Biological Relevance

Updated Apr 17, 2026

Carboxylic acid derivatives are the workhorse functional groups of biochemistry. Peptide bonds (amides) connect amino acids into proteins. Thioesters (like acetyl-CoA) activate carboxylic acids for transfer reactions. Esters link fatty acids into triglycerides and phospholipids. Understanding their organic chemistry unlocks their biological function.

Peptide bond formation showing two amino acids joining via amide bond and releasing water — Peptide bond = amide bond. The COOH of one amino acid condenses with the NH₂ of another, releasing water. The resulting amide is planar due to nitrogen lone pair delocalization into the carbonyl. *Credit: Wikimedia Commons, CC BY-SA*

Peptide Bonds: The Amide Backbone of Proteins

Every protein is a chain of amino acids connected by amide bonds (peptide bonds). The properties of amides explain protein behavior:

Planarity. Amide nitrogen’s lone pair delocalizes into the carbonyl, giving the C-N bond ~40% double-bond character. Six atoms (Cα, C=O, N, H, Cα) lie in a plane. This restricts rotation around C-N, which is why protein structure is dominated by the Ramachandran phi/psi plot showing allowed combinations of backbone torsion angles.
Stability. Amide hydrolysis has a half-life of hundreds of years at physiological conditions. Proteins last long enough to fold, function, and be recycled by cellular proteases.
Hydrogen bonding. The amide -NH can donate H-bonds; the amide C=O can accept them. This is the basis of alpha-helix and beta-sheet secondary structure.

Thioesters: Activated Biological Acyl Groups

Coenzyme A (CoA) has a terminal -SH group. When a carboxylic acid is attached to it, the result is a thioester: R-CO-S-CoA. Key example: acetyl-CoA (CH₃CO-S-CoA).

Thioesters are more reactive than regular esters because:

Sulfur’s 3p orbitals overlap poorly with C’s 2p orbitals, so S donates weakly into the carbonyl via resonance (compared to O in a regular ester).
Thioester’s -SCoA LG is a softer, more polarizable anion, making it a slightly better LG than alkoxide.

Net effect: thioesters are about $10^{3}$ to $10^{5}$ more reactive than regular esters. This makes them ideal for biological acyl transfer reactions - they are reactive enough to transfer the acyl group but stable enough to exist as discrete metabolic intermediates.

Acetyl-CoA is the “activated acetate” of metabolism. It:

Enters the Krebs cycle by condensing with oxaloacetate → citrate (Claisen-like condensation).
Starts fatty acid synthesis.
Acetylates histones (gene regulation) and neurotransmitters.
Is used for cholesterol and isoprenoid biosynthesis.

Aspirin: Pharmacological Use of an Ester Group

Aspirin molecular structure showing the acetyl ester group on salicylic acid — Aspirin (acetylsalicylic acid) contains both a carboxylic acid and an acetyl ester. The ester is the key pharmacophore: aspirin transfers its acetyl group to Ser530 of COX-1, permanently inhibiting the enzyme. *Credit: Wikimedia Commons, CC BY-SA*

Aspirin (acetylsalicylic acid) is synthesized from salicylic acid + acetic anhydride, installing an acetyl ester on the phenolic oxygen. The original -COOH of salicylic acid is preserved.

Aspirin’s biological mechanism: the ester is a reactive acyl donor that transfers the acetyl group to a specific serine residue (Ser530) in cyclooxygenase (COX-1 and COX-2) enzymes. This acylation covalently inhibits the enzyme, preventing prostaglandin synthesis and giving aspirin its anti-inflammatory, analgesic, and antipyretic effects.

The mechanism: aspirin’s acetyl-OR group (where R is the phenolate of salicylic acid) transfers to the enzyme’s serine -OH. The leaving group is salicylate itself (the free acid).

This is a specific example of an organic chemistry acetylation happening inside the human body, exploiting the same principles you learned about ester reactivity.

Other Ester-Based Pharmaceuticals

Procaine, lidocaine: local anesthetics with ester or amide functional groups. Hydrolysis rate of the linker determines duration of action.
Cocaine: an ester alkaloid. Cleaved by plasma esterases, which is why cocaine has a short half-life.
Statins (pravastatin, simvastatin, atorvastatin): HMG-CoA reductase inhibitors; some are administered as lactone prodrugs that are hydrolyzed in the liver to the active acid form.

Biological Amides Beyond Proteins

Amide bonds also appear in:

Penicillin and related beta-lactam antibiotics (4-ring amide is the key pharmacophore).
Acetaminophen (an amide linker): N-acetyl-para-aminophenol, a common analgesic.
Urea: diamide of carbonic acid, the nitrogen-disposal product in mammals.
DNA and RNA: no amides in the backbone (phosphate esters), but base-sugar linkages and some other structural features are amides.

Why is acetyl-CoA (a thioester) more chemically reactive than a typical ester, and how does this reactivity make it the "currency" of acyl transfer in metabolism?

Click to reveal answer

The S in a thioester donates less electron density into the carbonyl via resonance than O in a regular ester (S's 3p orbitals overlap poorly with C's 2p orbital). This keeps the thioester carbonyl more electrophilic. Thioesters are ~10³ times more reactive than esters but ~10² less reactive than acyl chlorides - perfect for biological acyl transfer. Cells can make acetyl-CoA, store it briefly, and transfer its acetyl group to many targets (citrate synthesis, fatty acid synthesis, acetylation, etc.).