N-methyltransferases and 7-methyl-N9-nucleoside hydrolase activity in Coffea arabica and the biosynthesis of caffeine |
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Authors: | Margaret F Roberts George R Wallert |
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Institution: | 1. Department of Pharmacognosy, The School of Pharmacy, University of London, 29–39 Brunswick Square, London, WC1N IAX, U.K.;2. Biochemistry Department, Oklahoma State University, Stillwater, OK 74074, U.S.A. |
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Abstract: | The incorporation of radioactivity from L-14CH3]-methionine into caffeine by coffee fruits was enhanced by additions of theobromine and paraxanthine but was reduced by additions of theophylline and caffeine. Cell-free extracts prepared from seedlings, partially ripe and unripe coffee fruits showed that only the unripe green fruits contained significant methyltransferase and 7-methyl-N9-nucleoside hydrolase activity. The cell-free extracts catalysed the transfer of methyl groups fromS-adenosyl-L-14CH3]-methionine to 7-methylxanthine, and 7-methylxanthosine, producing theobromine and to theobromine producing caffeine. The two enzymic methylations exhibited a sharp pH max at 8.5 and a similar pattern of effects with metal chelators, thiol reagents and Mg2+ ions, which were slightly stimulating though not essential to enzyme activity. Paraxanthine (1,7-dimethylxanthine) was sh own to be the most active among methylxanthines as methyl acceptors; however its formation from 1-methylxanthine and 7-methylxanthine was not detectable, and biosynthesis from paraxanthine in the intact plant would therefore appear not to occur. The apparent Km values are as follows: 7-methylxanthine 0.2 mM, theobromine 0.2 mM, paraxanthine 0.07 mM and S-adenosyl-L-methionine with each substrate 0.01 mM. The results suggest the pathway for caffeine biosynthesis in Coffea arabica is: 7-methylxanthosine → 7-methylxanthine → theobromine → caffeine. |
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Keywords: | Rubiaceae biosynthesis caffeine theobromine: theophylline paraxanthine enzymology |
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