Staphylococcal secondary structure

The radius of gyration of staphylococcal nuclease (SNase), measured by SAXS, increased from 1.6 nm in the native state to 3.3 nm in 8 M urea [103], The latter value is however well below the one (4.3 nm) that is expected for (Rg)e [97], showing that SNase is one of the few proteins that are not completely unfolded in 8 M urea. SNaseD, the truncated form of SNase lacking the C-terminal 13 residues, is devoid of secondary structure under physiological conditions but is rather compact (/ g = 2 nm) for most of its mutated forms, Rg was found to be around 1.7 nm, and for a few ones it is up to 3.5... 

Table 1. Secondary structure contents of staphylococcal a-toxin (Tobkes et al., 1985)...

Middlebrook, J.L., Spero, L. and Argos, P. 1980. The secondary structures of staphylococcal enterotoxins A,... 

Fig. 2). The staphylococcal enzyme may appear to be more akin in its mode of action to the spleen enzyme because they both hydrolyze DNA and RNA to 3 -nucleotides, whereas the venom enzyme releases 5 -nucleotides. However, their mode of action and specificity are quite different, and the structural requirements of the staphylococcal enzyme substrates are perhaps more nearly similar to those of the venom enzyme. The principal difference is that the staphylococcal enzyme cleaves the diester bond between the phosphate and the 5 -carbon of the sugar, whereas the venom enzyme cleaves on the other side of the phosphate, that is, between the phosphate and the nonspecific hydroxylic component of the diester bond. In contrast to both spleen and venom diesterases, the primary product released by staphylococcal nuclease hydrolysis is a derivative bearing a hydroxyl group (on the 5 position) rather than a phosphoryl group. Therefore, the 3 -phosphoryl product formed from polynucleotide hydrolysis is a secondary consequence of such cleavage. 

---