The X-ray crystal structure of the sulfide derivative of ferric Lucina
pectinata hemoglobin component I (HbI) has been determined at 1.9 Ang
strom resolution (R-factor 0.186). The heme pocket structural organiza
tion of HbI is in keeping with its ligand binding properties. The fast
sulfide association rate constant can be related to the presence of G
ln(64)E7, as the heme distal residue, together with the protein struct
ural properties in the CD-E distal region. Moreover, the very high sul
fide affinity for HbI is reflected by the exceptionally slow ligand di
ssociation rate. The stabilization of the heme-bound sulfide molecule
is achieved through hydrogen bonding to Gln(64)E7, as well as by finel
y tuned aromatic-electrostatic interactions with the clustered residue
s Phe(29)B10, Phe(43)CD1 and Phe(68)E11. Such a peculiar arrangement o
f phenylalanyl residues at the distal ligand binding site has not been
observed before in the globin family, and is unique to HbI, a protein
functionally devoted to sulfide transport. (C) 1996 Academic Press Li
mited