Physical properties of small functional units obtained from gastropod mollusc haemocyanins by tryptic digestion.

The haemocyanins of gastropod molluscs are giant proteins of mol.wt. approx. 9000000 whose function is the transport of oxygen in the blood. It is well established that the functional unit, which binds one molecule of 02, is a pair of copper atoms surrounded by a folded polypeptide chain to form a compact domain of mol.wt. approx. 50000 (Van Breemen et al., 1977). However, treatment of the protein with such reagents as 6~-guanidinium chloride or sodium dodecyl sulphate fails to produce a subunit of this size, but appears to reveal a minimum polypeptide chain of molecular weight in the region of 300000 (Wood &Peacocke, 1973; Brouwer & Kuiper, 1973). Functional subunits of mol.wt. 50000 have so far only been obtained by limited proteolytic digestion of the native protein (Pearson & Wood, 1974). Electron-microscopic studies have revealed necklace-like strings of linked domains and give valuable clues as to how the native molecule is constructed (Van Breeman et al., 1977). Moreover work on the limited proteolytic digestion of Helix pomatia 8-haemocyanin and Lymnaea stagnalis haemocyanin suggests that the domains may differ from one another in their structure and properties (Gielens et al., 1977; Wood & Mosby, 1977). In the present work we have studied the effect of trypsin hydrolysis on the haemocyanins from a marine gastropod, Buccinum undatum, and from a freshwater gastropod, Lymnaea stagnalis, which are known to differ in their oxygen-binding properties as well as in amino acid and carbohydrate composition (Hall & Wood, 1976; Hall et al., 1977). It is hoped that studies on the fundamental building-blocks of the molecule will