Intermediate trapping via a conformational switch in the Na(+)-activated tryptophan synthase bienzyme complex

Area of Science:

• Biochemistry
• Enzyme kinetics
• Protein conformational dynamics

Background:

• Tryptophan synthase is a bienzyme complex facilitating indole channeling between alpha and beta active sites through a 25 Å tunnel.
• Enzyme activity relies on conformational switching between low-activity open states for substrate binding and high-activity closed states for reaction.

Purpose of the Study:

• To investigate the relationship between chemical reaction steps and conformational transitions in the tryptophan synthase catalytic cycle.
• To elucidate the role of alpha-site substrate binding in modulating beta-site reaction dynamics and product channeling.

Main Methods:

• Utilized dihydroiso-L-tryptophan (DIT) as an L-Trp analogue to probe the catalytic cycle.
• Performed kinetic analyses of DIT cleavage in the presence and absence of the alpha-site substrate analogue, alpha-D,L-glycerol phosphate (GP).
• Monitored the formation and dissipation of key reaction intermediates, including the quinonoid species (E(Q)(indoline)) and the alpha-aminoacrylate Schiff base (E(A-A)).

Main Results:

• DIT cleavage in the absence of GP shows slow accumulation of E(Q)(indoline) due to slow conversion of E(A-A).
• In the presence of GP, DIT cleavage leads to rapid formation and decay of E(Q)(indoline), indicating efficient reaction through the quinonoid state.
• GP binding to the alpha-site traps the released indoline within the enzyme complex by inducing a closed conformation, favoring E(Q)(indoline) accumulation.

Conclusions:

• The reaction of DIT proceeds rapidly through the quinonoid state to form indoline and E(A-A), irrespective of GP presence.
• Indoline sequestration within the closed enzyme complex, induced by GP binding, significantly enhances E(Q)(indoline) stability.
• Conformational switching to a closed state is crucial for trapping reaction intermediates and influencing the overall catalytic efficiency of tryptophan synthase.