A mechanistic model of the cysteine synthase complex

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Plants and bacteria convert inorganic sulfur into organic compounds, primarily cysteine.
• Cysteine biosynthesis relies on the cysteine synthase (CS) complex, comprising serine acetyl transferase (SAT) and O-acetyl-serine-(thiol)-lyase) (OAS-TL).
• The precise function, regulation, and structure of the CS complex remain incompletely understood.

Purpose of the Study:

• To computationally model the structure and interactions within the mitochondrial Arabidopsis thaliana cysteine synthase (CS) complex.
• To elucidate the mechanism of reciprocal regulation between SAT and OAS-TL enzymes.
• To provide mechanistic insights into CS complexation and its impact on cysteine biosynthesis.

Main Methods:

• Comparative modeling and de novo structure prediction for SAT and OAS-TL enzymes.
• Rigid-body Brownian dynamics simulations to model diffusional encounter complexes.
• Molecular dynamics simulations to refine encounter complexes into bound structures.
• Binding energy calculations and computational mutagenesis to assess complex stability and formation.

Main Results:

• A stoichiometric ratio of 2 OAS-TL dimers to 1 SAT hexamer is energetically favored for the CS complex, aligning with experimental observations.
• Computational mutagenesis identified key OAS-TL residues critical for enzyme association, with alterations leading to reduced binding affinity.
• Cross-species binding analysis suggested that interactions between Arabidopsis thaliana OAS-TL and E. coli SAT could impair cysteine production.

Conclusions:

• The study presents a plausible structural model for the Arabidopsis thaliana CS complex, offering insights into its formation and regulation.
• The findings highlight the importance of specific enzyme-enzyme interactions and stoichiometry in controlling cysteine biosynthesis.
• The research provides a foundation for further experimental validation and understanding of sulfur assimilation pathways.