title: Cloning of Arabidopsis serotonin N-acetyltransferase and its role with caffeic acid O-methyltransferase in the biosynthesis of melatonin in vitro despite their different subcellular localizations.
abstract: Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme in melatonin biosynthesis. We cloned SNAT from Arabidopsis thaliana (AtSNAT) and functionally characterized this enzyme for the first time from dicotyledonous plants. Similar to rice SNAT, AtSNAT was found to localize to chloroplasts with peak enzyme activity at 45 °C (Km , 309 μm; Vmax , 1400 pmol/min/mg protein). AtSNAT also catalyzed 5-methoxytryptamine (5-MT) into melatonin with high catalytic activity (Km , 51 μm; Vmax , 5300 pmol/min/mg protein). In contrast, Arabidopsis caffeic acid O-methyltransferase (AtCOMT) localized to the cytoplasm. Interestingly, AtCOMT can methylate serotonin into 5-MT with low catalytic activity (Km , 3.396 mm; Vmax , 528 pmol/min/mg protein). These data suggest that serotonin can be converted into either N-acetylserotonin by SNAT or into 5-MT by COMT, after which it is metabolized into melatonin by COMT or SNAT, respectively. To support this hypothesis, serotonin was incubated in the presence of both AtSNAT and AtCOMT enzymes. In addition to melatonin production, the production of major intermediates depended on incubation temperatures; N-acetylserotonin was predominantly produced at high temperatures (45 °C), while low temperatures (37 °C) favored the production of 5-MT. Our results provide biochemical evidence for the presence of a serotonin O-methylation pathway in plant melatonin biosynthesis.
authors.affiliation: Department of Biotechnology, Bioenergy Research Center, Chonnam National University, Gwangju, South Korea.
authors.forename: Hyoung Yool
authors.initials: HY
authors.lastname: Lee
authors.forename: Yeong
authors.initials: Y
authors.lastname: Byeon
authors.forename: Kyungjin
authors.initials: K
authors.lastname: Lee
authors.forename: Hye-Jung
authors.initials: HJ
authors.lastname: Lee
authors.forename: Kyoungwhan
authors.initials: K
authors.lastname: Back
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology*; D047068:Arylalkylamine N-Acetyltransferase / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology; D003001:Cloning, Molecular; D003593:Cytoplasm / Q000201:enzymology; D066298:In Vitro Techniques; D008550:Melatonin / Q000096:biosynthesis*; D008780:Methyltransferases / Q000378:metabolism*; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013347:Subcellular Fractions
chemical_list: D008780:Methyltransferases; C019608:caffeate O-methyltransferase; D047068:Arylalkylamine N-Acetyltransferase; D008550:Melatonin
keywords: 5-methoxytryptamine; Arabidopsis thaliana; caffeic acid O-methyltransferase; serotonin N-acetyltransferase; substrate inhibition