Prochirality

Prochiral sp3 Centers

Imagine a carbon atom with two seemingly identical hydrogens attached (prochiral hydrogens). Although they appear the same, these hydrogens occupy slightly different positions in space. Enzymes, nature's master catalysts, can exploit this subtle difference.

Here's why: If we replace one of these hydrogens with a heavier isotope, deuterium (²H), in our minds, the carbon becomes a chiral center. A chiral center has four distinct groups attached, and as a result, the molecule itself becomes non-superimposable on its mirror image. In the case of ethanol, replacing the "red" hydrogen with deuterium would create an R-configured chiral center (due to the higher priority of deuterium compared to hydrogen).

This concept of prochirality helps us understand how enzymes can differentiate between these seemingly identical hydrogens. By recognizing the spatial arrangement, enzymes can selectively react with one hydrogen over the other, leading to specific products in biological processes.

Prochiral sp2 Centers/Faces

While sp2-hybridized carbons with a trigonal planar geometry aren't chiral centers themselves, they can be something called prochiral centers if they have three different substituents. This means that even though the central carbon might seem symmetrical, there are actually two distinct faces to this arrangement. These faces are important because enzymes, biological catalysts, can often tell them apart.

We use the terms "re" and "si" to refer to these two faces of a prochiral sp2-hybridized group. Imagine looking down on the flat molecule from the viewpoint of the sp2-carbon. We can assign priorities to the three substituents like we do in the R/S system for chiral centers. If the order of increasing priority goes clockwise around the molecule, that's the "re" face. If it goes counter-clockwise, that's the "si" face.

This distinction between re and si faces is crucial because it allows enzymes to control how they react with these molecules. By recognizing the different spatial environments of the re and si faces, enzymes can selectively interact with one side over the other, leading to specific products in biological processes.