Methionine (Met, M) - Structure, pKa, pI, and Codons
What is methionine and what is its side chain?
Methionine (Met, M) has the side chain -CH₂CH₂SCH₃ - 2-(methylthio)ethyl; a thioether. It is classified as nonpolar, aliphatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.74.
| Three-letter code | Met |
| One-letter code | M |
| Molecular formula | C₅H₁₁NO₂S |
| Molecular weight | 149.21 g/mol |
| Residue mass | 131.19 Da |
| Side chain | -CH₂CH₂SCH₃ - 2-(methylthio)ethyl; a thioether. |
| Classification | Nonpolar, aliphatic |
| pKa (alpha-COOH) | 2.28 |
| pKa (alpha-NH3+) | 9.21 |
| Isoelectric point (pI) | 5.74 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | +1.9 |
| Configuration | L-form is (S) |
| Codons | AUG (1) |
Side chain
The R group of methionine is -CH₂CH₂SCH₃. 2-(methylthio)ethyl; a thioether. 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
Methionine has 2 ionizable groups: alpha-COOH (pKa 2.28), alpha-NH3+ (pKa 9.21). 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 = (2.28 + 9.21) / 2 = 5.74
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-methionine is (S). Still (S) despite containing sulfur, because its sulfur sits two bonds from the alpha carbon and is never reached before CIP priorities are decided. Contrast with cysteine.
Genetic code
1 codon encode methionine: AUG. Having only one codon makes it unusually sensitive to point mutations, since any single-base change produces a different amino acid.
Features
- Sulfur-containing
MCAT notes
- Encoded by AUG, the start codon. Every eukaryotic protein begins with methionine (bacteria use formyl-methionine), often removed post-translationally.
- Nonpolar despite containing sulfur, because a thioether is unreactive - unlike cysteine's thiol.
- As S-adenosylmethionine (SAM) it is the body's chief methyl donor.
- One of only two amino acids with a single codon, making it highly sensitive to point mutations.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View methionine in 3D