Rubisco supplies pyruvate for the 2-C-methyl-D-erythritol-4-phosphate pathway

Area of Science:

• Plant biochemistry
• Metabolic pathways
• Photosynthesis

Background:

• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is crucial for carbon fixation.
• The source of chloroplast pyruvate for biosynthesis has been debated, termed the 'pyruvate paradox'.
• The 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway requires pyruvate for plastidic isoprenoid biosynthesis.

Purpose of the Study:

• To identify the primary source of chloroplast pyruvate in photosynthetically active plant tissues.
• To investigate the role of Rubisco's side reaction in supplying pyruvate for metabolic pathways.
• To understand the regulation of pyruvate production under varying oxygen conditions.

Main Methods:

• Utilized 13C-labeling studies in intact Arabidopsis plants.
• Performed metabolome analysis of mutants deficient in pyruvate transport.
• Conducted biochemical characterization of isolated chloroplasts.
• Assessed Rubisco activity in vitro.

Main Results:

• Rubisco's β-elimination activity produces significant amounts of pyruvate in chloroplasts.
• This Rubisco-derived pyruvate is a key precursor for isoprenoid, fatty acid, and branched-chain amino acid synthesis.
• Pyruvate sourcing shifts developmentally, with seedlings relying on exogenous pyruvate and adult plants on Rubisco production.
• Low oxygen levels enhance Rubisco activity, increasing pyruvate production and MEP pathway flux.

Conclusions:

• Rubisco's side reaction is the primary source of chloroplast pyruvate, resolving the 'pyruvate paradox'.
• This finding directly links carbon assimilation via the Calvin-Benson-Bassham cycle to the MEP pathway.
• The study provides crucial insights for refining metabolic models of plant central metabolism.