A novel domain arrangement in a monomeric cyclodextrin-hydrolyzing enzyme from the hyperthermophile Pyrococcus furiosus.

PFTA (Pyrococcus furiosus thermostable amylase) is a hyperthermophilic amylase isolated from the archaeon Pyrococcus furiosus. This enzyme possesses characteristics of both α-amylase- and cyclodextrin (CD)-hydrolyzing enzymes, allowing it to degrade pullulan, CD and acarbose—activities that are absent in most α-amylases—without the transferring activity that is common in CD-hydrolyzing enzymes. The crystal structure of PFTA revealed a unique monomeric subunit with an extended N-terminal region and an N′-domain folded into its own active site—a significantly altered domain configuration relative to that of the conventional dimeric CD-hydrolyzing amylases in glycoside hydrolase family 13. The active site is formed by the interface of the N′-domain and the catalytic domain and exhibits a broad and wide-open geometry without the concave pocket that is commonly found in the active sites of maltogenic amylases. The mutation of a residue (Gly415 to Glu) located at the domain interface between the N′- and catalytic domains yielded an enzyme that produced a significantly higher purity maltoheptaose (G7) from β-CD, supporting the involvement of this interface in substrate recognition and indicating that this mutant enzyme is a suitable candidate for the production of pure G7. The unique configuration of the active site distinguishes this archaic monomeric enzyme from classical bacterial CD-hydrolyzing amylases and provides a molecular basis for its enzymatic characteristics and for its potential use in industrial applications.