Haemoglobin involvment in the NO mediated regulation of blood flow in Ascaris lumbricoides.

One of the main examples of such mechanism is a parasitic worm present in about one billion humans humans around the world. It mainly infests intestines but is also present in jejunum. Ascaris lumbricoides lives in the anaerobic environment inside its host. The worm has octameric haemoglobin which has 25 000 times the affinity for oxygen compared to human haemoglobin. It is belived that the reason for such a high affinity level is not to transport oxygen but to eliminate it because even the smallest amount is very toxic to the worm. The aborbtion of the oxygen by the haemoglobin is enzymatically driven by nitric oxide3.

The site of action were the reaction occurs is in the distal haem pocket. It consist of iron, oxygen and thiol and E7 glutamine all which react with nitric oxide3. The glutamine is responsible for a thousand times increase in reactivity of oxidized myoglobin with nitric oxide in Ascaris lumbricoides. While E7 histidine present in higher eukaryotes produces normal reactivity. Another key feature of the worm is the close proximity of cysteine the ligand binding site. Nitric oxide is transferred between haem and thiol to form S-nitrosothiol (SNO). A number of experiments were carried out by Minning et. al in order to prove that nitric oxide does play a role in the enzymatic absorbtion of oxygen by Ascaris haemoglobin. The overall mechanism was determined and can be seen in the form of equations below3.

AH (Fe2+)O2 ←→ AH (Fe3+)O2-         (reaction 1), where AH stand for Ascaris haemoglobin

AH (Fe3+)O2- + NO ←→ AH (Fe3+) + NO3- (reaction 2), where NO enters into distil pocket.

AH (Fe3+) + NO ←→ AH (Fe3+)NO        (reaction 3)

       (methaemoglobin)

        AH (Fe3+)NO + (Cys)S- ←→ AH (Fe2+) + (Cys)SNO        (reaction 4)

                             Cysteine c

        

        AH (Fe2+) + O2  ←→ AH (Fe2+)O2                                 (reaction 5)

                                    Unstable complex which decomposes as the reaction occurs

                                    Product wasn’t detected in the spectroscopy

AH (Fe2+)O2 + (Cys)SNO ←→         AH (Fe3+) + (Cys)S + NO3-                                                              (reaction 6)

AH (Fe3+) + (Cys)S + e- ←→ AH (Fe3+) + (Cys)S-                (reaction 7)         reaction 7 is the source of electron for the  reaction 6

(Cys)S + NADPH ←→ (Cys)SH + NADP                 (reaction 8)

                addition of NADPH decreases haem-bound NO and SNO and NO3-

NADP + O2 ←→ NADP+ + O2                (reaction 9)

        AH (Fe2+) O2 + SNO (E15 Cys) ←→ AH (Fe2+)OO.ONS (E15 Cys)

AH (Fe2+) O2 was in reaction 5 unstable however in the above reaction the distal pocket of enzyme facillitate the above reaction        

        AH (Fe2+)OO.ONS (E15 Cys) + e- ←→ AH (Fe3+)(E15 Cys)S- + NO-                                

        

The equations above demostrate that Ascaris lumbricoides haemoglobin uses oxygen and nitric oxide in an NADPH-dependent way.

The role of Ascaris haemoglobin could be not just the removal of oxygen from the atmosphere but also to protect the organism from nitric oxide present in the host or generated as the result of host defence mechanisms.

(conclusion)

Reference:

Stuart A. Lipton; Nature 413, 118-121 (2001)

Imai Kiyohiro; Nature 401, 437-439 (1999)

Dena M. Minning, Andrew J. Gow, Joseph Bonaventura, Rod Braun, Mark Dewhirst, Daniel E. Goldberg and Jonathan S. Stamler; Nature 401, 497 (1999)

Human Physiology; Vander, Sherman and Luciano; McGrawHill, 8th ed.

        

 

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