The pathway of formate formation and coenzyme reduction during carbon monoxide respiration in Methanosarcina acetivorans

产甲烷古生菌被认为只通过电子传输链来保存能量，而电子传输链是由导致甲烷形成(甲烷生成)的反应提供燃料的。在甲烷链霉菌C2a在一氧化碳(CO)上生长的过程中，甲烷是通过已知的二氧化碳还原途径产生的，但在没有任何氢酶活性的情况下，如何还原辅因子F420是未知的，这是该途径中两个反应所必需的。令人惊讶的是，甲硫化物、乙酸盐和甲酸盐也是在碳酸营养条件下产生的。前两种代谢产物是如何形成的，是通过我们实验室的遗传和生理分析确定的。然而，甲酸盐的形成途径仍然难以捉摸，其形成速度甚至超过了产甲烷的速度，但甲酸脱氢酶显然没有参与其中。本项目的目的是阐明乙酰食单胞菌中CO生成甲酸盐的途径，从遗传和生化水平上表征所涉及的因子(S)，并表征辅酶F420和呼吸链中辅酶B和辅酶M的异二硫键还原的还原当量的途径。将采用突变分析和生化技术来实现这一目标。醋酸链霉菌的共代谢将作为甲烷八叠球菌代谢灵活性的范例，这在产甲烷菌中是无与伦比的，显然远远超出了人们所接受的教科书知识。

Methanogenic Archaea are thought to exclusively conserve energy through an electron transport chain fueled by reactions leading to methane formation (methanogenesis). During growth of Methanosarcina acetivorans C2A on carbon monoxide (CO) methane is generated via the known CO2 reduction pathway but how - in the absence of any hydrogenase activity - M. acetivorans reduces cofactor F420, which is required for two reactions in the pathway, is unknown. Surprisingly, methylsulfides, acetate and formate are also produced under carboxidotrophic conditions. How the former two metabolites are formed was established through genetic and physiological analyses in our laboratory. However, the pathway of formate formation, the rate of which exceeds even that of methanogenesis, remains elusive but formate dehydrogenase is apparently not involved. The aim of the proposed project is to elucidate the pathway of formate formation from CO in M. acetivorans, to characterize the factor(s) involved both on the genetic and on the biochemical level, and to characterize the path reducing equivalents take for reduction of coenzyme F420 and the heterodisulfide of coenzyme B and coenzyme M in the respiratory chain. Mutational analysis and biochemical techniques will be employed to achieve this goal. CO metabolism of M. acetivorans will serve as a paradigm for the metabolic flexibility of Methanosarcina species, which is unsurpassed among methanogens, and apparently far exceeds what is accepted as text-book knowledge.