Systematics of 2H patterns in natural compounds and its importance for the elucidation of biosynthetic pathways

Abstract

The relative global 2H-content of natural plant products is correlated to that of the primary hydrogen source, i.e. water, to the site of their biosynthesis (C3-, C4- and CAM-plants; chloroplasts, cytosol), and to their biosynthetic pathways. A relative global 2H-content sequence can be established in the order phenylpropanoids > carbohydrates > bulk material > hydrolysable lipids > steroids. A detailed analysis of the 2H-patterns of the main groups of secondary compounds reveals regularities, in that they are correlated to the primary precursors and to the origin of hydrogen from four main pools with the mean δ2H-values [‰]V−SMOW: leaf H2O ∼+30; carbohydrates ∼−70; NADPH ∼−250; flavoproteins ∼−350. Aside from the 2H-discrimination between these pools, kinetic isotope effects on defined reactions only become effective in connection with metabolic branching events. So, the 2H-pattern of natural aromatic compounds can be correlated to the 2H-pattern of the precursor carbohydrates and a reduction step in the course of the shikimic acid pathway, furthermore to the implication of the NIH-shift. The pattern of aromatic compounds from the polyketide is different from that of the shikimate pathway. The alternating 2H-abundance of fatty acid chains is caused by the origin of their hydrogen atoms from carbohydrates and from NADPH, directly or via a flavoprotein, respectively. This is similar for isoprenoids, and the natural 2H-patterns permit their assignment to the mevalonate or non-mevalonate biosynthetic pathway. Generally, the correlations and regularities of the 2H-patterns of organic compounds found are a new reliable tool for the elucidation of biosynthetic pathways and origin assignments.