The blood parasite Dicrocoelium dendriticum expresses a hemoglobin protein that

Question

The blood parasite Dicrocoelium dendriticum expresses a hemoglobin protein that is similar in sequence to
human hemoglobin. D. dendriticum hemoglobin (Dd 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 Dd Hb varies widely with pH (Smit, J. et al. (1986). Eur. J. Biochem. 155, 231-237.).

pH        p50 (mmHg)
4.0          0.016
8.0           0.15

Do you expect Dd Hb to show cooperative oxygen binding, given the information above?

Explanation / Answer

Hemoglobin is the  is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates and tissues of some invertebrates. It exists in two states, Tense state (T) and realxed state (R). When oxygen binds to one monomer of haemoglobin, the configuration shifts from the T (tense) state to the R (relaxed) state which promotes the binding of oxygen to the remaining three monomer's heme groups, thus saturating the hemoglobin molecule with oxygen. Hence in humans the binding is co-operative.

The P50 is the oxygen tension at which hemoglobin is 50% saturated.

Studies have shown that the haemoglobin of Dicrocoelium dendriticum has the highest dioxygen affinity (p50 at 250C between 0.016mm of Hg and 0.15mm of Hg. This molecule is monomeric irrespective of pH and ligation states.

Dicrocoelium dendriticum shows an Acid Bohr effect only. Bohr effect is the physiological phenomenon which states that hemoglobin's oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide. Mammalian haemoglobin shows both an acid and an alkaline Bohr effect. For the alkaline or normal Bohr effect the affinity increases with rising pH. In the acid or inverse Bohr effect affinity decreases with pH.

Monomeric haemoglobins show no homotropic effect and structural changes are restricted to the tertiary structure level. All the p50 values have to be compared to the lowest value reported for a cooperative haemoglobin i.e 0.0015 mm of Hg or the octameric Hb in the perienteric fluid of Ascaris lumbricoides. In contrast to this, Dd Hb shows no cooperativity and is monomeric throughtout, as mentioned. Also, studies have shown that O2 dissociation curves of this haemoglobin show no sign of h value(Hill co-efficient) below 1, which shows that it has functional homegeneity. hence, Dd Hb will not show co-operativity, and is a monomeric Hb throughout pH and ligation and moreover, it has a very high dioxygen affinity.

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