The crystal structure of a crustacean prophenoloxidase provides a clue to understanding the functionality of the type 3 copper proteins.

UNLABELLED: Phenoloxidase (PO), which is classified as a type 3 copper protein, catalyzes the hydroxylation of monophenol to o-diphenol and subsequent oxidation to the corresponding o-quinone. The geometry and coordination environment of the active site of the arthropod PO are very similar to those of the arthropod hemocyanin (Hc). However, unlike the POs, Hc is an oxygen carrier in crustaceans, and does not possess PO activity in general. Recently, we identified a new type of proPO from a crustacean and designated it proPOß. This enzyme has many characteristics that are rather similar to those of Hc, such as its maturation, localization, and oligomeric state. Here, we determined the crystal structure of proPOß prepared from the hemolymph of kuruma prawns (Marsupenaeus japonicus) at 1.8-Å resolution. M. japonicus proPOß forms a homohexamer rather similar to that of arthropod Hc. The geometry of the active copper site in proPOß is nearly identical to that of arthropod Hc. Furthermore, the well-characterized 'place-holder' phenylalanine is present (Phe72). However, the accessibility to the active site differs in several ways. First, another phenylalanine, which shields the active site by interacting with a copper-coordinated histidine in crustacean Hc, is replaced by valine in the proPOß structure. Second, two tyrosines, Tyr208 and Tyr209, both of which are absent in Hc, show the alternative conformations and form a pathway providing access to the reaction center. Thus, the present crystal structure clarifies the similarities and differences in the activity of two closely related proteins, PO and Hc. DATABASE: Structural data are available in the RSCB protein data bank under the accession number 3WKY. ray crystallography (View interaction).

Mass spectrometry based N- and C-terminal sequence determination of a hepatopancreas-type prophenoloxidase from the kuruma prawn, Marsupenaeus japonicus.

We previously identified and characterized a novel hepatopancreas-type prophenoloxidase from kuruma prawn, Marsupenaeus japonicus. In the characterization, this enzyme was indicated to have a feature of a signal peptide at its N-terminus. The putative primary structure was then proposed but its N- and C-terminal sequences remained undetermined. In the present study, the N- and C-terminal amino acid sequences of this prophenoloxidase were determined by de novo sequencing methods using matrix-assisted laser desorption ionization mass spectrometry. The sequence analyses revealed that the N-terminus of the prophenoloxidase was processed, whereas the C-terminus was not. This finding suggests that this enzyme has a signal peptide, and that it is synthesized at the endoplasmic reticulum in hepatopancreas cells and secreted to hemolymph plasma, similar to the case of hemocyanin, another member of the class III copper proteins.

A novel type of prophenoloxidase from the kuruma prawn Marsupenaeus japonicus contributes to the melanization of plasma in crustaceans.

Melanization is one of the major immune responses in arthropods. Prophenoloxidases (proPOs) catalyze the oxidation of mono- or o-diphenols, a reaction that is the key initial step of melanin formation. Well-characterized proPOs from crustaceans are synthesized in haemocytes and are released into plasma in response to microbial attack. However, PO activity does exist in the plasma of haemolymph without pathogenic infections. Here, we demonstrate that a novel type of proPO contributes to such PO activity in the plasma fraction of haemolymph of crustaceans. The novel enzyme, which was purified from the plasma of the kuruma prawn (Marsupenaeus japonicus), possessed strong and specific monophenol and o-diphenol oxidation activity compared with that of known haemocyte-type proPO. Amino acid sequence analyses indicated that this enzyme was distinct from the known proPO. The cDNA sequence and deduced amino acid sequence of this enzyme has a putative binuclear copper center, and showed approximately 30% and 20% identity with the primary structures of reported proPO and haemocyanin sequences of the kuruma prawn, respectively. Reverse transcription PCR analysis showed that this enzyme was synthesized in the hepatopancreas rather than in haemocytes. Although the primary structure and enzymatic properties of this novel enzyme suggested that it is a phenoloxidase, its biogenesis, tissue distribution, and oligomeric state resemble those of haemocyanin, which belongs to the same protein family (type III copper protein). This novel proPO enzyme may share a role with the already characterized version, itself a major component of the innate immune system in crustaceans.