5. The blood parasite Dicrocoelium dendriticum expresses a hemoglobin protein th

Question

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

Explanation / Answer

In Dicrocoelium dendriticum hemoglobin (DDHb) the replacement of the distal histidine by a glycine is accompanied by very high affinity for oxygen [5]. The protein also displays an acid Bohr effect [6,7]. The kinetics of ligand binding to DDHb are characterized by very fast and pHindependent association rates. In contrast, progressive lowering of the ligand dissociation velocities is observed if the pH is brought below 7 [8]. An analysis of the EPR properties of nitrosyl DD-Hb seemed most pertinent in order to gain more insights into the molecular basis of the functional behavior of this protein.

In DD-Hb the trend is opposite, i.e. the ligand dissociation rates decrease by lowering the pH, the association rates being unaffected [g]. In the nitrosyl derivative of Aplysia myoglobin the 3-line hyperfine spectrum is completely formed only at very low pH and is associated with an increase of the ligand binding rates [16]. In conclusion, in different proteins, similar EPR spectral transitions seem to be associated with different ligand binding properties, thus indicating that information on the local structure around the heme is not sufficient to explain the functional behavior of the macromolecule. However, the results on DD-Hb support the idea that changes at the proximal side of the heme may play a very important role in the regulation  of the ligand dissociation kinetics.

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