Photo-production of lactate from glyoxylate: how minerals can facilitate energy storage in a prebiotic world

Marcelo I. Guzman, Scot T. Martin
2010 Chemical Communications  
The reaction of glyoxylate with carbon dioxide to produce lactate is promoted when zinc sulfide is irradiated by ultraviolet light. These results, representing a model for the action of colloidal mineral semiconductors on early Earth, complete a consecutive series that culminates in entry-point molecules of the reductive tricarboxylic acid cycle. In a series of chemical steps that fix CO 2 , many organisms during the history of Earth have used the reductive tricarboxylic acid (rTCA) cycle to
more » ... re energy, ultimately as carbohydrates, fats, and proteins. 1 As such, the rTCA cycle is one of the most ancient and fundamental of all biochemical pathways, and the rTCA cycle accordingly has been proposed as a candidate mechanism for carbon fixation and energy storage at the time life originated (Scheme 1A). 2-5 Moreover, products of the rTCA cycle are common metabolites that serve as feedstock for further biosynthesis and assembly. A fundamental challenge is to provide synthetic access from consecutive reactions starting from CO 2 to the compounds that start the cycle. 3,6 Here, we demonstrate for the first time that the C 2 compound glyoxylate (HCOCOO À ) reacts with CO 2 to produce the C 3 compound lactate (CH 3 -HCOH-COO À ) in 15% yield through a ZnS-photo-promoted reaction. Photo-generated conduction-band electrons (e À CB ) and valence-band holes (h + VB ) of semiconductors can facilitate rapid reactions promoted by radical formation and energy storage. 7 The example of the present study, the mineral sphalerite (ZnS), is expected to have been plentiful in Earth's early seas because of its formation in anoxic conditions from the mix of zinc and sulfur ejected by extremely active hydrothermal vents. 2,3,8 Formate (HCOO À ) and formaldehyde (CH 2 O) are the main first-generation C 1 products of the photochemistry of CO 2 in the presence of ZnS. Glycolate, oxalate, and glyoxylate are first-generation C 2 coupling products, 9,10 promoted by the CO 2 radical formed by electron transfer from the conduction band of ZnS to CO 2 . 11-13 Once a pool of C 2 molecules is present, second-generation coupling products are produced. 2,3,14 Scheme 1 represents reactions that can be promoted by ZnS. Electrons are excited into the conduction band of ZnS by absorption of photons having wavelengths shorter than 344 nm (3.6 eV). Their standard reduction potential is À1.04 V vs. NHE. The resulting overall energy of glyoxylate to lactate promoted by ZnS is À951 kJ mol À1 based on the thermodynamic data catalogued in ref. 15. From this reaction, the energy storage per organic carbon atom increases by 56.7 kJ mol À1 . The highly reducing conduction-band electron also provides an overpotential that can promote reactions that are otherwise kinetically sluggish. These reactions were studied in a series of laboratory experiments. Colloidal ZnS was prepared by the addition of Scheme 1 (A) The rTCA cycle in a form highlighting the electrontransfer elements. Blue and orange indicate pathways that provide exit points from the cycle and that are useful for further synthesis. The combination of the rTCA cycle and its exit products operates as a factory for the synthesis of major classes of biologically important molecules (shown in red). (B) Abiotic anaplerotic-like steps that start from CO 2 and produce pyruvate, a species that serves as an entry point into the rTCA cycle.
doi:10.1039/b924179e pmid:20234927 fatcat:n2cobooqcbaqvizlxzuqgrvzqa