Thioester
Thioesters are compounds resulting from the bonding of sulfur with an acyl group (an alkyl group attached to a carbon-oxygen double bond). The thioester bond is viewed by some biochemists to have been critical in the origin of life; so much so that one Nobel Prize winning scientist (Belgium's Christian de Duve) posits a "Thioester World", which preceded and developed into an "RNA World", itself the immediate precursor to the appearance of entities we would call "organisms".
As de Duve explains, "A thioester forms when a thiol (whose general form is written as an organic group, R, bonded with sulfur and hydrogen, hence R-SH) joins with a carboxylic acid (R'-COOH). A molecule of water (H2O) is released in the process, and what remains is a thioester: R-S-CO-R'... The thioester bond is what biochemists call a high-energy bond, equivalent to the phosphate bonds in adenosine triphosphate (ATP), which is the main supplier of energy in all living organisms... It is revealing that thioesters are obligatory intermediates in several key processes in which ATP is either used or regenerated. Thioesters are involved in the synthesis of all esters, including those found in complex lipids. They also participate in the synthesis of a number of other cellular components, including peptides, fatty acids, sterols, terpenes, porphyrins and others. In addition, thioesters are formed as key intermediates in several particularly ancient processes that result in the assembly of ATP. In both these instances, the thioester is closer than ATP to the process that uses or yields energy. In other words, thioesters could have actually played the role of ATP in a thioester world initially devoid of ATP. Eventually, [these] thioesters could have served to usher in ATP through its ability to support the formation of bonds between phosphate groups."
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