Functional unit of the Rapana thomasiana (Grosse) (marine snail, gastropod) hemocyanin

cDNA Sequence, Protein Structure, and Evolution of the Single Hemocyanin from Aplysia californica, an Opisthobranch Gastropod

By protein immunobiochemistry and cDNA sequencing, we have found only a single hemocyanin polypeptide in an opisthobranch gastropod, the sea hare Aplysia californica, which contrasts with previously studied prosobranch gastropods, which express two distinct isoforms of this extracellular respiratory protein. We have cloned and sequenced the cDNA encoding the complete polypeptide of Aplysia californica hemocyanin (AcH). The cDNA comprises 11,433 bp, encompassing a 5'UTR of 77 bp, a 3'UTR of 1057 bp, and an open reading frame for a signal peptide of 20 amino acids plus a polypeptide of 3412 amino acids (Mr ca. 387 kDa). This polypeptide is the subunit of the cylindrical native hemocyanin (Mr ca. 8 MDa). It comprises eight different functional units (FUs: a, b, c, d, e, f, g, h) that have been identified immunobiochemically after limited proteolysis of AcH purified from the hemolymph. Each FU shows a highly conserved copper-A and copper-B site for reversible oxygen binding. FU AcH-h carries a specific C-terminal extension of ca. 100 amino acids that include two cysteines that may be utilized for disulfide bridge formation. Potential N-glycosylation sites are present in six FUs but lacking in AcH-b and AcH-c. On the basis of multiple sequence alignments, phylogenetic trees and a statistically firm molecular clock were calculated. The latter suggests that the last common ancestor of Haliotis and Aplysia lived 373+/-47 million years ago, in convincing agreement with fossil records from the early Devonian. However, the gene duplication yielding the two distinct hemocyanin isoforms found today in Haliotis tuberculata occurred 343+/-43 million years ago.

Stopped-flow Fluorescence Studies of Inhibitor Binding to Tyrosinase from Streptomyces antibioticus

Tyrosinase (Ty) is a type 3 copper protein involved in the rate-limiting step of melanin synthesis. It is shown that the endogenous Trp fluorescence of tyrosinase from Streptomyces antibioticus is remarkably sensitive to the redox state. The fluorescence emission intensity of the [(Cu(I) Cu(I)] reduced species is more than twice that of the oxygen-bound [Cu(II)-O(2)(2-)-Cu(II)] form. The emission intensity of the oxidized [Cu(II)-OH(-)-Cu(II)] protein (Ty(met)) appears to be dependent on an acid-base equilibrium with a pK(a) value of 4.5 +/- 0.1. The binding of fluoride was studied under pseudo first-order conditions using stopped-flow fluorescence spectroscopy. The kinetic parameters k(on), K(d), and the fraction of fluorescence emission quenched upon fluoride binding show a similar pH dependence as above with an average pK(a) value of 4.62 +/- 0.05. Both observations are related to the dissociation of Cu(2)-bridging hydroxide at low pH. It is further shown that Ty is rapidly inactivated at low pH and that halide protects the enzyme from this inactivation. All results support the hypothesis that halide displaces hydroxide as the Cu(2)-bridging ligand in Ty(met). The relevance of the experimental findings for the catalytic cycle is discussed. The data are consistent with the data obtained from other techniques, validating the use of fluorescence quenching as a sensitive and effective tool in studying ligand binding and substrate conversion.

Arrangement of functional units within the Rapana thomasiana hemocyanin subunit RtH2

For the determination of the number and linear sequential arrangement of functional units (FUs) within the polypeptide chain of the Rapana hemocyanin subunit RtH2, a panel of mono-, di-, tri- and penta-FU fragments was generated by limited proteolysis of the purified subunit with four different enzymes. The individual cleavage products were isolated, characterized by SDS-PAGE and N-terminally sequenced. Most of the information about the FU sequential arrangement within RtH2 was obtained after limited proteolysis of the subunit with plasmin. Overall correlation of the data revealed the sequential order of the eight FUs within the polypeptide chain of RtH2, termed RtH2-a to RtH2-h. The sites, most sensitive to proteolytic cleavage with plasmin, are located at the C-terminus, between the FUs ef, fg and gh. A second main cleavage site was observed between the FUs cd. Endoproteinase GluC hydrolyzes these sites, too, but produces exclusively a mixture of mono-, di- and tri-FU fragments. The most stable fragments, the trimer abc and the dimer gh, are found in all cleavage mixtures of RtH2 studied. RtH2-h is compared with the corresponding h-FUs of the gastropodan hemocyanins of Pila leopoldvillensis, Helix pomatia, Megathura crenulata and Haliotis tuberculata, and a remarkable similarity is observed between them: an increased M(r) of approximately 65000 instead of approximately 50000, estimated for an average FU, suggesting that the sequence of RtH2-h is elongated by about 95 amino acid residues at the C-terminal part of the molecule, as found for beta(c)-HpH, HtH1 and HtH2.

Low-resolution molecular structures of isolated functional units from arthropodan and molluscan hemocyanin

Synchrotron x-ray scattering measurements were performed on dilute solutions of the purified hemocyanin subunit (Bsin1) from scorpion (Buthus sindicus) and the N-terminal functional unit (Rta) from a marine snail (Rapana thomasiana). The model-independent approach based on spherical harmonics was applied to calculate the molecular envelopes directly from the scattering profiles. Their molecular shapes in solution could be restored at 2-nm resolution. We show that these units represent stable, globular building blocks of the two hemocyanin families and emphasize their conformational differences on a subunit level. Because no crystallographic or electron microscopy data are available for isolated functional units, this study provides for the first time structural information for isolated, monomeric functional subunits from both hemocyanin families. This has been made possible through the use of low protein concentrations (< or = 1 mg/ml). The observed structural differences may offer advantages in building very different overall molecular architectures of hemocyanin by the two phyla.

Hemocyanin subunit organization of the gastropod Rapana thomasiana

RtH1 and RtH2, the two hemocyanin isoforms of the prosobranch gastropod Rapana thomasiana, have been purified by anion-exchange chromatography and studied by SDS-PAGE and immunoelectrophoresis. Both subunit types are built up of eight functional units (FUs). Under reducing conditions subunit RtH2 splits into two fragments, RtH2-a-f and RtH2-gh, suggesting the presence of a disulfide bridge between FU2-f and FU2-g. By proteolytic cleavage of the subunits into three-, two-, and single-FU fragments, purification of fragments by HPLC, N-terminal sequencing of the peptides, and crossed-line immunoelectrophoresis, FUs-a-h of RtH2 and FU-a, FU-d, FU-e, and FU-f of RtH1 were identified and correlated to the eight-FUs pattern of immunoelectrophoresis. FU-a, FU-e, and FU-f of RtH1 and RtH2 are very closely related immunologically. RtH1 and RtH2 both correspond immunologically to KLH2, one of the two hemocyanin isoforms of the prosobranch gastropod Megathura crenulata.

Complete amino acid sequence of dioxygen-binding functional unit of the Rapana thomasiana hemocyanin

The complete amino acid sequence of the Rapana thomasiana hemocyanin N-terminal functional unit Rta was determined by direct sequencing and matrix-assisted laser desorption ionization mass spectrometry of the protein and peptides obtained by cleavage with EndoLysC proteinase, TPCK-trypsin and cyanogen bromide. The single polypeptide chain consists of 407 residues. This is the first report on the primary structure of a dioxygen-binding unit from a marine gastropod hemocyanin and of an N-terminal domain from a molluscan dioxygen carrier. Comparison with the sequences of other molluscan hemocyanin functional units shows an average identity of 48 +/- 5 %. Inspection of the Rta sequence revealed residues 27 and 250 as carbohydrate attachment sites. Conclusions about the molecular evolution of the molluscan hemocyanin dioxygen-binding functional units are made.

Amino-terminal oxygen-binding functional unit of the Rapana thomasiana grosse (gastropod) hemocyanin: carbohydrate content, monosaccharide composition and amino acid sequence studies

The amino-terminal oxygen-binding unit Rta of the Rapana thomasiana hemocyanin is a glycoprotein with a carbohydrate content of 4.8% (w/w). Sugar analysis revealed as monosaccharide constituents xylose, fucose, 3-O-methylgalactose, mannose, galactose, N-acetylgalactosamine and N-acetylglucosamine residues. On subtracting the carbohydrate contribution from the molecular mass of 49,698 Da, determined by laser desorption mass spectrometry for Rta, an M(r) value of 47,318 Da was determined for the polypeptide part of the functional unit. The Rapana hemocyanin oxygen-binding unit Rta contains 400 residues in a single polypeptide chain. The nearly complete amino acid sequence (about 90%) is determined. This is the first report on a sequence of a marine gastropod oxygen-binding unit and also on a molluscan hemocyanin amino-terminal unit. Comparison of the Rta sequence with those of other molluscan hemocyanin units, localized in the C-terminus or in the middle of the respective multidomain polypeptide chains, revealed 42-46% homology (52-55%, including isofunctional residues). Probably, all molluscan oxygen-binding units evolved from a common ancestral gene.