Serine (Ser, S) - Structure, pKa, pI, and Codons
What is serine and what is its side chain?
Serine (Ser, S) has the side chain -CH₂OH - Hydroxymethyl; a primary alcohol. It is classified as polar, uncharged, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.68.
| Three-letter code | Ser |
| One-letter code | S |
| Molecular formula | C₃H₇NO₃ |
| Molecular weight | 105.09 g/mol |
| Residue mass | 87.08 Da |
| Side chain | -CH₂OH - Hydroxymethyl; a primary alcohol. |
| Classification | Polar, uncharged |
| pKa (alpha-COOH) | 2.21 |
| pKa (alpha-NH3+) | 9.15 |
| Isoelectric point (pI) | 5.68 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Conditionally essential |
| Hydropathy (Kyte-Doolittle) | -0.8 |
| Configuration | L-form is (S) |
| Codons | UCU, UCC, UCA, UCG, AGU, AGC (6) |
Side chain
The R group of serine is -CH₂OH. Hydroxymethyl; a primary alcohol. This is what distinguishes it from the other 19 amino acids, since all of them share the same backbone: an alpha carbon bonded to an amino group, a carboxyl group, and a hydrogen.
Ionization and isoelectric point
Serine has 2 ionizable groups: alpha-COOH (pKa 2.21), alpha-NH3+ (pKa 9.15). The isoelectric point is the pH at which the molecule carries no net charge, found by averaging the two pKa values that bracket the neutral species.
pI = (2.21 + 9.15) / 2 = 5.68
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-serine is (S).
Genetic code
6 codons encode serine: UCU, UCC, UCA, UCG, AGU, AGC. This is the maximum degeneracy in the genetic code.
Features
- Hydroxyl-containing
- Phosphorylatable
- O-linked glycosylation site
MCAT notes
- Small and polar; a phosphorylation site and an O-linked glycosylation site.
- The nucleophile in the Ser-His-Asp catalytic triad of serine proteases (chymotrypsin, trypsin, elastase) and of acetylcholinesterase.
- Precursor to glycine and cysteine.
- One of six amino acids with maximum codon degeneracy, and its codons are not contiguous (UCN plus AGU/AGC).
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View serine in 3D