Primary and tertiary structures of the first domain of Panulirus interruptus hemocyanin and comparison of arthropod hemocyanins

The amino acid sequence of the first domain (positions 1–175) of Panulirus interruptus hemocyanin subunit a has been determined. The sequence of residues 1–158 (18‐kDa fragment obtained by limited proteolysis) was derived from peptides obtained by digestion of this fragment with CNBr and trypsin and by subdigestion of these peptides with other enzymes. The peptides were sequenced automatically or manually. The amino acid sequence has been fitted into the electron‐density map at 0.32‐nm resolution. The residues of domain 1 are folded into a larger, mainly helical, globular part, containing one disulfide bridge, and a smaller part near the molecular twofold axis. The latter part consists of an α helix and a β strand which contains a covalently attached carbohydrate moiety. The sites susceptible to limited proteolytic cleavage of the subunit are discussed. Comparison of the N‐terminal sequence with those of other arthropod hemocyanins revealed, besides an N‐terminal extension of five residues, the presence of a 21‐residue loop (positions 22–42) in the crustacean sequences. This loop contains helix 1.2, a less defined region in the electron‐density map. It is absent in chelicerate sequences. Strong evidence is presented that: (a) the structure of the first 21 residues (including helix 1.1) is the same in all arthropod hemocyanins with known amino acid sequence; (b) a stretch containing about 15 residues (including part of helix 1.3) following the 21‐residue loop has a different structure in crustaceans and chelicerates; (c) the rest of domain 1 has the same structure again. It is shown that all conserved residues are in the contact region with the other two domains.