Effects of extracellular pH on the metabolic pathways in sulfur-deprived, H2-producing Chlamydomonas reinhardtii cultures
Effects of extracellular pH on the metabolic pathways in sulfur-deprived, H2-producing Chlamydomonas reinhardtii cultures
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DOI：
10.1093/pcp/pcg020
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发表时间：
2003-02-01
影响因子：
4.9
通讯作者：
Ghirardi, ML
中科院分区：
文献类型：
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作者：
Kosourov, S;Seibert, M;Ghirardi, ML
通过在无硫的介质中培养细胞，可以获得绿藻Chlamydomonas rehardtii持续的光产氢[Ghirardi等人。(2000B)Trends BiotechnoL 18：506；Melis et al.(2000)植物生理学。122：127]。目前的工作集中在(A)不同的胞外初始PHS对缺硫藻细胞光系统II(PSII)失活和对O-2敏感的H-2产生活性的影响，以及(B)不同pH条件下H-2产生与光合作用、有氧和厌氧代谢的关系。H-2产生的最大速率和产率出现在缺硫开始时的pH为7.7时，当初始pH降低到6.5或增加到8.2时，H-2产生的速率和产率都下降。光合作用产生氢气的pH曲线与PSII剩余活性相关(最适pH 7.3-7.9)，但与光系统I的光合作用电子传递或淀粉和蛋白质降解的pH曲线无关。在此pH范围内的体外氢酶活性远高于实际的H-2产生速率，表明氢酶活性本身并不受限制。淀粉和蛋白质的分解代谢产生甲酸盐、乙酸盐和乙醇；为H-2光产生贡献一些还原剂，如3-(3，4-二氯苯基)1，1-二甲基脲和2，5-dibromo-6-isopropyl-3-methyl-1，4-benzoquinone抑制结果所示；是呼吸过程中去除光合成产生的O-2的主要还原剂来源。碳平衡表明，在不同的pH下，替代代谢途径占主导地位，这些途径取决于是否存在剩余的光合作用活性。
Sustained photoproduction of H, by the green alga, Chlamydomonas reinhardtii, can be obtained by incubating cells in sulfur-deprived medium [Ghirardi et al. (2000b) Trends BiotechnoL 18: 506; Melis et al. (2000) Plant Physiol. 122: 127]. The current work focuses on (a) the effects of different initial extracellular pHs on the inactivation of photosystem II (PSII) and O-2-sensitive H-2-production activity in sulfur-deprived algal cells and (b) the relationships among H-2-production, photosynthetic, aerobic and anaerobic metabolisms under different pH regimens. The maximum rate and yield of H-2 production occur when the pH at the start of the sulfur deprivation period is 7.7 and decrease when the initial pH is lowered to 6.5 or increased to 8.2. The pH profile of hydrogen photoproduction correlates with that of the residual PSII activity (optimum pH 7.3-7.9), but not with the pH profiles of photosynthetic electron transport through photosystem I or of starch and protein degradation. In vitro hydrogenase activity over this pH range is much higher than the actual in situ rates of H-2 production, indicating that hydrogenase activity per se is not limiting. Starch and protein catabolisms generate formate, acetate and ethanol; contribute some reductant for H-2 photoproduction, as indicated by 3-(3,4-dichlorophenyl)1,1-dimethylurea and 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone inhibition results; and are the primary sources of reductant for respiratory processes that remove photo-synthetically generated O-2. Carbon balances demonstrate that alternative metabolic pathways predominate at different pHs, and these depend on whether residual photosynthetic activity is present or not.