Mechanism of alkane formation by cyanobacterial aldehyde decarbonylase

在化学部生命过程化学奖中，来自密歇根大学的E.Neil Marsh博士将阐明从蓝藻中新发现的一种碳氢化合物形成酶--乙醛脱羧酶(CAD)的机制。产生碳氢化合物的生物合成途径最近已经成为人们强烈关注的主题，因为它们产生新生物燃料的潜力已经被认识到。乙醛脱羧酶是一类研究较多的非血红素二铁双加氧酶。初步研究表明，CAD催化了一种最不寻常的反应，包括将长链脂肪醛转化为相应的烷烃和甲酸盐。为了研究其机理，将合成可作为机理抑制剂的替代底物，以获得反应中可能存在的自由基中间体的证据。将研究还原系统在反应中的作用，并将鉴定宿主蓝藻P.marinus中天然还原系统的组成。定点突变将被用来研究活性位点残基在决定底物专一性和控制反应机制中的作用。最后，将测量动力学同位素效应，以深入了解CAD中决定速度的步骤以及导致碳-碳键断裂的过渡态的性质。迫切需要开发新的途径，以碳氢化合物为基础生产生物燃料，作为化石燃料的可持续替代品。蓝藻的乙醛脱羧酶产生碳氢化合物，在生物燃料应用中引起了特别的兴趣，但其极低的活性是其使用的主要绊脚石。了解这种酶的工作原理是设计更好的酶或合成催化剂的第一步，这种酶或合成催化剂可以更有效地完成碳氢化合物的合成。这反过来可能允许开发新的生物燃料路线，可以取代目前的化石燃料，包括汽油、柴油和喷气燃料。

In this award from the Chemistry of Life Processes in the Chemistry Division, Dr. E. Neil Marsh, from the University of Michigan, will elucidate the mechanism of a newly-discovered hydrocarbon-forming enzyme from cyanobacteria, aldehyde decarbonylase (cAD). Biosynthetic pathways that generate hydrocarbons have become the subject of intense interest recently as their potential to generate new biofuels has been recognized. Aldehyde decarbonylase is a member of a well-studied class of enzymes, the non-heme di-iron dioxygenases. Preliminary studies have shown that cAD catalyzes a most unusual reaction involving the conversion of long-chain fatty aldehydes to the corresponding alkanes and formate. To investigate the mechanism, alternative substrates that may act as mechanism-based inhibitors will be synthesized to gain evidence for proposed radical intermediates in the reaction. The role of the reducing system in the reaction will be investigated and the components of the native reducing system in the host cyanobacterium P. marinus, will be identified. Site-directed mutagenesis will be used to investigate the role of active site residues in determining substrate specificity and controlling the mechanism of the reaction. Lastly, kinetic isotope effects will be measured to provide insight into the rate determining step in cAD and the nature of the transition state leading to carbon-carbon bond cleavage.There is a pressing need to develop new pathways to produce biofuels based on hydrocarbons as sustainable alternatives to fossil fuels. The cyanobacterial enzyme aldehyde decarbonylase produces hydrocarbons and has attracted particular interest for biofuels applications; yet its extremely low activity presents a major stumbling block to its use. Understanding how the enzyme works is the first step to designing better enzymes, or synthetic catalysts, that accomplish the synthesis of hydrocarbons more efficiently. This, in turn, may allow the development of new routes to biofuels that can substitute for current fossil fuels including gasoline, diesel and jet fuels.