Накопление витамина С в сочных плодах: биосинтез и рециркуляция, гены и ферменты

L-аскорбиновая кислота (витамин C) – вторичный метаболит растений, выполняющий множество разнообразных функций как в растительных тканях, так и в организме человека. Основным источником витамина С в питании человека служат растения, и прежде всего плоды цитрусовых, шиповника, перца, смородины, томата, клубники, папайи, киви. Однако, несмотря на то что L-аскорбиновая кислота – важное биологически активное вещество, путь ее биосинтеза в растительной клетке был описан лишь в 2007 г. на примере модельного растения Arabidopsis thaliana. В настоящем обзоре рассмотрены известные на сегодняшний день пути биосинтеза L-аскорбиновой кислоты в тканях растений. Это L-галактозный, L-гулозный, галактуроновый и мио-инозитоловый пути. Наиболее изучен из них L-галактозный путь (путь Смирнова–Уилера), для которого определены все ферменты, катализирующие последовательную цепь реакций. Для других путей известна лишь предположительная последовательность метаболитов, при этом многие ферменты, катализирующие их превращение, еще не выявлены. Выделены ключевые гены, которые участвуют в биосинтезе и накоплении аскорбиновой кислоты в сочных плодах. Среди них ферменты L-галактозного пути (ГДФ-маннозофосфорилаза (GMP, VTC1), ГДФ-D-маннозо-3’5’-эпимераза (GME), ГДФ-L-галактозофосфорилаза (GGP, VTC2/VTC5), L-галактозо-1-фосфатфосфатаза (GPP/VTC4), L-галактозо-1-дегидрогеназа (GalDH) и L-галактоно1,4-лактондегидрогеназа (GalLDH)); ферменты D-галактуронового пути (NADPH-зависимая D-галактуронатредуктаза (GalUR)) и ферменты рециркуляции АК (дегидроаскорбатредуктаза (DHAR1) и монодегидро аскорбатредуктаза (MDHAR)). До сих пор нет однозначного описания всех путей биосинтеза и накопления L-аскорбиновой кислоты в плодах. В настоящее время нельзя однозначно утверждать, что какой-то из четырех известных путей биосинтеза аскорбиновой кислоты является преобладающим в плодах растений. Так, в плодах персика и киви основным является L-галактозный путь, тогда как в плодах винограда и клубники – по всей видимости, D-галактуроновый. В то же время у ряда растений, например цитрусовых или томата, по мере созревания плодов может происходить смена различных путей биосинтеза. Отмечается, что уровни накопления аскорбиновой кислоты зависят не только от биосинтеза, но и от скорости ее окисления и рециркуляции.

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