The discovery of nature's biosynthetic pathways

Area of Science:

• Biochemistry
• Organic Chemistry
• Molecular Biology

Background:

• Urogen-III is a vital intermediate in the biosynthesis of essential natural pigments, including haem, chlorophyll, and cytochromes.
• The complex synthesis of urogen-III's unique structure has been a long-standing puzzle in biological chemistry.

Purpose of the Study:

• To elucidate the biosynthetic pathway of urogen-III, focusing on the rearrangement mechanism and stereochemistry.
• To identify and characterize intermediates in the pathway from urogen-III to cobyrinic acid, a precursor to vitamin B12.

Main Methods:

• Utilized 13C-labelling studies to track the rearrangement steps in urogen-III biosynthesis.
• Employed stereospecific deuterium and tritium labelling to determine the absolute stereochemistry of haem vinyl group formation.
• Isolated and structurally characterized an octacarboxylic isobacteriochlorin intermediate from a vitamin B12-producing organism.

Main Results:

• Demonstrated that urogen-III synthesis involves a single intramolecular rearrangement occurring after the formation of the linear tetrapyrrole.
• Elucidated the absolute stereochemistry of the enzymatic reactions forming the vinyl groups of haem.
• Identified sirohydrochlorin as a key intermediate between urogen-III and cobyrinic acid, confirming its structure and stereochemistry.

Conclusions:

• The findings provide critical insights into the intricate enzymatic mechanisms underlying tetrapyrrole biosynthesis.
• The identification of sirohydrochlorin contributes to understanding the evolution of metabolic pathways for vital biomolecules.
• This research clarifies a significant gap in knowledge regarding the biosynthesis of essential biological pigments and cofactors.