Biological Elimination Reactions

Core Concepts

What enzymes cannot do — and what they do instead

At physiological pH there is no free hydroxide or alkoxide; a strong base would attack everything. Enzymes instead use general base catalysis: a side chain (e.g., histidine's imidazole) removes the β-hydrogen, accepting the proton into a relay. General acid catalysis or a metal ion (Mg²⁺, Zn²⁺, Fe–S) activates the leaving group: a protonated hydroxyl is water — an excellent leaving group. The enzyme makes the leaving group better, the β-H more acidic, and the intermediate more stable, all at once: a "push–pull" design, the enzymatic version of E1cB.

The E1cB logic: stabilize the carbanion first

Enzymatic eliminations usually begin by deprotonating a β-hydrogen made acidic by an adjacent electron-withdrawing group. In carbonyl chemistry this is the α-position: deprotonation gives an enolate, its negative charge delocalized onto oxygen. Enzymes stabilize enolates by hydrogen-bonding the developing oxyanion and coordinating metal ions, lowering the α-H pKa from ~19–20 to a value a neutral base can reach. Only then does the leaving group depart. Because deprotonation is usually the slow step, a primary deuterium isotope effect — the same fingerprint as E2 — identifies the rate-determining step.

Poor leaving groups and their activation

Biology's leaving groups are mostly poor by laboratory standards: hydroxide, alkoxide, phosphate, pyrophosphate, ammonia (as NH₄⁺), even carboxylates. Each is activated characteristically: OH is protonated to water (enolase, fumarase, aconitase); phosphate/pyrophosphate leave as stable anions assisted by Mg²⁺; NH₃ from an α-amino group departs after the β-H is removed, with the C–N electrons forming an imine — PLP or an electrophilic cofactor (MIO in histidine and phenylalanine ammonia-lyases) makes the β-H acidic enough to start.

Cofactors extend the chemistry

Pyridoxal phosphate (PLP) is the workhorse of amino-acid eliminations. PLP forms a Schiff base (imine) with the amino acid; the imine is an electron sink that lowers the pKa of the α-hydrogen from ~30 toward 7, so a general base can remove it. The resulting carbanion eliminates the β-substituent (OH, OR, or another group); the imine hydrolyzes to the keto acid and ammonia. Serine dehydratase, threonine dehydratase, and cystathionine β-lyase all work this way.

Stereospecificity

Enzymatic eliminations are often anti and stereospecific: the enzyme removes a specific hydrogen (one prochiral face) and the leaving group from the opposite face, giving one alkene geometry. Fumarase dehydrates L-malate to trans-fumarate with complete control, removing the pro-R hydrogen — the biological echo of E2's anti-periplanar requirement.

How It Works / Step-by-Step Process

1. Identify what will be lost: a β-H and a leaving group (OH, phosphate, NH₃, OR).
2. Find how the leaving group is activated: protonation, metal coordination, or a stable anion.
3. Find what makes the β-H acidic: a carbonyl, imine (PLP), or electrophilic cofactor (MIO).
4. Deprotonate first (E1cB) to form the stabilized enolate; watch for a primary KIE.
5. Eject the leaving group, form the alkene, note stereochemistry.

Worked Example: Enolase — Dehydration to Phosphoenolpyruvate

In glycolysis, enolase converts 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP), the pathway's highest-energy phosphate:

2-phosphoglycerate ^enolase⟶ phosphoenolpyruvate + H₂O

In structure terms, 2-PG is HOOC-CH(OPO₃^2-)-CH₂OH; PEP is HOOC-C(OPO₃^2-)=CH₂. A glutamate side chain (general base) removes the proton from C2 — α to the carboxylate — forming a carbanion/enolate stabilized by two Mg²⁺ ions coordinated to the carboxylate and phosphate. The β-hydroxyl at C3 leaves as water (assisted by Mg²⁺ and protonation), and C2–C3 becomes a double bond: E1cB in action.

Worked Example: Fumarase — a Stereospecific Anti Dehydration

In the citric acid cycle, fumarase dehydrates L-malate to fumarate:

HOOC-CH(OH)-CH₂-COOH ^fumarase⟶ HOOC-CH=CH-COOH + H₂O

The reaction is stereospecific: fumarase removes the pro-R hydrogen of the methylene carbon and the hydroxyl from the opposite face (anti elimination), giving exclusively the trans alkene, fumarate — never maleate. An aspartate side chain is the general base, and the hydroxyl leaves as water after protonation — E1cB deprotonation, poor-LG activation (OH → H₂O), and anti, stereospecific alkene formation.

Worked Example: Histidine Ammonia-Lyase — Losing Ammonia

Histidine ammonia-lyase (histidase) eliminates ammonia from L-histidine, giving urocanate in histidine catabolism:

L-histidine ^histidase⟶ urocanate + NH₃

The amino group is the leaving group — very poor in the flask — and the β-hydrogen sits on a non-acidic carbon. The enzyme's electrophilic cofactor MIO reacts with the substrate to make the β-H acidic; a general base removes it, and the C–N electrons form the double bond as ammonia departs, giving the trans alkene. (MIO details are still studied; the E1cB framework is the standard model.)

Common Confusions