Acetohydroxy Acid Synthase Activity in Chlorella emersonii under Auto- and Heterotrophic Growth Conditions

Area of Science:

• Biochemistry
• Enzymology
• Algal Physiology

Background:

• Acetohydroxyacid synthase (AHAS) is a key enzyme in the biosynthesis of branched-chain amino acids.
• Understanding AHAS regulation is crucial for metabolic studies in algae.
• Chlorella emersonii serves as a model organism for studying algal biochemistry.

Purpose of the Study:

• To investigate Acetohydroxyacid synthase (AHAS) activity and regulation in Chlorella emersonii.
• To compare AHAS activity in autotrophically and heterotrophically grown cells.
• To determine the direct target of sulfometuron methyl (SMM) in C. emersonii.

Main Methods:

• Enzyme assays were performed on crude extracts from Chlorella emersonii.
• Kinetic parameters (Km) for pyruvate and substrate preference were determined.
• Sulfometuron methyl (SMM) and valine inhibition studies were conducted.
• Isolation and characterization of SMM-resistant mutants were performed.

Main Results:

• A single AHAS enzyme was identified, with consistent kinetic properties in both autotrophic and heterotrophic conditions.
• The enzyme exhibited a Km for pyruvate of 9 mM and a 22-fold preference for 2-ketobutyrate.
• Sulfometuron methyl (SMM) caused 50% inhibition at 0.3 μM, and valine caused 50% inhibition at 0.5 mM.
• SMM-resistant mutants possessed SMM-resistant AHAS activity, indicating AHAS as the sole target of SMM.

Conclusions:

• Acetohydroxyacid synthase (AHAS) is the direct and sole target of sulfometuron methyl (SMM) in Chlorella emersonii.
• The same AHAS enzyme functions under both autotrophic and heterotrophic growth conditions.
• Valine and isoleucine partially alleviate SMM's inhibition of biomass increase but not cell division.