No abstract

Summary As in many other hemoglobins, no direct route for migration of ligands between solvent and active site is evident from crystal structures of Scapharca inaequivalvis dimeric HbI. Xenon (Xe) and organic halide binding experiments along with computational analysis presented here reveal protein cavities as potential ligand migration routes. Time-resolved crystallographic experiments show that photodissociated carbon monoxide (CO) docks within 5ns at the distal pocket B-site and at more remote Xe4 and Xe2 cavities. CO rebinding is not affected by the presence of dichloroethane within the major Xe4 protein cavity, demonstrating that this cavity is not on the major exit pathway. The crystal lattice has a substantial influence on ligand migration, suggesting that significant conformational rearrangements may be required for ligand exit. Taken together, these results are consistent with a distal histidine gate as one important ligand entry and exit route, despite its participation in the dimeric interface.

A direct transfer of the reduced flavin mononucleotide (FMNH(2)) cofactor of Vibrio harveyi NADPH:FMN oxidoreductase (FRP) to luciferase for the coupled bioluminescence reaction has been indicated by recent kinetic studies [Lei, B., and Tu, S.-C. (1998) Biochemistry 37, 14623-14629; Jeffers, C., and Tu, S.-C. (2001) Biochemistry 40, 1749-1754]. For such a mechanism, a complex formation of luciferase with FRP is essential, but until now, no evidence for such a complex has been reported. In this work, FRP was labeled at 1:1 molar ratio with the fluorophore eosin. The labeled enzyme was about 30% active in either the reductase single-enzyme or the luciferase-coupled assay. The labeled FRP in either the holo- or apoenzyme form was similar to the native FRP in undergoing a monomer-dimer equilibrium. By measuring the steady-state fluorescence anisotropy of eosin-labeled FRP, it was shown that luciferase formed a complex at 1:1 molar ratio with the monomer of either the apoenzyme or the holoenzyme form of FRP with K(d) values of 7 and 11 microM, respectively. Neither the holo- nor the apoenzyme of the labeled FRP in the dimeric form was effective in complexing with luciferase. At maximal in vivo bioluminescence, the V. harveyi cellular contents of luciferase and FRP were estimated to be 172 and 3 microM, respectively. The vast majority of FRP would be trapped in the luciferase/FRP complex. Plausible physiological significance of such a finding is discussed.