Chemistry of Thioesters and Acyl Phosphates: Biological Carboxylic Acid Derivatives

Core Concepts

Structure: thioesters and acyl phosphates

• Thioester: a carbonyl bonded to S–R′ (the sulfur analog of an ester): RCO–SR′. Acetyl-CoA is CH3CO–S–CoA — an acetyl group attached through a thioester link to the thiol (–SH) at the end of coenzyme A.
• Acyl phosphate: a carbonyl bonded to O–PO₃²⁻: RCO–O–PO₃²⁻. Acetyl phosphate is CH3CO–O–PO₃²⁻; 1,3-BPG carries an acyl phosphate at C-1 and a phosphate ester at C-3.
• Compare with phosphate esters (R–O–PO₃²⁻, no carbonyl) and ordinary esters (RCO–OR′).

Why thioesters are "high-energy"

Two electronic factors make a thioester carbonyl more reactive than an O-ester carbonyl:

• Weaker resonance stabilization. Sulfur's lone pairs sit in 3p orbitals, which overlap less efficiently with the carbonyl π system than oxygen's 2p lone pairs do. The thioester carbonyl is therefore less resonance-stabilized, more electrophilic, and closer in character to a ketone.
• A better leaving group. Thiolate (R′S⁻) is a much better leaving group than alkoxide (R′O⁻), so acyl transfer from a thioester is faster and more favorable.

The thermodynamic result: hydrolysis of acetyl-CoA has ΔG°′ ≈ −31 kJ/mol (literature values range from about −31 to −36 kJ/mol), comparable to ATP hydrolysis (−30.5 kJ/mol) and well below the roughly −20 kJ/mol typical of an O-ester. "High-energy" does not mean the bond is intrinsically strong — it means the products (acetate + free thiol) are far more stable than the reactant, so hydrolysis releases free energy that enzymes can couple to other reactions.

Acyl phosphates: the mixed anhydrides of metabolism

Acyl phosphate hydrolysis is even more exergonic (about −49 kJ/mol for 1,3-BPG), enough to drive ADP phosphorylation thermodynamically. In glycolysis, phosphoglycerate kinase catalyzes 1,3-BPG + ADP → 3-phosphoglycerate + ATP — a substrate-level phosphorylation.

Acetyl-CoA: the acyl-carrier hub

Coenzyme A is built from adenosine diphosphate linked to pantothenate (vitamin B5), ending in a thiol (–SH); the acetyl group rides as CH3CO–S–CoA. Major roles:

• Citric acid cycle: citrate synthase condenses acetyl-CoA with oxaloacetate (a carbon–carbon bond-forming reaction that begins with enolate attack — see Chapters 22–23).
• Fatty acid synthesis: acetyl-CoA carboxylase makes malonyl-CoA, the two-carbon donor carried by acyl carrier protein (ACP) thioesters.
• β-Oxidation: fatty acids are activated as fatty acyl-CoAs before oxidation.
• Acetylation: acetyl-CoA acetylates proteins (histones) and xenobiotics.

Reaction chemistry: acyl transfer from a thioester

Thioesters undergo the same nucleophilic acyl substitution as other derivatives, but faster than O-esters. A nucleophile (water, alcohol, amine, or thiol) attacks the carbonyl carbon; the tetrahedral intermediate collapses, expelling the thiolate (CoA-S⁻) and transferring the acyl group. Examples: transesterification to carnitine (mitochondrial acyl transport) and the Claisen condensation of acetyl-CoA in fatty acid synthase (Chapter 23).

How It Works / Step-by-Step Process

Acyl transfer from acetyl-CoA to an alcohol (transesterification), described in words:

1. The alcohol's oxygen lone pair attacks the thioester carbonyl carbon (a curved arrow from the O lone pair to C).
2. The C=O π bond moves onto oxygen, forming a tetrahedral alkoxide intermediate.
3. The intermediate collapses: the C–S bond electrons move onto sulfur, expelling the thiolate leaving group (CoA-S⁻).
4. Proton transfer completes the O-ester product and regenerates coenzyme A.

Net: R′OH + CH3CO–S–CoA → CH3CO–OR′ + CoA–SH; water, an amine, or another thiol in place of the alcohol gives hydrolysis, amide formation, or thioester exchange.

General lab-safety principle: thioesters and acylating agents are moisture-sensitive and some are irritants or lachrymators. Follow institutional chemical hygiene rules — fume hood, gloves, goggles — and consult the safety data sheet before handling.

Common Confusions