Mazzafera P, Wingsle G, Olsson O, Sandberg G
S -Adenosyl -L -Methionine -Theobromine 1 -N -Methyltransferase, an Enzyme Catalyzing the Synthesis of Caffeine in Coffee
Phytochemistry: 1994 37:1577-1584
This paper presents data on the purification and N-terminal sequence of S-adenosyl-L-methionine: theobromine 1-N-methyltransferase (STM), the enzyme responsible for the methylation of theobromine leading to caffeine formation in coffee. STM was purified from developing endosperms of immature fruits by DEAF-cellulose, hydrophobic interaction and affinity chromatography, using S-adenosyl-L-homocysteine as a ligand. The enzyme showed an apparent M(r) of ca 54 000 and ca 60 000 determined by gel filtration and SDS-PAGE, respectively. A pI of 5.1 was obtained by liquid chromatofocusing and 4.8 by isoelectrofocusing in polyacrylamide gel electrophoresis. Using theobromine as substrate, the K-m value for S-adenosyl-L-methionine was 10 mu M, being competitively inhibited by S-adenosyl-L-homocysteine (K-i = 4.6 mu M). STM is a bifunctional enzyme since it also methylated 7-methylxanthine, the immediate precursor of theobromine in the caffeine biosynthesis pathway. The specific activity of STM with 7-methylxanthine was ca 55% of that determined with theobromine. The K-m values obtained for theobromine and 7-methylxanthine were 0.196 and 0.496 mM, respectively. STM was also purified from leaves using the same procedures used for endosperms, plus an additional chromatography on a Mono Q column, theobromine was used as substrate. Finally, the N-terminal sequence for the first 20 amino acids was obtained for STM purified from endosperms. No similarities were found with other methyltransferase sequences or other known proteins.
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