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General structure of a thioester.

Thioesters are compounds resulting from the bonding of sulfur with an acyl group with the general formula R-S-CO-R'. They are the product of esterification between a carboxylic acid and a thiol (as opposed to an alcohol in regular esters).

Some biochemists believe that the thioester bond was critical for the origin of life. 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 sulfhydryl (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.

• Acetyl-CoA
• Malonyl-CoA

Thionoesters are isomeric with thioesters. In a thionoester, sulfur replaces the carbonyl oxygen in an ester. Methylthionobenzoate is C₆H₅C(S)OCH₃. Such compounds are typically prepared by the reaction of the thioacyl chloride with an alcohol, but they can also be made by the reaction of Lawesson's reagent with esters.^[1]

1. ^  de Duve, Christian (Sept./Oct. 1995), American Scientist