In asymmetric catalysis, we use the terms "pro-R" and "pro-S" to refer to the stereocenters created in symmetrical molecules by the action of chiral enzymes. Usually, these mechanisms have more than one step and at least one step involves an intermediate generated from the substrate, that is approaching the chiral center on either the pro-R or pro-S faces. Most often, this sort of chiral induction is invoked for radical processes in which the radical can take one of two conformations (pro-R and pro-S) in its approach towards the chiral catalyst. For aconitase, the mechanism involves dehydration to form a double bond and then rehydration to move the hydroxyl. During rehydration, the olefin can present one of two faces towards the chiral center - one leads to the R form and the other leads to the S form.
