We propose a new model for prochirality that satisfies all known examples: the prochiral plane. This plane contains the prochiral carbon and defines two separate faces for chemical modification. We extend this to enzyme catalysis, replacing the "three point attachment" hypothesis and its variants. Once a prochiral substrate is fixed on an enzyme surface, the asymmetry of the enzyme provides reactants exclusively on one side of the prochiral plane, producing an enantiomerically pure chiral product. The aconitase reaction is detailed as an example, using molecular modeling and its known enzymatic mechanism. We show that the prochiral substrate for this enzyme is not citrate, but rather cis-aconitate. The number of interaction points of cis-aconitate is not relevant to prochirality, but rather to substrate specificity. A second detailed example is the enzyme fumarase; here the substrate fumarate has only two binding sites, but is nonetheless fixed onto the enzyme and has a defined prochiral plane. We also provide a literature survey of more prochiral substrates, all of which have sp2 hybridized carbon and contain a prochiral plane. An example of a prochiral unnatural substrate for sphingosine kinase 2, fingolimod, has an sp3 hybridized prochiral carbon and also contains a prochiral plane. Finally, we provide an intuitive example of a prochiral physical object, a coffee cup, interacting with one hand and lip.
Keywords: Aconitase; Fumarase; Molecular modeling; Prochiral plane; Stereospecificity.
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