Plant tropane alkaloid biosynthesis evolved independently in the Solanaceae and Erythroxylaceae

Abstract

The pharmacologically important tropane alkaloids have a scattered distribution among angiosperm families, like many other groups of secondary metabolites. To determine whether tropane alkaloids have evolved repeatedly in different lineages or arise from an ancestral pathway that has been lost in most lines, we investigated the tropinone-reduction step of their biosynthesis. In species of the Solanaceae, which produce compounds such as atropine and scopolamine, this reaction is known to be catalyzed by enzymes of the short-chain dehydrogenase/reductase family. However, in Erythroxylum coca (Erythroxylaceae), which accumulates cocaine and other tropane alkaloids, no proteins of the shortchain dehydrogenase/reductase family were found that could catalyze this reaction. Instead, purification of E. coca tropinonereduction activity and cloning of the corresponding gene revealed that a protein of the aldo-keto reductase family carries out this reaction in E. coca. This protein, designated methylecgonone reductase, converts methylecgonone to methylecgonine, the penultimate step in cocaine biosynthesis. The protein has highest sequence similarity to other aldo-keto reductases, such as chalcone reductase, an enzyme of flavonoid biosynthesis, and codeinone reductase, an enzyme of morphine alkaloid biosynthesis. Methylecgonone reductase reduces methylecgonone (2-carbomethoxy-3-tropinone) stereospecifically to 2-carbomethoxy-3β-tropine (methylecgonine), and has its highest activity, protein level, and gene transcript level in young, expanding leaves of E. coca. This enzyme is not found at all in root tissues, which are the site of tropane alkaloid biosynthesis in the Solanaceae. This evidence supports the theory that the ability to produce tropane alkaloids has arisen more than once during the evolution of the angiosperms.

A Root-Expressed l-Phenylalanine:4-Hydroxyphenylpyruvate Aminotransferase Is Required for Tropane Alkaloid Biosynthesis in Atropa belladonna

“…However, recent evidence suggests independent evolution of pathways leading to the reduction of tropinone in the Solanaceae and Erythroxylaceae (Jirschitzka et al, 2012). Decoration of the tropine skeleton by a phenyllactate molecule to form littorine, and subsequently hyoscyamine and scopolamine, is a reaction that predominantly occurs in specific genera of the Solanaceae, and the identity of the enzymes involved in these conversions are largely unresolved (Griffin and Lin, 2000;Humphrey and O'Hagan, 2001).…”

Tropane and Granatane Alkaloid Biosynthesis: A Systematic Analysis

“…Namely, have these biosynthetic pathways arisen from a common ancestor or have they arisen independently in several cases? Recent evidence in TA biosynthesis suggests that certain biosynthetic steps have multiple origins [5]. By extension, this could also mean that GA biosynthesis has arisen independently more than once.…”

“…TR I only converts the 3-keto function to a product that has a 3α-configuration (Scheme 3). This produces tropine (13) (3α-tropanol), which serves as a precursor for a wide range of esterified TAs [5]. On the contrary, TR II produces an alcohol solely with a 3β-configuration called pseudotropine (3β-tropanol).…”

“…This would explain the presence of the carbomethoxy group found in cocaine (1) as well as other tropane alkaloids produced by plants found in the Erythroxylaceae. The resulting compound methylecgonone (21), contains a keto function at the C3 position and a carbomethoxy group at the C2 position [5]. Reduction at C3 is necessary for ester formation to occur.…”

“…Despite their similarities, TAs and GAs show different distribution patterns across the plant kingdom. The N-methyl-8-azabicyclo[3.2.1]-octane core structure of TAs is found in over 200 alkaloids [2,5,6] (Figure 1). In contrast, N-methyl-9-azabicyclo [3.3.1]-nonane, the core scaffold of GAs, appears in considerably fewer alkaloid metabolites ( Figure 1).…”