The biosynthetic route from ornithine to proline

Area of Science:

• Biochemistry
• Plant Physiology
• Metabolic Pathways

Background:

• Ornithine is a precursor amino acid in plants.
• The metabolic conversion of ornithine to proline is crucial for plant development.
• The established pathway for this conversion involves glutamic gamma-semialdehyde.

Purpose of the Study:

• To elucidate the precise metabolic pathway of ornithine to proline conversion in plants.
• To identify the specific hydrogen atoms involved and their fate during the conversion.
• To validate or refute the currently accepted metabolic route.

Main Methods:

• Utilizing isotopically labeled ornithine (DL-[2-3H,5-14C]- and DL-[(RS)-5-3H,5-14C]) as tracers.
• Conducting metabolic conversion experiments in three distinct plant species: Nicotiana tabacum, Datura stramonium, and Lupinus angustifolius.
• Analyzing the retention or loss of specific hydrogen isotopes (delta-hydrogen and alpha-hydrogen) in the resulting proline molecules.

Main Results:

• Tracer experiments demonstrated the retention of delta-hydrogen atoms during ornithine to proline conversion.
• A significant loss of the alpha-hydrogen atom was observed in the same metabolic process.
• These findings were consistent across all three tested plant species.

Conclusions:

• The observed hydrogen atom retention and loss patterns indicate a metabolic route proceeding via alpha-keto-delta-aminovaleric acid (5-amino-2-oxopentanoic acid).
• This proposed pathway, involving alpha-keto-delta-aminovaleric acid, is supported by the experimental data.
• The established route via glutamic gamma-semialdehyde (2-amino-5-oxopentanoic acid) is disproven by these findings, necessitating a revision of the understanding of ornithine metabolism in plants.