The o ‐diphenol oxidase activity of arthropod hemocyanin

205

139

An enzyme generally catalyzes one well defined reaction with high specificity and efficiency. We report here in contrast that the copper protein hemocyanin of the tarantula Eurypelma californicum exhibits two different functions. These occur at the same active site. While hemocyanin usually is an oxygen carrier, its function can be transformed totally to monophenoloxidase and o-diphenoloxidase activity after limited proteolysis with trypsin or chymotrypsin. N-acetyldopamine (NADA) is more effectively oxidized than L-dopa or dopamine. This irreversible functional switch of tarantula hemocyanin function is limited to the two subunits b and c of its seven subunit types. A conserved phenylalanine in the hemocyanin molecule acts as a placeholder for other substrates that are phenylalanine derivatives. The proteolytic cleavage removes an N-terminal fragment, including the critical phenylalanine residue, which opens an entrance for substrates. Therefore no new arrangement of the active site, with its two copper atoms and the ؊ 2 : 2 bound O 2 molecule, is necessary to develop the catalytic function.

Section: Resultssupporting

confidence: 74%

Section: Discussionmentioning

confidence: 73%

Section: Discussionmentioning

confidence: 85%

See 1 more Smart Citation

Tarantula Hemocyanin Shows Phenoloxidase Activity

Decker

Rimke

1998

205

139

“…The high degree of similarity between the active sites of phenoloxidases and hemocyanins of arthropods and molluscs is also supported by the capability of hemocyanins to exhibit O-diphenoloxidase activity after exposure to chaotrophic salts, SDS, or low pH values (22,23). The hemocyanin from a chelicerate, the tarantula Eurypelma californicum, has shown phenoloxidase activity after proteolysis (24).…”

mentioning

confidence: 91%

SDS-induced Phenoloxidase Activity of Hemocyanins fromLimulus polyphemus, Eurypelma californicum, andCancer magister

Decker

Ryan

Jaenicke

et al. 2001

170

Phenoloxidase, widely distributed among animals, plants, and fungi, is involved in many biologically essential functions including sclerotization and host defense. In chelicerates, the oxygen carrier hemocyanin seems to function as the phenoloxidase. Here, we show that hemocyanins from two ancient chelicerates, the horseshoe crab Limulus polyphemus and the tarantula Eurypelma californicum, exhibit O-diphenoloxidase activity induced by submicellar concentrations of SDS, a reagent frequently used to identify phenoloxidase activity. The enzymatic activity seems to be restricted to only a few of the heterogeneous subunits. These active subunit types share similar topological positions in the quaternary structures as linkers of the two tightly connected 2 ؋ 6-mers. Because no other phenoloxidase activity was found in the hemolymph of these animals, their hemocyanins may act as a phenoloxidase and thus be involved in the primary immune response and sclerotization of the cuticle. In contrast, hemolymph of a more recent arthropod, the crab Cancer magister, contains both hemocyanin with weak phenoloxidase activity and another hemolymph protein with relatively strong phenoloxidase activity. The chelicerate hemocyanin subunits showing phenoloxidase activity may have evolved into a separate phenoloxidase in crustaceans.

“…Arthropod prophenoloxidases have been sequenced, indicating that they are closely related to hemocyanins (15)(16)(17). Furthermore, phenoloxidase activities have been reported for arthropod and molluscan hemocyanins (18,19). Prophenoloxidases have not been identified in chelicerates, but the induction of prophenoloxidase activity in hemolymph is evident by treatment with either detergents or lipids in the American horseshoe crab Limulus polyphemus (20).…”

mentioning

confidence: 99%

Functional Conversion of Hemocyanin to Phenoloxidase by Horseshoe Crab Antimicrobial Peptides

Nagai

Osaki

Kawabata

2001

180

110

Arthropod hemocyanins and phenoloxidases serve different physiological functions as oxygen transporters and enzymes involved in defense reactions, respectively. However, they are equipped with a structurally similar oxygen-binding center. We have shown that the clotting enzyme of the horseshoe crab, Tachypleus tridentatus, functionally converts hemocyanin to phenoloxidase by forming a complex without proteolytic cleavage (Nagai, T., and Kawabata, S. (2000)