l-Threonine Transaldolase Activity Is Enabled by a Persistent Catalytic Intermediate.

Area of Science:

• Biochemistry
• Enzymology
• Natural Product Biosynthesis

Background:

• l-Threonine transaldolases (lTTAs) are pyridoxal-5'-phosphate (PLP)-dependent enzymes involved in β-hydroxy amino acid biosynthesis.
• ObiH is an lTTA crucial for synthesizing the β-lactone obafluorin, but its catalytic mechanism is poorly understood.
• ObiH purification yields a mixture of states, including an inactive PLP cofactor form, requiring photoexcitation for activation.

Purpose of the Study:

• To elucidate the catalytic mechanism of ObiH, an lTTA.
• To understand the structural basis for ObiH's unique reactivity in β-hydroxy amino acid synthesis.
• To explore ObiH's synthetic utility for creating novel amino acid derivatives.

Main Methods:

• UV-vis spectroscopy to analyze reaction intermediates and kinetics.
• X-ray crystallography to determine the high-resolution structure of ObiH bound to PLP.
• Molecular dynamics simulations and mutagenesis studies to investigate structural dynamics and functional roles.

Main Results:

• ObiH catalyzes the retro-aldol cleavage of l-threonine, forming a stable glycyl quinonoid intermediate (t½ ≈ 3 h).
• Kinetically disfavored protonation of the intermediate allows efficient reaction with aldehydes, yielding β-hydroxy amino acids.
• The crystal structure reveals a unique active site with a Glu residue instead of the conserved Asp, influencing cofactor interaction.
• Structural rearrangement upon l-threonine binding was observed via simulations and mutagenesis, explaining substrate entry.

Conclusions:

• The unique active site configuration and dynamic rearrangement explain the distinct reactivity of lTTAs like ObiH.
• ObiH demonstrates synthetic potential, enabling efficient one-step synthesis of (2S,3R)-β-hydroxyleucine.
• This work provides mechanistic insights and a structural foundation for engineering lTTAs for biocatalysis.