Biological Carboxylic Acids
Carboxylic acids are ubiquitous in biology. Nearly every metabolic pathway involves carboxylate-bearing intermediates, and many of the molecules you recognize from biochemistry (amino acids, fatty acids, Krebs cycle intermediates) are really just carboxylic acids with specific substitutions.
Fatty Acids
A fatty acid is a long-chain carboxylic acid. Typical structures: CH₃-(CH₂)_n-COOH, where n is 12, 14, 16, 18, or higher. They are the building blocks of triglycerides (fats and oils) and phospholipids (membrane components).
Saturated fatty acids (no double bonds) pack tightly and are solid at room temperature (e.g., stearic acid, palmitic acid). Unsaturated fatty acids (cis double bonds) have kinks that prevent tight packing, so they are liquid at room temperature (oleic acid, linoleic acid in olive oil).
At physiological pH, fatty acids exist as fatty acid anions (pKa of COOH ~4-5; pH 7 gives 99.9% deprotonated). In cell membranes, fatty acid tails are esterified into triglycerides or phospholipids, and the COOH is no longer free.
Soap and Micelles
Soap is a salt of a fatty acid (typically a sodium or potassium salt of C12-C18 fatty acids). The molecule has:
- A polar “head” (the COO⁻ Na⁺).
- A long hydrophobic “tail” (the alkyl chain).
In water, soap molecules aggregate into micelles: spherical clusters with the hydrophobic tails pointing inward and the polar heads pointing outward into water. Micelles can solubilize oils and grease by trapping them in the hydrophobic interior.
This amphipathic behavior - one polar head + one hydrophobic tail - is the foundation of biological membranes. Phospholipid bilayers in cell membranes are a more elaborate version of the same chemistry, with a polar phosphate head group and two fatty acid tails.
Amino Acids as Dicarboxylic-Like Systems
An amino acid has both a COOH (pKa ~2) and an NH₃⁺ (pKa ~9). At physiological pH, the COOH is fully deprotonated (to COO⁻) and the NH₃⁺ is fully protonated, giving the zwitterion:
⁺H₃N-CH(R)-COO⁻
This zwitterionic structure has no net charge (positive on N balances negative on O). The ISO-electric point (pI) is the pH at which the amino acid has zero net charge on average. For simple amino acids with a non-ionizable side chain: pI = (pKa₁ + pKa₂) / 2.
Amino acids are biology’s dipeptide-forming units. A peptide bond is an amide bond (covered in Ch 9): the COOH of one amino acid + the NH₂ of the next → peptide bond + H₂O. This is another carboxylic-acid-to-amide conversion.
The Krebs Cycle: A Parade of Carboxylates
The citric acid cycle (Krebs cycle, TCA cycle) is full of carboxylic acid/carboxylate intermediates. Each has a specific structure:
| Intermediate | Structure (as free acid) |
|---|---|
| Pyruvate | CH₃-CO-COOH |
| Acetyl-CoA | CH₃-CO-S-CoA |
| Citrate | HOOC-CH₂-C(OH)(COOH)-CH₂-COOH (tricarboxylic!) |
| Isocitrate | Similar, with OH shifted |
| Alpha-ketoglutarate | HOOC-CO-CH₂-CH₂-COOH |
| Succinyl-CoA | HOOC-CH₂-CH₂-CO-S-CoA |
| Succinate | HOOC-(CH₂)₂-COOH |
| Fumarate | trans-HOOC-CH=CH-COOH |
| Malate | HOOC-CH(OH)-CH₂-COOH |
| Oxaloacetate | HOOC-CO-CH₂-COOH |
At pH 7, every one of these is the anion of its corresponding carboxylic acid - citrate has three negative charges; the others have two. Enzymes like aconitase, fumarase, and the dehydrogenases work with the anionic forms.
Carboxylic Acids in Pharmaceuticals
Many drugs are carboxylic acids:
- Aspirin (acetylsalicylic acid): contains a COOH and an ester. Both are important in its mechanism.
- Ibuprofen (a 2-arylpropionic acid): the COOH is a key pharmacophore.
- Penicillin (beta-lactam antibiotics): the free COOH is essential for binding to the bacterial target enzyme.
- Statins (cholesterol drugs): carboxylic acid form is active; ester prodrug form is more oral-bioavailable.
The COOH group’s ionization at physiological pH affects both pharmacokinetics (absorption, distribution) and target engagement. Most COOH-containing drugs are given as sodium or potassium salts to improve solubility.
The Acetyl-CoA Connection
Acetyl-CoA (CH₃-CO-S-CoA) is biology’s activated carboxylic acid. It is a thioester of acetic acid and coenzyme A. Key properties:
- More reactive than a regular ester because the thioester’s S-R group is a better leaving group than O-R (S is less electronegative, less lone-pair donation into carbonyl).
- Used for: citrate synthesis (condenses with oxaloacetate in the Krebs cycle), fatty acid synthesis (enters FAS), cholesterol synthesis, ketone body formation, acetylation of histones (gene regulation).
Biology uses thioesters instead of direct carboxylic acids because they are better electrophiles for acyl transfer reactions. The ATP-driven activation of acetate to acetyl-CoA is essentially a biological equivalent of SOCl₂ activation.