Consider the reaction in glycolysis, catalyzed by the enzyme enolase.
a) Propose a reasonable mechanism for this enzymatic reaction, assuming it occurs in more than one step. Use B to denote a generic basic residue in the enzyme (don’t worry about specifics). What type of elimination is occurring here?
b) Enolase uses Mg2+ ions. What might be a possible role of these Mg2+ ions with reference to your mechanism from part a?
c) Why is OH protonated as it leaves in the mechanism from part a?
d) Draw a mechanism for the reverse reaction. What type of reaction is occurring here?
Solution
a) E1cB is the type of elimination
b) The Mg2+ stabilizes the negatively charged intermediate (see mechanism in part a)
c) OH- is a bad leaving group (strong base; unstable). Therefore, it is protonated while leaving so that it leaves as H2O, which is a good (stable) leaving group.
d) 1,4-addition (i.e. conjugate addition)
