Biological Enolates

Updated Apr 17, 2026

Every reaction in Chapter 7 has a biological counterpart. Enzymes use enolate chemistry to do clean carbon-carbon bond formation, racemization, decarboxylation, and chain elongation. The mechanism is exactly what you just learned in the lab version - the enzyme just provides positioning, acid-base catalysis, and sometimes a cofactor.

Aldolase in Glycolysis

Glycolysis breaks fructose-1,6-bisphosphate (FBP) into two three-carbon units (DHAP and G3P). The enzyme aldolase performs a retro-aldol - the reverse of the aldol condensation:

Forward direction (in liver gluconeogenesis): DHAP + G3P → FBP. An aldol condensation joining two trioses.
Reverse direction (in glycolysis): FBP → DHAP + G3P. A retro-aldol splitting a hexose.

Class I aldolases (in animals, plants, higher organisms) use a lysine side-chain to form a Schiff base with the C2 carbonyl of the substrate. This turns the ketone into an iminium, which amplifies the alpha-acidity for easier enolate formation. Then the retro-aldol cleaves the C3-C4 bond, giving an iminium-stabilized enamine (the DHAP-lysine complex) and G3P.

Class II aldolases (in bacteria, fungi) use a zinc(II) cofactor that serves a similar role: it coordinates to the C2 oxygen and polarizes the C=O, promoting enolate formation.

Both mechanisms recapitulate the retro-aldol chemistry you learned with a mechanistic twist (Schiff base or metal coordination) for additional acceleration.

Fatty Acid Synthesis: The Claisen in Biology

Fatty acid biosynthesis extends a growing fatty-acyl chain by two carbons per cycle. The key C-C bond-forming step is a Claisen-like condensation:

Acetyl-CoA (CH₃-CO-S-CoA) and malonyl-CoA (HOOC-CH₂-CO-S-CoA) are loaded onto acyl carrier protein (ACP).
The beta-ketoacyl-ACP synthase (KS) forms a Claisen-like bond between the acetyl alpha-carbon and the malonyl carbonyl carbon.
Malonyl’s carboxylate is released as CO₂ (decarboxylation during the condensation, providing thermodynamic drive).
Product: 3-ketobutyryl-ACP (a beta-keto thioester).

This is a Claisen with a “malonate trick”: the malonate donor has an extra COOH that leaves as CO₂, making the reaction exergonic. Without the CO₂ loss, the Claisen equilibrium would not be favorable at physiological conditions.

PLP Chemistry: Schiff Bases with Amino Acids

Pyridoxal phosphate (PLP, vitamin B6) uses Schiff base chemistry (Chapter 6.8) to catalyze transamination, racemization, and decarboxylation reactions of amino acids.

Mechanism of transamination (the classic example):

Amino acid’s -NH₂ forms a Schiff base with PLP’s -CHO.
The resulting PLP-imine is a quinone-like electron sink that activates the alpha-H of the amino acid.
A base in the active site removes the alpha-H, forming a resonance-stabilized carbanion (the “quinonoid intermediate”).
Reprotonation at a different carbon gives a different tautomer (ketimine), which then hydrolyzes to release the amino acid as an alpha-keto acid + PLP-NH₂ (PMP).

This is basically enolate chemistry on a Schiff-base-activated alpha-carbon, with the PLP ring providing the electron sink.

Alanine racemase (found in bacteria) uses PLP the same way but reprotonates from the opposite face, giving the opposite enantiomer. This is how bacteria make D-alanine for their cell walls.

Acetaldehyde in Ethanol Metabolism

Ethanol metabolism: ethanol → acetaldehyde → acetate, catalyzed by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). Both steps use NAD⁺ as the hydride acceptor.

The acetaldehyde intermediate is potentially toxic. Its alpha-H is acidic (pKa ~17), and it can undergo aldol-like reactions with itself or with proteins’ amines, forming DNA adducts and protein crosslinks that contribute to alcohol-related tissue damage.

Ketone Bodies

Beta-hydroxybutyrate and acetoacetate are the ketone bodies produced in the liver during fasting. Their synthesis from acetyl-CoA involves a Claisen-like condensation of two acetyl-CoA molecules (forming acetoacetyl-CoA), followed by another acetyl-CoA addition (forming HMG-CoA), and cleavage to give acetoacetate + acetyl-CoA.

These ketone bodies are shipped to peripheral tissues (including the brain during prolonged fasting), where they are broken back down to acetyl-CoA via reverse aldol-like cleavage. The reversibility and compatibility with physiological conditions (near neutral pH, 37°C) reflect the generally reversible nature of aldol-Claisen chemistry.

Alpha-Keto Acid Decarboxylation

Beta-keto carboxylic acids decarboxylate via a 6-membered cyclic transition state, with the carbonyl oxygen abstracting a proton from the carboxylic acid. This is the same chemistry that drives the acetoacetic ester synthesis’s decarboxylation step (Section 7.9). In biology, pyruvate decarboxylation (by pyruvate dehydrogenase, PDH) involves this chemistry with thiamine pyrophosphate (TPP) as a cofactor.

Explain why fatty acid biosynthesis uses malonyl-CoA (instead of another acetyl-CoA) as the donor in the KS-catalyzed Claisen condensation.

Click to reveal answer

Malonyl-CoA's extra carboxylate (-COOH) is lost as CO₂ during the condensation, providing a large thermodynamic driving force. If two acetyl-CoA molecules just condensed together (as in a regular Claisen), the reaction would be thermodynamically unfavorable at physiological conditions (cellular pH and temperature are not extreme). The "malonate trick" - using a carboxylate that will leave as CO₂ - converts an unfavorable Claisen into a strongly exergonic one. This is why malonyl-CoA is the true substrate for fatty acid elongation, not two acetyl-CoAs.