Structure-based calculation of multi-donor multi-acceptor fluorescence resonance energy transfer in the 4x6-mer tarantula hemocyanin

Hemocyanins are oxygen carriers of arthropods and molluscs. The oxygen is bound between two copper ions, forming a Cu(II)-O-2(2-)-Cu(II) complex. The oxygenated active sites create two spectroscopic signals indicating the oxygen load of the hemocyanins: first, an absorption band at 340 nm. which is due to a ligand-to-metal charge transfer complex, and second, a strong quenching of the intrinsic tryptophan fluorescence, the cause of which has not been definitively identified. We showed for the 4x6-mer hemocyanin of the tarantula Eurypelma californicum that the fluorescence quenching of oxygenated hemocyanin is caused exclusively by fluorescence resonance energy transfer (FRET). The tarantula hemocyanin consists of 24 subunits containing 148 tryptophans acting as donors and 24 active sites as acceptors. The donor-acceptor distances are determined on the basis of a closely related crystal structure of the horseshoe crab Limulus polyphemus hemocyanin subunit II (68-79% homology). Calculation of the expected fluorescence quenching and the measured transfer efficiency coincided extraordinary well, so that the fluorescence quenching of oxygenated tarantula hemocyanin can be completely explained by Forster transfer. This results explain for the first time, on a molecular basis, why fluorescence quantum yield can be used as an intrinsic signal for oxygen load of at least one arthropod hemocyanin, in particular that from the tarantula.