A monofunctional and thermostable prephenate dehydratase from the archaeon Methanocaldococcus jannaschii.
Prephenate dehydratase (PDT) is an important but poorly characterized enzyme that is involved in the production of L-phenylalanine. Multiple-sequence alignments and a phylogenetic tree suggest that the PDT family has a common structural fold. On the basis of its sequence, the PDT from the extreme thermophile Methanocaldococcus jannaschii (MjPDT) was chosen as a promising representative of this family for pursuing structural and functional studies. The corresponding pheA gene was cloned and expressed in Escherichia coli. It encodes a monofunctional and thermostable enzyme with an N-terminal catalytic domain and a C-terminal regulatory ACT domain. Biophysical characterization suggests a dimeric (62 kDa) protein with mixed alpha/beta secondary structure elements. MjPDT unfolds in a two-state manner (Tm = 94 degrees C), and its free energy of unfolding [DeltaGU(H2O)] is 32.0 kcal/mol. The purified enzyme catalyzes the conversion of prephenate to phenylpyruvate according to Michaelis-Menten kinetics (kcat = 12.3 s-1 and Km = 22 microM at 30 degrees C), and its activity is pH-independent over the range of pH 5-10. It is feedback-inhibited by L-phenylalanine (Ki = 0.5 microM), but not by L-tyrosine or L-tryptophan. Comparison of its activation parameters (DeltaH(++)= 15 kcal/mol and DeltaS(++)= -3 cal mol-1 K-1) with those for the spontaneous reaction (DeltaH(++) = 17 kcal/mol and DeltaS(++)= -28 cal mol-1 K-1) suggests that MjPDT functions largely as an entropy trap. By providing a highly preorganized microenvironment for the dehydration-decarboxylation sequence, the enzyme may avoid the extensive solvent reorganization that accompanies formation of the carbocationic intermediate in the uncatalyzed reaction.
Kleeb
Andreas C
AC
Laboratory of Organic Chemistry, ETH Zurich, Hönggerberg HCI F 339, CH-8093 Zurich, Switzerland.
Kast
Peter
P
Hilvert
Donald
D
D000595:Amino Acid Sequence; D001483:Base Sequence; D002384:Catalysis; D003001:Cloning, Molecular; D017931:DNA Primers; D004795:Enzyme Stability; D007700:Kinetics; D017017:Methanococcus / Q000201:enzymology*; D008969:Molecular Sequence Data; D011302:Prephenate Dehydratase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013816:Thermodynamics
D017931:DNA Primers; D011994:Recombinant Proteins; D011302:Prephenate Dehydratase