Tropinone reductases, enzymes at the branch point of tropane alkaloid metabolism

• Published in: Phytochemistry (Oxford) Vol. 67; no. 4; pp. 327 - 337
• Main Author: Drager, B
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 01.02.2006; Elsevier
• Subjects: alcohol oxidoreductases; Alcohol Oxidoreductases - chemistry; Alcohol Oxidoreductases - genetics; Alcohol Oxidoreductases - metabolism; Alkaloids - metabolism; Amino Acid Sequence; Atropine - metabolism; Belladonna Alkaloids - chemistry; Belladonna Alkaloids - metabolism; Binding Sites; Biological and medical sciences; Calystegine; enzyme activity; enzyme substrates; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation; Hyoscyamine; Kinetics; literature reviews; Metabolism; Metabolism. Physicochemical requirements; Molecular Sequence Data; NADP - metabolism; Plant physiology and development; protein structure; pseudotropine; Scopolamine; Scopolamine Hydrobromide - metabolism; Short-chain dehydrogenase; Solanaceae; Solanaceae - metabolism; Stereoisomerism; Substrate Specificity; Tropane alkaloid; tropane alkaloids; Tropanes - metabolism; tropinone; Tropinone reductase; Hyoscyamine; Short-chain dehydrogenase; Scopolamine; Tropane alkaloid; Calystegine; Solanaceae; Tropinone reductase; Enzyme; Metabolism; Tropane derivatives; EC 1.1.1.236; Characterization; Alkaloid; Enzymatic activity; Dicotyledones; Angiospermae; Spermatophyta; Oxidoreductases; Kinetics
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2005.12.001

Two specific tropinone reductases produce tropine or pseudotropine. Enzymes were isolated, characterised, cloned, and expressed from Solanaceae. Stereospecificity of reduction is achieved by binding tropinone in the enzymes’ active centres in opposite orientation. Two stereospecific oxidoreductases constitute a branch point in tropane alkaloid metabolism. Products of tropane metabolism are the alkaloids hyoscyamine, scopolamine, cocaine, and polyhydroxylated nortropane alkaloids, the calystegines. Both tropinone reductases reduce the precursor tropinone to yield either tropine or pseudotropine. In Solanaceae, tropine is incorporated into hyoscyamine and scopolamine; pseudotropine is the first specific metabolite on the way to the calystegines. Isolation, cloning and heterologous expression of both tropinone reductases enabled kinetic characterisation, protein crystallisation, and structure elucidation. Stereospecificity of reduction is achieved by binding tropinone in the respective enzyme active centre in opposite orientation. Immunolocalisation of both enzyme proteins in cultured roots revealed a tissue-specific protein accumulation. Metabolite flux through both arms of the tropane alkaloid pathway appears to be regulated by the activity of both enzymes and by their access to the precursor tropinone. Both tropinone reductases are NADPH-dependent short-chain dehydrogenases with amino acid sequence similarity of more than 50% suggesting their descent from a common ancestor. Putative tropinone reductase sequences annotated in plant genomes other that Solanaceae await functional characterisation.