maenas (intertidal zone) and Segonzacia mesatlantica - Station ...
maenas (intertidal zone) and Segonzacia mesatlantica - Station ...
maenas (intertidal zone) and Segonzacia mesatlantica - Station ...
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5.4. MANUSCRIT : RESPIRATORY ADAPTATIONS OF S. MESATLANTICA 209<br />
the species. Different thermal stability is not involved since it was shown that no dissociation occurs<br />
upon incubation at 35°C (Chausson, 2001). In S. <strong>mesatlantica</strong> we observed a correlation between high<br />
dodecamer proportion <strong>and</strong> high protein content : this enables to decrease blood viscosity <strong>and</strong> osmotic<br />
pressure at the very high protein contents observed in some crabs (up to 200 µg/µl <strong>and</strong> 282 µg/µl),<br />
compared to what it would be with the same protein quantity under the hexameric form. It must also<br />
be noted that freezing can cause partial dissociation of dodecamers, as observed for several decapod<br />
species Morris (1988). The correlation between sodium <strong>and</strong> protein concentration <strong>and</strong> the dodecamer<br />
proportion can possibly reflect stabilizing effect of one complex or the other during freezing. The ideal<br />
control experiment would be to perform SEC experiments on board on freshly sampled hemolymph.<br />
An interesting alternative explanation is that hydrostatic pressure could have an effect on oligomerization.<br />
Elevated hydrostatic pressure can modify chemical equilibria <strong>and</strong> rate constants depending<br />
on the specific volumes <strong>and</strong> activation volumes of reactions <strong>and</strong> has an effect on ionic interactions,<br />
hydrogen bonds <strong>and</strong> hydrophobic cores. Pressure can stabilize or destabilize protein conformation<br />
depending on the type of interaction involved in the tertiary structure. Dissociation of oligomeric<br />
proteins is usually promoted by high pressures (Gross et Jaenicke, 1994, Boonyaratanakornkit et al.,<br />
2002). Glossoscolex paulistus giant hemoglobin is dissociated at pressures above 1 kbar (equivalent<br />
to 10 000 m depth) (Silva et al., 1989). Lactate dehydrogenase apoenzyme is progressively dissociated/inactivated<br />
at pressures comprised between 0 <strong>and</strong> 80 MPa (0 <strong>and</strong> 800 bar, equivalent to 8 000 m<br />
depth) (Gross et Jaenicke, 1994). To our knowledge, all deep-sea crabs studied to date have higher<br />
hexamer proportions than shallow-water crabs (including for a bathyal species non-endemic from<br />
hydrothermal vents, Chaceon affinis (Chausson, 2001)). These proportions could reflect a specific<br />
molecular adaptation of the oligomeric complexes to deep-sea pressure.<br />
Functional properties<br />
S. <strong>mesatlantica</strong> Hc had a very high affinity for O 2 <strong>and</strong> exhibited a strong Bohr effect. Along with<br />
no or inverse temperature effect on affinity, this is a common feature of hydrothermal-vent crustacean<br />
Hc <strong>and</strong> is adaptive in hypoxic environments for efficient loading at the gills <strong>and</strong> unloading near the<br />
metabolizing tissues (Hourdez et Lallier, 2007).<br />
The strong lactate effect observed <strong>and</strong> the absence of urate effect for S. <strong>mesatlantica</strong> is not typical<br />
of hydrothermal species. Cyanagraea praedator Hc is insensitive to lactate (Chausson, 2001) whereas<br />
lactate sensitivity is small for Bythograea thermydron Hc ( ∆log(P 50)<br />
∆log[lactate]<br />
=-0.06 (S<strong>and</strong>ers et Childress,<br />
1992)), moderate for Rimicaris exoculata Hc (-0.12 (Lallier et Truchot, 1997)) <strong>and</strong> Chorocaris chacei<br />
(-0.16 (Lallier et al., 1998)). From our data, L-lactate is not a physiological modulator of Hc for S.<br />
<strong>mesatlantica</strong> given the occurrence of a strong lactate effect, a high basal lactate level <strong>and</strong> no lactate