21.8 Chemistry of Thioesters and Acyl Phosphates: Biological Carboxylic Acid Derivatives

Thioesters and Acyl Phosphates

Thioesters and acyl phosphates are the carboxylic acid derivatives that biology actually uses to get work done. While acyl chlorides and anhydrides are too reactive and harsh for a cellular environment, thioesters like Acetyl-CoA hit a sweet spot: reactive enough to drive acyl substitution reactions, but stable enough to exist in aqueous solution. Understanding why these molecules are so reactive connects directly to the reactivity trends you've already learned for carboxylic acid derivatives.

Structure and Function of Thioesters

A thioester looks just like a regular ester, except the bridging oxygen is replaced by sulfur. The general structure is $\text{R-C(=O)-S-R'}$, where R and R' are alkyl or aryl groups.

Acetyl Coenzyme A (Acetyl-CoA) is the most important thioester in biology. It consists of an acetyl group ($\text{CH}_3\text{C(=O)-}$) bonded to the large Coenzyme A molecule through a thioester linkage. Acetyl-CoA feeds into the citric acid cycle, fatty acid synthesis, and dozens of other metabolic pathways.

Why are thioesters more reactive than ordinary oxygen esters? Two reasons work together:

• Weaker overlap with sulfur. Sulfur's 3p orbitals are larger and overlap poorly with the carbonyl's 2p orbital. That means sulfur donates less electron density into the carbonyl compared to oxygen in a regular ester. The carbonyl carbon stays more electrophilic and more open to nucleophilic attack.
• Better leaving group. The thiolate ($\text{RS}^-$) is a better leaving group than an alkoxide ($\text{RO}^-$) because sulfur stabilizes negative charge more effectively due to its larger size and greater polarizability.

Both factors make the tetrahedral intermediate form more easily and collapse more readily, which is exactly what you need for nucleophilic acyl substitution.

Reactivity of Acyl CoA vs. Carboxylic Acids

The full reactivity ranking of carboxylic acid derivatives, from most to least reactive toward nucleophilic acyl substitution:

1. Acyl chlorides ($\text{R-C(=O)-Cl}$)
2. Anhydrides ($\text{R-C(=O)-O-C(=O)-R}$)
3. Thioesters ($\text{R-C(=O)-S-R'}$)
4. Esters ($\text{R-C(=O)-O-R'}$)
5. Amides ($\text{R-C(=O)-NH}_2$)
6. Carboxylate ions / Carboxylic acids ($\text{R-C(=O)-O}^-$ / $\text{R-C(=O)-OH}$)

Notice that thioesters sit right in the middle. They're more reactive than esters and amides but less reactive than acyl chlorides and anhydrides. This is the biological sweet spot: reactive enough to transfer acyl groups efficiently, yet stable enough to be stored and transported within cells.

Carboxylic acids themselves are poor acyl transfer agents because the hydroxide ion ($\text{HO}^-$) is a terrible leaving group, and resonance donation from the $\text{-OH}$ oxygen stabilizes the carbonyl and reduces its electrophilicity. Biology solves this problem by first activating carboxylic acids as thioesters or acyl phosphates before using them in reactions.

Nucleophilic Acyl Substitution in Biology

The mechanism follows the same two-step pattern you've seen for all acyl substitutions: nucleophilic addition to the carbonyl, then elimination of the leaving group.

Example: Formation of N-Acetylglucosamine (GlcNAc)

Glucosamine reacts with acetyl-CoA to form GlcNAc through these steps:

1. The amine group ($\text{-NH}_2$) of glucosamine acts as the nucleophile and attacks the electrophilic carbonyl carbon of acetyl-CoA.
2. A tetrahedral intermediate forms.
3. The thiolate of Coenzyme A ($\text{CoA-S}^-$) departs as the leaving group.
4. The product is an amide bond: the acetyl group ($\text{CH}_3\text{C(=O)-}$) is now attached to the nitrogen of glucosamine.

GlcNAc is a building block of chitin, the structural polysaccharide in arthropod exoskeletons and fungal cell walls, and it's also found on cell surface glycoproteins that play roles in cell recognition and signaling.

Other biologically important molecules formed by nucleophilic acyl substitution from thioesters:

• Acetylcholine — a neurotransmitter synthesized from acetyl-CoA and choline
• Acetylated proteins — acetyl groups added to lysine residues regulate gene expression and cellular signaling
• Fatty acid chains — built by repeated condensation of acetyl-CoA (as malonyl-CoA) units, each step involving acyl transfer from a thioester

Biological Reactions and Catalysis

Several recurring reaction types involve thioesters and acyl phosphates in metabolism:

• Acylation is the transfer of an acyl group ($\text{R-C(=O)-}$) to a nucleophile. Enzymes called acyltransferases catalyze these reactions with high specificity, ensuring the right substrate gets acylated at the right time.
• Hydrolysis breaks a thioester or acyl phosphate by reaction with water, releasing the free carboxylic acid. Thioesterases catalyze this process when the cell needs to "deactivate" an acyl group or release a finished product (for example, releasing a completed fatty acid from fatty acid synthase).
• Enzyme catalysis is essential here because, even though thioesters are relatively reactive, uncatalyzed reactions in water would be far too slow for cellular needs. Enzymes stabilize transition states and position substrates precisely to accelerate these acyl substitution reactions by many orders of magnitude.
• Metabolic pathways like the citric acid cycle and fatty acid synthesis use thioesters and acyl phosphates as activated intermediates and energy carriers, linking the chemistry of acyl substitution directly to how cells extract and store energy.