Chapter 15 · Chapter 16
Biomolecules
Four families build every living thing. What each one does follows directly from the functional groups it carries.
| Amino acid | Codes | Side chain | Class | Mass | pKa | pI |
|---|---|---|---|---|---|---|
| GlycineThe smallest side chain is a single hydrogen, so glycine fits where nothing else will and is the only achiral amino acid. | Gly · G | –H | Nonpolar | 75.07 | — | 5.97 |
| AlanineA simple methyl group; the baseline hydrophobic residue. | Ala · A | –CH₃ | Nonpolar | 89.09 | — | 6.00 |
| ValineessentialBranched and bulky. The sickle cell mutation swaps glutamate for valine. | Val · V | –CH(CH₃)₂ | Nonpolar | 117.15 | — | 5.96 |
| LeucineessentialOne of the most common residues in proteins. | Leu · L | –CH₂CH(CH₃)₂ | Nonpolar | 131.17 | — | 5.98 |
| IsoleucineessentialAn isomer of leucine with a second chiral centre. | Ile · I | –CH(CH₃)CH₂CH₃ | Nonpolar | 131.17 | — | 6.02 |
| ProlineIts side chain loops back onto the nitrogen, which kinks the chain and breaks helices. | Pro · P | ring to the backbone N | Nonpolar | 115.13 | — | 6.30 |
| PhenylalanineessentialPeople with PKU cannot process it, which is why diet drinks carry a warning. | Phe · F | –CH₂C₆H₅ | Aromatic | 165.19 | — | 5.48 |
| TryptophanessentialThe largest residue, and the precursor of serotonin. | Trp · W | indole ring | Aromatic | 204.23 | — | 5.89 |
| MethionineessentialEvery protein starts here — AUG is the start codon. | Met · M | –CH₂CH₂SCH₃ | Nonpolar | 149.21 | — | 5.74 |
| SerineA small hydroxyl that enzymes phosphorylate to switch proteins on and off. | Ser · S | –CH₂OH | Polar, uncharged | 105.09 | — | 5.68 |
| ThreonineessentialAnother phosphorylation site, with a second chiral centre. | Thr · T | –CH(OH)CH₃ | Polar, uncharged | 119.12 | — | 5.60 |
| CysteineTwo cysteines oxidise into a disulfide bridge, the covalent staple that holds a folded protein together. | Cys · C | –CH₂SH | Polar, uncharged | 121.16 | 8.18 | 5.07 |
| TyrosineAromatic and weakly acidic; the precursor of adrenaline and thyroid hormone. | Tyr · Y | –CH₂C₆H₄OH | Aromatic | 181.19 | 10.07 | 5.66 |
| AsparagineThe amide of aspartate. First isolated from asparagus. | Asn · N | –CH₂CONH₂ | Polar, uncharged | 132.12 | — | 5.41 |
| GlutamineThe most abundant free amino acid in blood. | Gln · Q | –CH₂CH₂CONH₂ | Polar, uncharged | 146.15 | — | 5.65 |
| Aspartic acidNegatively charged at body pH. Half of aspartame. | Asp · D | –CH₂COOH | Acidic, negative | 133.10 | 3.65 | 2.77 |
| Glutamic acidNegatively charged at body pH, and the source of umami taste as MSG. | Glu · E | –CH₂CH₂COOH | Acidic, negative | 147.13 | 4.25 | 3.22 |
| LysineessentialPositively charged at body pH; the site collagen cross-links through. | Lys · K | –(CH₂)₄NH₂ | Basic, positive | 146.19 | 10.53 | 9.74 |
| ArginineThe most basic residue — its guanidinium group stays charged at any biological pH. | Arg · R | –(CH₂)₃NHC(NH)NH₂ | Basic, positive | 174.20 | 12.48 | 10.76 |
| HistidineessentialIts pKa sits near body pH, so it can gain or lose a proton on demand — which is why it does the work in so many enzyme active sites. | His · H | imidazole ring | Basic, positive | 155.15 | 6 | 7.59 |
Essential means dietary
Nine of the twenty cannot be made by the body and have to come from food. The other eleven are built from intermediates you already have.
Why histidine turns up in active sites
Its side chain pKa of 6.0 sits close to body pH, so it can pick up or drop a proton as the reaction needs. No other residue is poised so evenly between both states.