A two-step process controls the formation of the bienzyme cysteine synthase complex

Area of Science:

• Biochemistry
• Enzymology
• Metabolic Regulation

Background:

• Enzyme activity regulation via multiprotein assemblies is crucial for biosynthetic pathways.
• Cysteine synthase (CS), composed of O-acetylserine sulfhydrylase (OASS) and serine acetyltransferase (SAT), is a key regulatory complex.
• CS enzymes are central to sulfur, carbon, and nitrogen assimilation.

Purpose of the Study:

• To elucidate the mechanism of cysteine synthase complex formation.
• To understand the regulatory roles of cysteine and bisulfide in CS assembly.
• To clarify the function of CS in sulfur assimilation control in bacteria and plants.

Main Methods:

• Stopped-flow fluorescence spectroscopy was employed.
• Enzyme interactions were studied at varying temperatures and pH.
• The influence of physiological regulators (cysteine and bisulfide) was investigated.

Main Results:

• CS complex formation proceeds in two steps: rapid encounter complex formation followed by slow isomerization.
• Bisulfide stabilizes the CS complex by reducing the rate of the reverse isomerization step.
• Cysteine slightly destabilizes the CS complex by affecting formation kinetics.

Conclusions:

• The study reveals a detailed mechanism for CS assembly and regulation.
• Understanding CS complex dynamics provides insights into sulfur assimilation regulation.
• The findings highlight the distinct roles of bisulfide and cysteine in modulating CS activity.