These two enzymes perform a similar reaction, but do it in stereochemically opposite ways. The crystallographic structures capture muconate lactonizing enzyme (MLE) from two different bacteria, after they have finished the reaction, shuffling around a few hydrogen atoms to form a ring and change a carbon-carbon double bond to a single bond. The catalytic amino acid is a lysine, shown here in turquoise. On the substrate, two hydrogen atoms are shown in light green and pink: in one enzyme, the pink one has been added, and in the other, the green one was moved in the reaction. The resulting product is the same, but the enzymes get there a different way. The amino acids shown in magenta (a glutamate in one enzyme and a histidine in the other) position the substrate in two very different orientations.

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