5 The blood parasite Dicrocoelium dendriticum expresses a he

5. The blood parasite Dicrocoelium dendriticum expresses a hemoglobin protein that is similar in sequence to human hemoglobin. D. dendriticum hemoglobin (Da Hb binds oxygen with much higher affinity than human hemoglobin. In contrast to human hemoglobin, research has shown that hemoglobin does not form quarternary structure, it exists only as a monomer in solution. The p50 value for DdHb varies widely with pH (Smit, J. et al. (1986). Eur. J. Biochem. 155, 231-237.). pH p50 mmH 4.0 0.016 8.0 0.15

Solution

The nitric oxide derivative of liver fluke (Dicrocoelium dendriticum) hemoglobin (DD-Hb) have been investigated in the pH range from 4.8 to 8.
In the neutral and alkaline regions the spectra have a rhombic shape, with gX = 2.09, gy, = 1.99 and gz, = 2.009, and a triplet hyperfine structure of 2.2 mT,due to the nitrogen of the bound NO molecule, in the center resonance.

By lowering the pH the EPR spectrum undergoes a reversible change showing a 3-line pattern in the high-field region. Such a spectrum is fully formed at pH 4.8 and is interpreted in terms of a dissociation of the proximal histidine from the heme iron.
The high pH form has a rhombic shape.
Lowering the pH produces a reversible transition towards a species characterized by an EPR spectrum with 3-line splitting in the high-field region. Such a pattern, which is taken as iagnostic for a pentacoordinated nitrosyl-heme complex, is completely formed at pH 4.8

In DD-Hb the trend is opposite, i.e. the ligand dissociation rates decrease by lowering the pH.

Dicrocoelium haemoglobin is a non-cooperative monomer. It has the highest O2 affinity so far known for any monomeric haemoglobin: its half-saturation pressure, p50 value, ranges at 25 degrees C from 0.016 mm Hg to 0.15 mm Hg dependent on pH. Dicrocoelium haemoglobin shows an acid Bohr effect only and as such it constitutes a new class of haemoglobins. Its log p50 versus pH plot (Bohr effect curve) is characterized by a large amplitude, delta log p50 = 0.96, and an inflection point (Bohr effect pK) at pH 5.0.

an increasing p50 would suggest that the affinity of hemoglobin for oxygen is decreasing - it takes more oxygen to achieve 50% saturation. So state with high affinity with oxygen is more stable.