Figure 2.23 Glycerol held on an imaginary enzyme surface, being selectively phosphorylated. The numbering of glycerol phosphate presents a problem, because \.-glycerol-3-phosphate is the same compound as D-glycerol-1-phosphate. To avoid confusion, for glycerol, the pro-S CH2 group is numbered 1 and the pro-R CH2 numbered 3. Therefore the compound shown is the 3-phosphate selectively phosphorylated, that symmetry is destroyed and chiral glycerol-3-phosphate is formed (Figure 2.23).

If a molecule is not chiral, it is designated as prochiral if a single substitution will make it chiral. Ethanol is an achiral compound, its CH2 group is described as prochiral. To distinguish between the two hydrogen atoms of this group, the Cahn-Ingold-Prelog rules are applied to describe the group, after each hydrogen in turn is notionally replaced by deuterium, and the priority rule applied (Figure 2.24). If the new chiral centre has the S configuration, then the hydrogen that has been replaced is pro-S, and, conversely, if the new centre with deuterium is R, then the hydrogen that was replaced is pro-R. The enzyme alcohol dehydrogenase a pro-chiral centre

OH ethanol first, replace this hydrogen with deuterium

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