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Proteins utilizing pyridoxal 5'-phosphate as a coenzyme constitute a large superfamily and are currently classified into three functional groups and five structural fold types. Despite the variability of sequences and catalyzed reactions, they share relevant structural, dynamic and functional properties. Therefore, they constitute an optimal system to investigate the relative influence of primary sequence and coenzyme interactions on folding pathways, structural stability and enzymatic function. O-Acetylserine sulfhydrylase is a dimeric pyridoxal 5'-phosphate dependent enzyme that catalyzes the synthesis of L-cysteine from O-acetylserine and sulfide. The time-resolved fluorescence study of O-acetylserine sulfhydrylase unfolding, here reported, indicates that the coenzyme stabilizes the protein structure. The dependence on denaturant concentration of tryptophan lifetimes in the holo- and apo-enzyme demonstrates that the interactions with the coenzyme stabilize the C-terminal domain to a higher extent with respect to the N-terminal domain. This result is discussed in terms of a linkage between the differential stabilization brought about by the coenzyme and the different degrees of conformational flexibility required by the specialized functional role of distinct protein regions.

The electrical conduction due to trapped ele£ trom with energy higher than the potential barrier (Poole-Frenkel effect) is claimed to take part in several materials /1/. Much less is known about the current due to electrons whose energy is lower than the trapping well. A classical model is here formulated to take into account such contribution and the theoretical results are compared with tho se experimental ly obtained in CdF 2 :Gd crystals.In these substances, where the electrical con duction is due to a trapping mechanism between lo calized levels, the mean free path of the carriers is determined by the random distribution of the � nized impurity centres. Their density can be equr lized to that of free electrons with energy £ in the conduction band, that is:24 If £�kT with the temperature T=lOOK, one has n�lO m -3 , indicating that the average distance between o charged Gd centres is d � 100 A. On the other hand the de-Broglie wavelength of the electron is: I A = h/((£-V (x» (2m»2 where £�kT and Vex) is the potential wave function: 2 Vex) = e /4rr IxlE ( 2) with � =9G o being the dielectric constant of the o ma terial /2/. Since A