产甲烷生化代谢途径研究进展

Research progress in biochemical pathways of methanogenesis

微生物产甲烷过程产生的甲烷约占全球甲烷产量的74%.产甲烷过程对生物燃气生产和全球气候变暖等都有着重要的意义.本文综述了产甲烷菌的具体生化代谢途径,其本质是产甲烷菌利用细胞内一系列特殊的酶和辅酶将CO2或甲基化合物中的甲基通过一系列的生物化学反应还原成甲烷.在这一过程中,产甲烷菌细胞能够形成钠离子或质子跨膜梯度,驱动细胞膜上的ATP合成酶将ADP转化成ATP以获得能量.根据底物类型的不同,可以将该过程分为3类:还原CO2途径、乙酸途径和甲基营养途径.还原CO2途径是以H2或甲酸作为主要的电子供体还原CO2产生甲烷,其中涉及到一个最新的发现——电子歧化途径;乙酸途径是乙酸被裂解产生甲基基团和羧基基团,随后,羧基基团被氧化产生电子供体H2用于还原甲基基团;甲基营养途径是以简单甲基化合物作为底物,以外界提供的H2或氧化甲基化合物自身产生的还原当量作为电子供体还原甲基化合物中的甲基基团.通过这3种途径产甲烷的过程中,每消耗1mol底物所产生AT P的顺序为还原CO2途径>甲基营养途径>乙酸途径.由于产甲烷菌自身难以分离培养,未来将主要通过现代的生物技术和计算机技术,包括基因工程和代谢模型构建等最新技术来研究产甲烷菌的生化代谢过程以及其与其它菌群之间的相互作用机制,以便将其应用于生产实践.

Microbial methanogenesis accounts for approximately 74% of natural methane emission.The process plays a major role in global warming and is important for bioenergy production.This paper reviews the biochemical pathways of methanogenesis.It is currently recognized that methanogenesis proceeds via three biochemical pathways depending on the carbon sources,including hydrogenotrophic,aceticlastic,and methylotrophic methanogenesis.Multiple enzymes and coenzymes are involved in the process,during which Na~＋ or proton gradient is created across the cell membrane,contributing to limited ATP synthesis.In the hydrogenotrophic pathway,CO2 is reduced to methane with H2 or formate as an electron donor.In the aceticlastic pathway,acetate is split into methyl and carboxyl group,then the carboxyl group is oxidized to produce H2 which is used as the electron donor to reduce methyl group.In the methylotrophic pathway,methyl group is reduced with external H2 or reducing equivalent from the oxidation of its own methyl group.The ATP gained from per mol substrate for different pathways are as follows：hydrogenotrophic 〉methylotrophic 〉aceticlastic pathway.Due to the unculturability of most archaeal methanogens,understandings of the biochemical pathways of methanogenesis and the relationships between methanogens and other microbial communities will have to depend on new technologies including bioinformatics,gene engineering and metabolic modelling.