Driving Parts of Krebs Cycle in Reverse through Mineral Photochemistry

Abstract

Scenarios for driving the Krebs cycle in reverse in the absence of enzymatic networks have received much attention in the literature of prebiotic evolution because repeated cycling provides a core mechanism for the synthesis of useful biomolecules from carbon dioxide. Here, we show that three of the five reductive steps in the cycle can be driven photochemically at high yield by a ZnS colloid. The results show the conversion of oxaloacetate to malate, fumarate to succinate, and oxoglutarate to oxalosuccinate. The experimental conditions of circumneutral pH and 288 K, as well as the ZnS semiconductor photocatalyst particles (sphalerite), are believed to have been prevalent in the waters of early Earth. These findings therefore both establish and constrain the plausibility of the occurrence of heretofore difficult chemical conversions in prebiotic metabolic systems.

Formamide and the origin of life

“…Acceleration of three of the five reduction reactions on this cycle (oxaloacetic to malic, fumaric to succinic and oxoglutaric to oxalosuccinic acids) occurs abiotically at the surface of the mineral ZnS by photochemistry [133], showing that catalyst-induced transformations may have been instrumental in the establishment of autocatalytic cycles.…”

“…Whether autocatalytic cycles have been able to establish themselves is matter of open debate [8,16,17,21,90,133,134]. In particular, the onset of metabolic cycles on the prebiotic Earth has been dubbed as implausible [134], mostly on the basis of kinetic considerations and on the difficulty of justifying both thermodynamically and kinetically the exclusion of side reactions that would disrupt the cycle.…”

Prebiotic Metabolism: Production by Mineral Photoelectrochemistry of α-Ketocarboxylic Acids in the Reductive Tricarboxylic Acid Cycle

“…The rectangle encloses the species of the rTCA cycle. Reactions labeled I were studied by Zhang and Martin (2006) and Guzman and Martin (2008). Reactions labeled II are reported in the present study.…”

“…In summary, colloidal photoelectrochemistry possibly had an active role on early Earth for converting and cycling small organic compounds essential for the origin of life (Zhang et al, 2004(Zhang et al, , 2007Zhang and Martin, 2006;Guzman and Martin, 2008). Energy from ultraviolet sunlight, catalysis by semiconductor minerals, and organic compounds could have interacted in a synergistic way in the prebiotic environment, promoting reactions otherwise unviable, and allowed fixation of carbon dioxide, which eventually evolved into metabolism as we know it today.…”

“…A nonenzymatic rTCA cycle might have functioned as an autocatalytic network of chemical reactions able to provide and self-sustain the biosynthetic pathways essential for life to originate (Wachtershauser, 1993;Morowitz et al, 2000;Smith and Morowitz, 2004). Zhang and Martin (2006) and Guzman and Martin (2008) showed that several difficult chemical steps of the rTCA cycle can be successfully driven by mineral photoelectrochemistry with the use of colloidal ZnS semiconductor particles. Photo-generated conduction-band electrons and valence-band holes of semiconductors can drive strongly endoergic reduction and oxidation reactions, respectively ( Fig.…”