Report: loss of hemoglobin H2S-binding function in annelids

Geoff Read at
Tue May 13 15:38:28 EST 2003

You saw it here first:

Bailly, X.; Leroy, R.; Carney, S.; Collin, O.; Zal, F.; Toulmond, A. ; Jollivet,
D. 2003: The loss of the hemoglobin H2S-binding function in annelids from
sulfide-free habitats reveals molecular adaptation driven by Darwinian positive
selection. Proceedings of the National Academy of Sciences 100(10)[May 
13, 2003]: 5885-5890.

ABSTRACT: The hemoglobin of the deep-sea hydrothermal vent 
vestimentiferan Riftia pachyptila (annelid) is able to bind toxic hydrogen 
sulfide (H2S) to free cysteine residues and to transport it to fuel 
endosymbiotic sulfide-oxidising bacteria. The cysteine residues are 
conserved key amino acids in annelid globins living in sulfide-rich 
environments, but are absent in annelid globins from sulfide-free 
environments. Synonymous and nonsynonymous substitution analysis from 
two different sets of orthologous annelid globin genes from sulfide rich and 
sulfide free environments have been performed to understand how the 
sulfide-binding function of hemoglobin appeared and has been maintained 
during the course of evolution. This study reveals that the sites occupied by 
free-cysteine residues in annelids living in sulfide-rich environments and 
occupied by other amino acids in annelids from sulfide-free environments, 
have undergone positive selection in annelids from sulfide-free 
environments. We assumed that the high reactivity of cysteine residues 
became a disadvantage when H2S disappeared because free cysteines 
without their natural ligand had the capacity to interact with other blood 
components, disturb homeostasis, reduce fitness and thus could have been 
counterselected. To our knowledge, we pointed out for the first time a case 
of function loss driven by molecular adaptation rather than genetic drift. If 
constraint relaxation (H2S disappearance) led to the loss of the sulfide- 
binding function in modern annelids from sulfide-free environments, our work 
suggests that adaptation to sulfide-rich environments is a plesiomorphic 
feature, and thus that the annelid ancestor could have emerged in a sulfide- 
rich environment.   

  Geoff Read < at>

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