Zhang Zhisheng, Mao Xingxue, Ou Juanying, Ye Nenghui, Zhang Jianhua, Peng Xinxiang
State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, South China Agricultural University, Guangzhou, China; Laboratory of Molecular Plant Physiology, College of Life Sciences, South China Agricultural University, Guangzhou, China; State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Hong Kong, China.
Institute of Rice Research, Guangdong Academy of Agricultural Sciences, Guangzhou, China.
J Photochem Photobiol B. 2015 Jan;142:110-7. doi: 10.1016/j.jphotobiol.2014.11.009. Epub 2014 Dec 12.
The metabolic function of glutamate:glyoxylate aminotransferase (GGAT) and serine:glyoxylate aminotransferase (SGAT) for photorespiration is still not exactly understood so far though it is mostly held that both enzymes may work in parallel in the reaction of glyoxylate to glycine during photorespiration of plants. Here, for the first time, we define the genes encoding GGAT and SGAT and report their biochemical and enzymatic properties in rice plants, in contrast to those from other plant species. Noticeably, GGAT exhibited approximately 18 fold higher catalytic efficiency (Kcat/Km) with glyoxylate and glutamate than SGAT with glyoxylate and serine, and additionally, rice leaves usually contain 3-4times higher abundance of glutamate relative to serine, implicating that GGAT may preferentially utilize glyoxylate to form glycine over SGAT. When SGAT or GGAT activity was regulated by gene transformation or nitrogen deficiency, respectively, it was observed that the glycine content was positively related to GGAT activities, while both serine and glycine contents were negatively related to SGAT activities. The results suggest that GGAT preferentially catalyzes the conversion of glyoxylate into glycine while SGAT is mainly responsible for the transamination reaction of serine to hydroxypyruvate in the photorespiratory pathway of rice.
乙醛酸转氨酶（GGAT）和丝氨酸：乙醛酸转氨酶（SGAT）在光呼吸中的代谢功能至今仍未完全明确，尽管大多观点认为在植物光呼吸过程中，这两种酶在乙醛酸转化为甘氨酸的反应中可能并行发挥作用。在此，我们首次鉴定了编码GGAT和SGAT的基因，并报道了它们在水稻植株中的生化和酶学特性，这与其他植物物种不同。值得注意的是，相对于SGAT催化乙醛酸和丝氨酸的反应，GGAT催化乙醛酸和谷氨酸的反应时，其催化效率（Kcat/Km）高出约18倍，此外，水稻叶片中谷氨酸的丰度通常比丝氨酸高3 - 4倍，这意味着相较于SGAT，GGAT可能更优先利用乙醛酸来形成甘氨酸。当分别通过基因转化或氮缺乏来调节SGAT或GGAT的活性时，观察到甘氨酸含量与GGAT活性呈正相关，而丝氨酸和甘氨酸含量均与SGAT活性呈负相关。结果表明，在水稻的光呼吸途径中，GGAT优先催化乙醛酸转化为甘氨酸，而SGAT主要负责丝氨酸向羟基丙酮酸的转氨反应。
