalasemia, porphyria etc are examples of haemoglobinopathies.