Tcherkez Guillaume
Laboratoire d'Ecophysiologie Végétale, CNRS UMR 8079, Bâtiment 362, Université Paris XI, 91405 Orsay, France. Email.
Funct Plant Biol. 2006 Oct;33(10):911-920. doi: 10.1071/FP06098.
Despite the intense effort developed over the past 10 years to determine the C / C isotope fractionation associated with photorespiration, much uncertainty remains about the amplitude, and even the sign, of the C / C isotope fractionation of glycine decarboxylase, the enzyme that produces CO during the photorespiratory cycle. In fact, leaf gas-exchange data have repeatedly indicated that CO evolved by photorespiration is depleted in C compared with the source material, while glycine decarboxylase has mostly favoured C in vitro. Here I give theoretical insights on the glycine decarboxylase reaction and show that (i), both photorespiration and glycine decarboxylation must favour the same carbon isotope - the in vitro measurements being probably adulterated by the high sensitivity of the enzyme to assay conditions and the possible reversibility of the reaction in these conditions, and (ii), simplified quantum chemistry considerations as well as comparisons with other pyridoxal 5'-phosphate-dependent decarboxylases indicate that the carbon isotope fractionation favour the C isotope by ~20‰, a value that is consistent with the value of the photorespiratory fractionation (f) obtained by gas-exchange experiments.
尽管在过去10年里人们付出了巨大努力来确定与光呼吸相关的碳/碳同位素分馏，但对于光呼吸循环中产生二氧化碳的甘氨酸脱羧酶的碳/碳同位素分馏的幅度，甚至符号，仍存在许多不确定性。事实上，叶片气体交换数据反复表明，与源物质相比，光呼吸释放的二氧化碳中碳含量较低，而甘氨酸脱羧酶在体外大多有利于碳。在此，我对甘氨酸脱羧酶反应给出了理论见解，并表明：（i）光呼吸和甘氨酸脱羧都必须有利于相同的碳同位素——体外测量可能因酶对测定条件的高敏感性以及这些条件下反应的可能可逆性而受到影响；（ii）简化的量子化学考虑以及与其他依赖磷酸吡哆醛的脱羧酶的比较表明，碳同位素分馏有利于碳同位素约20‰，这一数值与气体交换实验获得的光呼吸分馏（f）值一致。
