S-SULFOHEMOGLOBIN AND DISULFIDE EXCHANGE - THE MECHANISMS OF SULFIDE BINDING BY RIFTIA-PACHYPTILA HEMOGLOBINS

The deep sea hydrothermal tube worm Riftia pachyptila possesses a mult ihemoglobin system with three different extracellular hemoglobins (Hbs ; V1, V2, and C1): two dissolved in the vascular blood, V1 and V2, and one in the coelomic fluid, C1. V1 consists of four heme-containing ch ains and four linker chains. The globin chains making up V2 and C1 are , with one exception, common to V1, Remarkably these Hbs are able to b ind oxygen and sulfide simultaneously and reversibly at two different sites. Two of the globin chains found in these three Riftia Hbs posses s one free Cys residue and for at least one of the globins, the b-Cys( 75) is conserved among vestimentifera (Lamellibrachia sp,) and pogonop hora (Oligobrachia mashikoi). By selectively blocking the free Cys wit h N-ethylmaleimide and using electrospray ionization mass spectrometry experiments, we show that these Cys residues are involved in sulfide binding by Riftia Hbs. Moreover, we also demonstrate that the larger V 1 Hb can form persulfide groups on its linker chains, a mechanism that can account for the higher sulfide-binding potential of this Hb.