Bridging geochemistry and biochemistryFrom non-enzymatic methane formation pathways to the evolutionary dawn of enzymatic nitrogen fixation

Life depends on iron-sulfur chemistry. Prior to the origin of life, the prebiotic iron-sulfur-world developed an essential set of non-enzymatic, metabolic core reactions. Driven by the redox-active properties of iron-sulfur minerals, a chemical evolution firstly led to an emergence of organic chemistry from inorganic chemistry. In a second step, iron-sulfur chemistry allowed for abiogenesis, the rise of organismal life from dead matter. Notably, these properties of iron-sulfur chemistry remained essential during the entire subsequent biological evolution, demonstrated by the vast and diverse set of metalloenzymes that are harbored by all living organisms until today. In general, this transition from geochemistry to biochemistry is characterized by a transition from slow, unspecific and surface-catalyzed reactions to an increasingly complex enzymatic network of highly efficient reactions, with a mutual exclusivity of a given reaction and the corresponding enzyme. However, this evolutionary process necessarily had to occur in a stepwise manner. It is the goal of this thesis to characterize several of these steps, taking place in the grey zone between geochemistry and biochemistry, between the non-enzymatic features of iron-sulfur chemistry and enzymatic catalysis. Furthermore, this thesis both proposes and demonstrates that only the mutual, stepwise and synergistic interplay of iron-sulfur chemistry and its surrounding biochemical environment expanded the chemical reaction space towards increasingly difficult reactions. During this evolutionary process, the complexity of (i) iron-sulfur chemistry, (ii) protein abundance, and (iii) biochemically achievable, metalloenzyme-catalyzed reactions and their efficiency is proposed to having occurred in an iterative process. In specific, this thesis investigates novel, non-enzymatic methane formation mechansisms and the evolutionary dawn of enzymatic nitrogen fixation. Notably, both processes were found to depend on the redox-active properties of iron and its sulfur-bound metal clusters. The observed methane formation pathways are driven by an interplay of ferrous iron and reactive oxygen species that was found to drive the oxidative demethylation of methylated sulfur compounds, resulting in methane formation. While this reaction is non-enzymatic and catalyzed be free iron, enzymatic nitrogen fixation represents the upper complexity end of organismal iron-sulfur chemistry, with a set of both several enzymes and the most complex metal clusters catalyzing a chemically far more challenging reaction, the reduction of inert dinitrogen to ammonia. Here, this thesis unravels how nitrogen fixation initially emerged, facilitated by a reduced set of enzymes and simpler metal clusters, with an iterative complexity increase gradually enhancing the desired reaction.