葡萄糖-6-磷酸脱氢酶基因的Candida tropicalis过量表达及其对木糖醇合成代谢的影响 高雪楠1,张 婵1,*,尹 胜2,张秋晨2,王成涛1,2,*,徐宝财1,2 (1.北京工商大学 北京市食品添加剂工程技术研究中心,北京 100048; 2. 北京工商大学 食品质量与安全北京实验室,北京 100048)
摘 要:研究葡萄糖-6-磷酸脱氢酶基因g6pd过量表达对Candida tropicalis木糖醇生物合成代谢的影响。克隆Candida tropicalis CT16的g6pd基因,并将其与表达载体pYES-pgk重组连接,构建重组载体pYES-pgk-g6pd,LiAc/ssDNA/PEG 关键词:Candida tropicalis;葡萄糖-6-磷酸脱氢酶;g6pd基因;过量表达;木糖醇
Cloning and Overexpression of Glucose-6-phosphate dehydrogenase in Candida tropicalis and Its Influence on Xylitol Biosynthesis
Gao Xue-nan1 , Zhang Chan1,*, Yin Sheng2, Zhang Qiu-chen2, Wang Cheng-tao1,2,*, Xu Bao-cai1,2 (1. Beijing Engineering and Technology Research Center of Food Additives, Beijing Technology and Business University,
Abstract: The influence of overexpression of glucose 6-phosphate dehydrogenase (G6PDH) gene, g6pd, on xylitol biosynthesis in Candida tropicalis was investigated. The gene g6pd was cloned from Candida tropicalis CT16 and inserted into a yeast expression vector pYES-pgk, generating a recombinant expression vector pYES-pgk-g6pd, which was then introduced into C. tropicalis CT16 by the LiAc/ssDNA/PEG transformation method, resulting in over-expression of the g6pd gene. The fermentation results showed that the G6PDH activity was increased by 300%, and a maximum xylitol yield of 79.90 g/L was achieved in the recombinant strain SYG5 harboring pYES-pgk-g6pd after 62 h of fermentation. Compared to the wild type strain C. tropicalis CT16, the yield and productive rate of xylitol in strain SYG5 were increased by 11.30% and 44.91%, respectively. These results indicate that increasing the expression level of the gene g6pd significantly enhances xylitol production and that the G6PDH plays a key role in the biosynthesis pathway of xylitol in C. tropicalis. Key words: Candida tropicalis; glucose 6-phosphate dehydrogenase (G6PDH); gene g6pd over-expression; xylose; xylitol 中图分类号:Q789 文献标志码:A 文章编号:1002-6630(2014)07-0102-05 doi:10.7506/spkx1002-6630-201407021 木糖醇是甜度最高的糖醇,其甜度相当于蔗糖,热量只有蔗糖的40%,并具有清凉感,因其具有防龋齿、洁齿、润喉、改善胃肠功能等特点,已广泛应用于口香糖、胶姆糖、太妃糖、软糖、巧克力、果冻、冷饮、口含片、漱口剂、润喉药物、止咳糖浆中;木糖醇代谢不需要胰岛素调节,不会导致人体血糖的剧烈变化,可作为糖尿病人的甜味剂;木糖醇可促进肝糖元合成,减少脂肪和肝组织中蛋白质消耗,具有改善肝功能和抗脂肪肝的作用,可作为肝炎、高血压、高血脂和年老体弱病人的专用输液剂。近年来,随着人们生活质量提高,木糖醇的健康功效越来越多被消费者所接受[1-3]。全球木糖醇年需求量10万t以上,我是世界木糖醇的主要生产和出口国,年产量占世界总产量约40%,我国木糖醇年需求量2.5万t以上。目前木糖醇主要通过化学合成制造,需使用有毒性的镍催化剂和高压氢气,要求原料木糖的纯度高,工艺复杂、安全性差、生产成本高、副产物成分复杂,环境污染严重,不符合节能减排、可持续发展的社会要求,成为目前国家严格限制发展的行业[4]。 已有研究发现,酵母菌可转化木糖生成木糖醇,该生化过程的原料木糖无需纯化制备,反应条件温和,无需耐高压设备,此“清洁”、“绿色”的生产技术成为近年研究热点[5-7]。木糖醇的生物合成与磷酸戊糖途径(pentose phosphate pathway,PPP)紧密相连,需要木糖还原酶、还原性辅酶NADPH等参与(木糖+NADPH 木糖还原酶 木糖醇+NADP+),此还原反应是木糖醇能否顺利连续合成的关键步骤[8-10],NADPH主要是通过PPP途径产生,当PPP途径受到破坏,木糖醇产量则明显降低[11]。关于增强PPP途径的代谢通量分配及NADPH的再生,改善PPP途径以提高木糖醇产率的研究已有一些报道[12-19]。Kang等[15]研究认为,以木糖为底物发酵时,PPP途径的多种酶活力远远高于以葡萄糖为底物时的情况。Choi等[16]研究认为,葡萄糖-6-磷酸脱氢酶(glucose 6-phosphatedehydrogenase,G6PDH)是PPP途径中催化生成NADPH的关键性调控限速酶,因此,提高G6PDH的酶活力是增加NADPH供应量的有效手段之一。Kwon等[17] 1 材料与方法 1.1 材料与试剂 C. tropicalis CT16 本实验室保存;Escherichia coli TOP10感受态细胞 天根生化科技(北京)有限公司;酵母表达载体pYES-pgk 本实验室构建保存。 酵母基因组提取试剂盒、酵母质粒提取试剂盒、抗生素G418 北京天根生化科技有限公司;DNA Marker 北京全式金公司;EX Taq DNA聚合酶、限制性内切酶、T4 DNA连接酶 日本TaKaRa公司;酵母提取物、胰蛋白胨和蛋白胨 英国Oxoid公司;木糖 山东福田科技集团;木糖醇(分析纯) 美国Sigma公司;NADP+、NADPH 半夏科技公司;其余试剂均为国产分析纯。 1.2 培养基 Luria-Bertani (LB)培养基(g/L):胰蛋白胨 10、酵母提取物5、NaCl 10;YPD (yeast extract peptone dextrose)培养基(g/L):葡萄糖20、蛋白胨20、酵母提取物10;木糖醇发酵培养基(g/L):木糖100、葡萄糖10、蛋白胨 1、酵母提取物0.5。 1.3 引物设计 根据GenBank中报道的C. tropicalis葡萄糖-6-磷酸脱氢酶基因g6pd编码序列(Accession No. XM_002548907),利用DNAMAN软件设计引物。 g6pd基因扩增引物:F-g6pd:5’-gg ggt acc atg tct tat gat tca ttc gg-3’(酶切位点KpnⅠ);R-g6pd:5’-gc tct aga tta gat ctt acc ttt gac at-3’(酶切位点XbaⅠ)。酵母转化子鉴定引物:F-T7:5’-CAG CTG TAA TAC GAC TCA CTA TAG GG-3’; 1.4 g6pd基因的PCR扩增 以C. tropicalis CT16的基因组为模板,设计引物F-g6pd和R-g6pd进行PCR扩增。反应体系如下:Ex Taq聚合酶0.3μL、模板2μL、上下游引物(10mmol/L)各1μL、dNTP混合物(2.5mmol/L)2μL、10×Ex Taq Buffer 2.5μL、ddH2O补至25μL。扩增条件:94℃预变性5min,94℃ 30s、55℃ 30s、72℃ 1.5min,30个循环,72℃ 10min。凝胶电泳检测PCR产物。将纯化的PCR产物送至上海生工(生物)工程技术有限公司测序,DNAMAN软件和NCBI(National Center for Biotechnology Information)网站在线BLAST程序(http://blast.ncbi. nlm.nih.gov/Blast.cgi)进行序列分析和同源性搜索比对。 1.5 表达载体pYES-pgk-g6pd的构建 表达载体pYES-pgk-g6pd的构建如图1所示。KpnⅠ和XbaⅠ分别对g6pd基因片段和pYES-pgk进行双酶切,纯化回收的酶切片断经T4 DNA Ligase于16℃连接过夜,连接产物转化E. coli TOP10感受态细胞,在添加抗生素G418的LB平板上筛选阳性重组子,37℃培养过夜,提取阳性菌落质粒鉴定。
图 1 表达载体pYES-pgk-g6pd的构建策略 Fig.1 Construction strategy of the expression vector pYES-pgk-g6pd 1.6 g6pd基因在C. tropicalis中的过量表达 利用LiAc/ssDNA/PEG方法将重组表达载体pYES-pgk-g6pd转化至C. tropicalis CT16感受态细胞中,在添加G418的YPD平板上筛选阳性重组子。提取重组子质粒,设计引物F-T7和R-CYC1进行PCR扩增,回收目的条带,酶切、测序鉴定重组子。C. tropicalis感受态细胞的制备及其转化方法参照文献[20]。 1.7 葡萄糖-6-磷酸脱氢酶(G6PDH)的酶活力检测 取30h的发酵液,1100r/min、4℃离心20min,灭菌蒸馏水洗涤、重悬、再离心,菌体重悬于缓冲液体系:10mmol/L β-巯基乙醇、2mmol/L甘氨酸、0.071mol/L Tris-HCl(pH7.5),并加入玻璃珠(体积比1∶1)振荡破碎,离心取上清液(粗酶液)用于酶活力分析[13]。 反应体系(2.56mL):2mL 70 mmol/L Tris-HCl(pH7.5)、0.4 mL 35 mmol/L MgCl2、20μL 131 mmol/L NADP+、40μL 500 mmol/L 6-磷酸-葡萄糖、0.1 mL粗酶液。30 ℃、340 nm测定NADPH吸收值变化。340 nm波长处每隔30s测定一次吸光度,反应10 min,以吸光度y与NADPH浓度x(mmol/L)作标准曲线。G6PDH酶活力定义:反应体系中每分钟还原1μL NADP+所需的酶量为1U。 1.8 发酵实验及木糖和木糖醇的高效液相色谱(high performance liquid chromatography,HPLC)检测 将C. tropicalis CT16野生型和重组菌株C. tropicalis SYG5 以3%的接种量分别接种到木糖醇发酵培养基,30 ℃、200r/min振荡培养,每隔10h取发酵液样品,8000r/min离心4 min,HPLC检测木糖及木糖醇浓度。HPLC(岛津LC-20A)检测条件为:色谱柱HPX-87H(300mm×7.8mm),示差折光检测器,柱温45 ℃,流动相0.5mmol/L H2SO4,流速0.5 mL/min,进样量20μL。 2 结果与分析 2.1 葡萄糖-6-磷酸脱氢酶基因g6pd的PCR扩增 以C. tropicalis CT16基因组为模板,扩增目的基因片段,电泳、测序。如图2所示,得到的PCR产物单一DNA片段大小约为1.5kb,与预期相符。将纯化的PCR产物测序,经DNAMAN软件和NCBI比对,与GenBank中报道的C. tropicalis基因g6pd(Accession No. XM_002548907)相似度达100%,表明g6pd基因扩增正确。
M.DNA Marker D2000;1.g6pd基因PCR产物。 图 2 g6pd基因PCR产物电泳图 Fig.2 Electrophoresis of the PCR product of the g6pd gene 2.2 表达载体pYES-pgk-g6pd的构建 使用KpnⅠ和XbaⅠ双酶切鉴定重组质粒,电泳检测如图3所示。重组质粒pYES-pgk-g6pd经双酶切得到6.0kb和1.5kb的清晰条带,与预期相符,证明表达载体构建正确。
M.DNA Marker Trans2k plus;1.pYES-pgk-g6pd双酶切产物;2.pYES-pgk双酶切产物。 图 3 KpnⅠ+XbaⅠ双酶切重组质粒pYES-pgk-g6pd电泳图 Fig.3 Electrophoresis of the plasmid pYES-pgk-g6pd double digested with KpnⅠ+ XbaⅠ 2.3 重组C. tropicalis的筛选、鉴定及酶活力分析 表达载体pYES-pgk-g6pd转入C. tropicalis CT16后,在含抗生素G418的YPD平板上进行筛选。从平板上挑取长势较好的单菌落,在含1mg/mL G418的培养基培养,筛选获得阳性转化子C. tropicalis SYG5。 提取C. tropicalis SYG5质粒,设计引物F-T7和R-CYC1-Age PCR扩增,电泳检测结果如图4所示。从转化子(工程菌)SYG5中扩增g6pd基因表达盒(包括启动子、g6pd基因编码序列和终止子)的特异性条带,大小约为1.9kb,与预期相符,且测序验证正确,表明重组质粒pYES-pgk-g6pd已转入C. tropicalis CT16。 分析30h发酵粗酶液的G6PDH酶活力,以吸光度y与NADPH浓度x(mmol/L)作标准曲线,获得方程y=0.0055x-0.0073(R2=0.9999)。C. tropicalis CT16 G6PDH的酶活力为786 U/L,C. tropicalis SYG5的G6PDH酶活力为3145U/L,说明g6pd基因的过量表达能显著增强C. tropicalis G6PDH的活性。
M.DNA Marker D2000;1. g6pd 基因表达盒的PCR产物。 图 4 重组菌株C. tropicalis SYG5中g6pd基因表达盒的PCR产物电泳图 Fig.4 Electrophoresis of the PCR product of the g6pd gene expression cassette from the recombinant strain C. tropicalis SYG5 2.4 野生菌和工程菌转化木糖醇能力比较 将野生型菌C. tropicalis CT16和重组工程菌
图 5 木糖和木糖醇的HPLC分析 Fig.5 HPLC analysis of xylose and xylitol 如图6所示,工程菌C. tropicalis SYG5的木糖醇生成量随发酵时间延长逐渐增多,摇瓶发酵62h时木糖醇质量浓度达到最高值79.90 g/L,木糖醇产率为1.29 g/(L•h);
图 6 C. tropicalis CT16与C. tropicalis SYG5摇瓶发酵实验中的木糖和木糖醇质量浓度动态变化曲线 Fig.6 Dynamic curves of xylitol production and xylose consumption in C. tropicalis CT16 and C. tropicalis SYG5 during fermentation on xylose and glucose in shaking flasks 3 结 论 本实验构建了葡萄糖-6-磷酸脱氢酶g6pd基因的重组表达载体pYES-pgk-g6pd,并将其过量表达于C. tropicalis CT16,探索g6pd基因过量表达对木糖醇合成代谢影响。结果表明,发酵62h时,携带重组质粒pYES-pgk-g6pd的工程菌C. Tropicalis SYG5摇瓶发酵木糖醇产量为79.90g/L, 参考文献: [1] Emodi A. Xylitol: its properties and food applications[J]. Food Technology, 1978, 32(1): 28-32. [2] Makinen K K. Xylitol and oral health[J]. Advance in Food Research, 1979, 25: 137-158. [3] Prakasham R S, Rao R S, Hobbs P J. Current trends in biotechnological production of xylitol and future prospects[J]. Current Trends in Biotechnology and Pharmacy, 2009, 3(1): 8-36. [4] 成英, 闫书磊, 明立雪. 木糖醇的生产工艺及应用研究进展[J]. 甘肃石油和化工, 2008, 22(3): 18-21. [5] Ko C H, Chiu P C, Yang C L, et al. Xylitol conversion by fermentation using five yeast strains and polyelectrolyte-assisted ultrafiltration[J]. Biotechnology Letters, 2008, 30(1): 81-86. [6] Sánchez S, Bravo V, García J F, et al. Fermentation of D-glucose and D-xylose mixtures by Candida tropicalis NBRC 0618 for xylitol production[J]. World Journal of Microbiology and Biotechnology, 2008, 24(5): 709-716. [7] IN S. Hydrolysate detoxification with activated charcoal for xylitol production by Candida guilliermondii[J]. Biotechnology Letters, 2001, 23(20): 1681-1684. [8] Moreira dos Santos M, Thygesen G, Kötter P, et al. Aerobic physiology of redox-engineered Saccharomyces cerevisiae strains modified in the ammonium assimilation for increased NADPH availability[J]. FEMS Yeast Research, 2003, 4(1): 59-68. [9] Kim J H, Han K C, Koh Y H, et al. Optimization of fed-batch fermentation for xylitol production by Candida tropicalis[J]. Journal of Industrial Microbiology and Biotechnology, 2002, 29(1): 16-19. [10] Nolleau V, Preziosi-Belloy L, Navarro J M. The reduction of xylose to xylitol by Candida guilliermondii and Candida parapsilosis: incidence of oxygen and pH[J]. Biotechnology Letters, 1995, 17(4): 417-422. [11] Leathers T D, Dien B S. Xylitol production from corn fibre hydrolysates by a two-stage fermentation process[J]. Process Biochemistry, 2000, 35(8): 765-769. [12] Oh Y J, LEE T H, Lee S H, et al. Dual modulation of glucose 6-phosphate metabolism to increase NADPH-dependent xylitol production in recombinant Saccharomyces cerevisiae[J]. Journal of Molecular Catalysis B: Enzymatic, 2007, 47(1): 37-42. [13] Chin J W, KHANKAL R, Monroe C A, et al. Analysis of NADPH supply during xylitol production by engineered Escherichia coli[J]. Biotechnology and Bioengineering, 2009, 102(1): 209-220. [14] Fernandes S, TUOHY M G, Murray P G. Cloning, heterologous expression, and characterization of the xylitol and L-arabitol dehydrogenase genes, Texdh and Telad, from the thermophilic fungus Talaromyces emersonii[J]. Biochemical Genetics, 2010, 48(5/6): 480-495. [15] Kang H Y, Kim Y S, Kim G J, et al. Screening and characterization of flocculent yeast, Candida sp. HY200, for the production of xylitol from D-xylose [J]. Journal of Microbiology and Biotechnology, 2005, 15(2): 362-367. [16] Choi J H, Moon K H, Ryu Y W, et al. Production of xylitol in cell recycle fermentations of Candida tropicalis[J]. Biotechnology Letters, 2000, 22(20): 1625-1628. [17] Kwon D H, Kim M D, Lee T H, et al. Elevation of glucose 6-phosphate dehydrogenase activity increases xylitol production in recombinant Saccharomyces cerevisiae[J]. Journal of Molecular Catalysis B: Enzymatic, 2006, 43(1): 86-89. [18] Verho R, Londesborough J, Penttilä M, et al. Engineering redox cofactor regeneration for improved pentose fermentation in Saccharomyces cerevisiae[J]. Applied and Environmental Microbiology, 2003, 69(10): 5892-5897. [19] Ahmad I, Shim W Y, Jeon W Y, et al. Enhancement of xylitol production in Candida tropicalis by co-expression of two genes involved in pentose phosphate pathway[J]. Bioprocess and Biosystems Engineering, 2012, 35(1/2): 199-204. [20] 安伯格 D C. 酵母遗传学方法实验指南[M]. 霍克克, 译. 2版. 北京: 科学出版社, 2009: 98-99. 收稿日期:2013-09-17 基金项目:国家高技术研究发展计划(863计划)项目(2012AA021502);北京市自然科学基金项目(5122008); 教育部科学技术研究重点项目(211101);北京市教委科技面上项目(KM201110011001) 作者简介:高雪楠(1988—),女,硕士研究生,研究方向为食品生物技术。E-mail:945310757@qq.com *通信作者:张婵(1984—),女,讲师,博士,研究方向为食品生物技术。E-mail:zhangchan@th.btbu.edu.cn 王成涛(1969—),男,教授,博士,研究方向为食品生物技术。E-mail:wct5566@163.com |