[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Amino acids: structure, charge and pKa [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # Amino acids: structure, charge and pKa The 20 amino acids share one backbone and differ in their side chain; their charge changes with pH, which decides how a protein folds and migrates. Updated 2026-11-01 ## Key facts - Each amino acid has an α carbon bonded to an amino group, a carboxyl group, a hydrogen and a side chain (R); in proteins they are all L-amino acids. - Typical pKa: α-carboxyl ≈ 2 and α-amino ≈ 9–10. At pH 7.4 nearly all exist as zwitterions (−COO⁻ and −NH₃⁺). - Ionisable side chains: Asp and Glu (≈ 4, acidic), His (≈ 6), Cys (≈ 8), Tyr (≈ 10), Lys (≈ 10.5) and Arg (≈ 12.5, basic). - Isoelectric point (pI): the pH at which net charge is zero; it is the mean of the two pKa values on either side of the neutral form (glycine ≈ 6). - Histidine is the only amino acid that buffers well at physiological pH (imidazole pKa ≈ 6). ## In detail - Henderson–Hasselbalch equation: pH = pKa + log ([base]/[acid]). When pH equals pKa, half of the group is protonated. - Below its pKa a group is protonated (carboxyl neutral, amino positive); above it, deprotonated (carboxyl negative, amino neutral). - To find the pI of an amino acid with an acidic side chain (Asp, Glu), average the two lowest pKa values; with a basic side chain (Lys, Arg, His), the two highest. - Side-chain classes: non-polar (Gly, Ala, Val, Leu, Ile, Met, Pro, Phe, Trp), polar uncharged (Ser, Thr, Cys, Tyr, Asn, Gln), acidic (Asp, Glu) and basic (Lys, Arg, His). - Glycine is the only amino acid with no chiral carbon; proline has its amino group inside a ring and breaks α-helices. - Essential (must come from the diet): phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine and lysine; arginine is conditionally essential during growth. - At pH 8.6 (electrophoresis) almost all plasma proteins carry a net negative charge and move towards the anode, faster the lower their pI. ## Memory aid Essential: “PVT TIM HaLL” (Phe, Val, Thr, Trp, Ile, Met, His, Leu, Lys). **Also called** Amino acids, Isoelectric point ## Clinical notes - Inborn errors of amino acid metabolism (phenylketonuria, maple syrup urine disease) are picked up by newborn screening. - Swapping a charged amino acid for an uncharged one changes a protein's pI: that is why HbS (Glu→Val) runs differently on electrophoresis. - The main buffer inside the red cell is the histidine residues of haemoglobin. ## Related conditions - [Phenylketonuria (PKU)](https://durgical.com/en/conditions/phenylketonuria/) - [Maple syrup urine disease](https://durgical.com/en/conditions/maple-syrup-urine-disease/) - [Cystinuria](https://durgical.com/en/conditions/cystinuria/) - [Hartnup disease](https://durgical.com/en/conditions/hartnup-disease/) ## Related - [Histidine](https://durgical.com/en/biochemistry/molecules/histidine/) - [Hydroxyproline](https://durgical.com/en/biochemistry/molecules/hydroxyproline/) - [Protein structure and folding](https://durgical.com/en/biochemistry/topics/protein-structure-and-folding/) - [Plasma proteins and electrophoresis](https://durgical.com/en/biochemistry/topics/plasma-proteins-and-electrophoresis/) - [Glutamate (glutamic acid)](https://durgical.com/en/biochemistry/molecules/glutamate/) - [Phenylalanine](https://durgical.com/en/biochemistry/molecules/phenylalanine/) - [Tyrosine](https://durgical.com/en/biochemistry/molecules/tyrosine/) - [Tryptophan](https://durgical.com/en/biochemistry/molecules/tryptophan/) - [Alanine](https://durgical.com/en/biochemistry/molecules/alanine/) ## Test yourself Think of the answer, then tap the question to see it. ## What net charge does glycine carry at pH 7.4? About zero: the carboxyl (pKa ≈ 2) is deprotonated and the amino group (pKa ≈ 9.6) protonated; it is a zwitterion with net charge ≈ 0. ## Why does histidine buffer well at physiological pH? Its imidazole has a pKa ≈ 6, close to 7.4, so it can take up or give off protons in that range. ## How is the isoelectric point of glutamic acid calculated? Average the two lowest pKa values (α-carboxyl and side-chain carboxyl), which flank the form with zero net charge: about 3.2. ## Which is the only amino acid without a chiral carbon? Glycine: its side chain is a hydrogen, so the α carbon carries two identical substituents. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/amino-acids-structure-charge-and-pka/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Amino acids: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002222.htm) (MedlinePlus, US National Library of Medicine, 2026); [What are proteins and what do they do?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/howgeneswork/protein/) (MedlinePlus, US National Library of Medicine, 2026); [L-Histidine (CID 6274)](https://pubchem.ncbi.nlm.nih.gov/compound/6274) (PubChem, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/