Half-Barrels Derived from a (β/α)(8) Barrel β-Glycosidase Undergo an Activation Process

The evolution of (β/α)(8) barrel proteins is currently thought to have involved the fusion of two (β/α)(4) half-barrels, thereby conferring stability on the protein structure. After the formation of a whole (β/α)(8) barrel, this structure could evolve and diverge to form fully active enzymes. Interestingly, we show here that isolated (β/α)(4) half-barrels derived from the N- and C-terminal domains of the β-glucosidase Sfβgly (Sfβgly-N: residues 1 to 265; Sfβgly-C: residues 266 to 509) undergo an activation process, which renders them catalytically active. The rate constants of the activation process were calculated to be 0.029 and 0.032 h(-1) for Sfβgly-N and Sfβgly-C, respectively. Moreover, the Sfβgly-N and Sfβgly-C activation processes were simultaneous with modifications in their initial structure, which reduced the exposure of their tryptophan residues. Importantly, this activation was also coincident with an increase in the sizes of Sfβgly-N and Sfβgly-C particles. These novel observations suggest that the change in catalytic activity associated with the transition from a half to whole (β/α)(8) barrel might also have driven such an evolutionary process.