13C isotope effect on the reaction catalyzed by prephenate dehydratase

Abstract

The 13C isotope effect for the conversion of prephenate to phenylpyruvate by the enzyme prephenate dehydratase from Methanocaldococcus jannaschii is 1.0334 ± 0.0006. The size of this isotope effect suggests that the reaction is concerted. From the X-ray structure of a related enzyme, it appears that the only residue capable of acting as the general acid needed for removal of the hydroxyl group is threonine-172, which is contained in a conserved TRF motif. The more favorable entropy of activation for the enzyme-catalyzed process (25 eu larger than for the acid-catalyzed reaction) has been explained by a preorganized microenvironment that obviates the need for extensive solvent reorganization. This is consistent with forced planarity of the ring and side chain, which would place the leaving carboxyl and hydroxyl out of plane. Such distortion of the substrate may be a major contributor to catalysis.

Redesign of MST enzymes to target lyase activity instead promotes mutase and dehydratase activities

“…Prephenate dehydratase activity is a concerted decarboxylation reaction assisted by general acid catalysis for removal of the hydroxyl group (Figure 6B). Aromatization is hypothesized to be promoted by a forcing of planarity of the ring (44) . One might imagine that a strategically placed general acid residue for the isochorismate synthase activity in the wildtype enzymes may serve a similar role in the variants for prephenate dehydratase activity (compare Figures 6A and 6B).…”

Structure-guided discovery of the metabolite carboxy-SAM that modulates tRNA function

“…There is precedence for decarboxylation of prephenate, as prephenate dehydratase (PDT) catalyzes the decarboxylation of prephenate with concomitant loss of hydroxide to generate phenylpyruvate in a fashion similar to that suggested for CmoA 8 . In the PDT-catalyzed reaction, elimination of the hydroxyl group as water is facilitated by the participation of Thr172 as a general acid to protonate the leaving hydroxide group.…”

“…The threonine side-chain is not sufficiently acidic to directly protonate the prephenate hydroxyl (i.e., the conjugate acid of the prephenate hydroxyl is expected to behave similarly to an alcohol, with a p K a of ~-2, while the p K a of threonine is ~16 in water); thus, other mechanisms must be operative as the major driving force for this reaction. Hilvert and Cleland proposed that geometric distortion drives decarboxylation and the favorable energetics associated with aromatization reduces the bond order of the hydroxyl, shifting its reactivity towards that of hydroxide and allowing for efficient general acid catalysis 8 . Similar considerations are relevant to the CmoA-catalyzed reaction.…”

Olefin Isomerization Regiochemistries during Tandem Action of BacA and BacB on Prephenate in Bacilysin Biosynthesis

“…When canonical prephenate decarboxylases liberate CO 2 from prephenate, the electrons that flow into the 1,4-cyclohexadiene ring lead to aromatization as the C 7 -OH is ejected and phenylpyruvate is formed (17, 18). By contrast, the BacA subfamily enzymes, during the comparable decarboxylation, capture the electrons by protonation at one end of the starting 1,4-diene (Figure 1B).…”