Temperature growth tests.

Anaerobic microbial degradation of the amino acids glycine and serine either occurs through enzymes of the so-called Stickland reaction or alternatively through the glycine cleavage system (GCS). In the mesophilic anaerobe Peptoclostridium acidaminophilum, initial glycine degradation proceeds through disproportionation to methylene tetrahydrofolate (THF) and acetyl-phosphate by GCS and glycine reductase. The thermophilic acetogen Thermacetogenium phaeum is able to utilize glycine, serine or threonine as sole carbon source, although it lacks genes for glycine reductase. In contrast, T. phaeum possesses genes for the GCS as well as for serine-converting enzymes, and the corresponding enzymes were specifically overabundant in the proteome and active. Among these enzymes, serine dehydratase was most active in serine-grown cells, even though its abundance in the proteome was comparably low. We suggest that two serine-converting enzyme systems (serine dehydratase, and the combination of glycine hydroxymethyltransferase and GCS) are used under different growth conditions: for breakdown of serine, T. phaeum most likely converts serine to pyruvate and ammonia by serine dehydratase, followed by acetate and ATP production via pyruvate dehydrogenase, phosphate acetyltransferase and acetate kinase. Electron carriers are then re-oxidized through CO2-fixation via the Wood-Ljungdahl pathway (WLP) of acetogenesis. When grown with glycine, the GCS most likely converts glycine to methylene-THF, which is then disproportionated to methenyl-THF and methyl-THF in the WLP. Glycine and serine both are excellent substrates for T. phaeum, yet in syntrophic cocultures with Methanothermobacter thermautotrophicus, acetate from glycine or serine degradation cannot be degraded further, as in syntrophic cultures with acetate as sole carbon source. This indicates an inhibitory or regulatory effect of glycine or serine degradation on acetate-degrading enzymes, resulting in the inability of T. phaeum to transition directly to syntrophic acetate oxidation after amino acid degradation.

氨基酸甘氨酸与丝氨酸的厌氧微生物降解，既可通过所谓施蒂克兰德反应（Stickland reaction）的相关酶类完成，亦可借助甘氨酸裂解系统（glycine cleavage system, GCS）实现。在嗜温厌氧菌解氨基拟梭菌（Peptoclostridium acidaminophilum）中，甘氨酸的初始降解经由GCS与甘氨酸还原酶催化的歧化反应，生成亚甲基四氢叶酸（methylene tetrahydrofolate, THF）与乙酰磷酸。嗜热产乙酸菌暗热产乙酸菌（Thermacetogenium phaeum）可利用甘氨酸、丝氨酸或苏氨酸作为唯一碳源生长，但其基因组中不携带甘氨酸还原酶的编码基因。与之相反，暗热产乙酸菌不仅拥有GCS的编码基因，同时还具备丝氨酸转化酶的编码基因，且其对应的酶类在蛋白质组中特异性富集并具有活性。在这些酶类中，丝氨酸脱水酶在以丝氨酸为唯一碳源培养的细胞中活性最高，尽管其在蛋白质组中的丰度相对较低。我们推测，两类丝氨酸转化酶系统（丝氨酸脱水酶，以及甘氨酸羟甲基转移酶与GCS的组合）会在不同生长条件下发挥功能：针对丝氨酸的降解，暗热产乙酸菌大概率通过丝氨酸脱水酶将丝氨酸转化为丙酮酸与氨，随后经由丙酮酸脱氢酶、磷酸乙酰转移酶与乙酸激酶完成乙酸与ATP的合成。电子载体随后通过产乙酸的伍德-隆德哈尔途径（Wood-Ljungdahl pathway, WLP）固定CO₂实现再氧化。当以甘氨酸为碳源培养时，GCS大概率将甘氨酸转化为亚甲基四氢叶酸，后者随后在伍德-隆德哈尔途径中经歧化反应生成次甲基四氢叶酸与甲基四氢叶酸。甘氨酸与丝氨酸均为暗热产乙酸菌的优质底物，但在与嗜热产甲烷杆菌（Methanothermobacter thermautotrophicus）的互养共培养体系中，由甘氨酸或丝氨酸降解产生的乙酸无法被进一步降解，这与以乙酸为唯一碳源的互养培养体系中的情况一致。这表明甘氨酸或丝氨酸的降解过程会对乙酸降解酶产生抑制或调控效应，使得暗热产乙酸菌无法在氨基酸降解后直接转向互养型乙酸氧化途径。