Hemoglobin, Myoglobin structure function and evolution
The function of the globin family is to bind and transport oxygen molecules. Each oxygen molecule binds to a heme group. The heme group consists of a ring of carbon, nitrogen and hydrogen atoms called ‘porphyrin’, with an atom of iron (fe2+) at its centre. A histidine side chain of the protein binds to the iron atom at the centre of the heme. Heme is non-covalently bound to the protein and gives blood its red colour. Globins consist of 7 or 8 alpha-helices and binds to the heme group in a pocket that is mainly hydrophobic.
Two types of globin proteins are myoglobin and haemoglobin. Myoglobin is a single-chain globular protein containing a heme prosthetic group. It has 8 alpha helices, a hydrophobic core and functions in muscles storing oxygen. Haemoglobin is a tetramer consisting of two copies of each two globin chains, the alpha- and beta chains. Each of the 4 subunits in haemoglobin binds to one heme group. The haemoglobin functions to transport oxygen from the lungs to other parts of the body. Haemoglobin and myoglobin structures were discovered by Mark Perutz and John Kendrew and co-workers.
Haemoglobin oxygen binding is cooperative. Cooperative is the binding of one ligand or substrate molecule that influences the affinity of the protein for additional molecules of the same substrate. The binding oxygen molecules to haemoglobin as a function to the oxygen pressure is sigmoidal, with relatively weak binding at low oxygen pressure. This is in contrast to myoglobin, which binds strongly at all oxygen levels. The binding of oxygen to haemoglobin has two conformational states, a T state (tight state/deoxygenated) where all the subunits have a low affinity for oxygen molecules and an R state (Relaxed state/oxygenated) with strong affinity to oxygen. When oxygen binds to the subunits in the T state, this triggers a conformational change to the R state. When the oxygen molecules are released, the...