A study of type-3 copper proteins from arthropods

Abstract

Arthropod hemocyanin and phenoloxidase are members of a group of proteins called the Type-3 copper oxygen-binding proteins, both possessing a highly conserved oxygen-binding site containing two copper atoms each coordinated by three histidine residues (Decker and Tuczek, 2000). Despite similarities in their active site, these proteins have very different physiological functions. Phenoloxidase possesses both tyrosinase and o-diphenoloxidase activity, and is predominantly involved in reactions which protect insects from infection (Kopàcek et al., 1995). Hemocyanin is a large multi-subunit protein with a primary function as a respiratory protein, reversibly binding and transporting molecular O2 (Decker and Rimke, 1998; Decker and Tuczek, 2000).

Recently, it has been demonstrated in vitro that arthropod hemocyanin possesses an inducible phenoloxidase activity when incubated with denaturants, detergents, phospholipids or proteolytic enzymes. This activity appears to be restricted to only a few subunit types, and it has been hypothesised that it may be accompanied by conformational change which opens the active site increasing access for larger phenolic substrates (Decker and Jaenicke, 2004; Decker et al., 2001; Decker and Tuczek, 2000). This possibly suggests a dual role of hemocyanin in arthropods.

The presented thesis deals with two distinct aims. The first was to isolate and sequence a phenoloxidase gene from the insect Spodoptera littoralis (Egyptian Cottonleaf Worm). Despite efforts, progress was hindered by a number of experimental problems which are outlined within the relevant chapters. The second aim was to characterise the mode of SDS induced phenoloxidase activity in arthropod hemocyanin from the ancient chelicerates Limulus polyphemus (horseshoe crab) and Eurypelma californicum (tarantula) and the more modern chelicerate Pandinus imperator (scorpion), using a number of biophysical techniques. The results indicated that the SDS induced phenoloxidase activity is associated with localised tertiary and secondary conformational changes in hemocyanin, most likely in the vicinity of the dicopper centre, thus enhancing access for larger phenolic substrates. Experiments indicate that copper remains associated with the protein during these structural changes; however the nature of the association is unclear. SDS concentrations approximating the CMC appeared critical in causing the necessary structural changes required for a significant increase in the detectable phenoloxidase activity to be exhibited.