Biochem · the 20 you have to know cold
Amino acid trainer
The MCAT expects you to know all 20 by structure, name, three-letter code, one-letter code, side-chain class and charge. Study the grid, then drill until you can name every one in a second. Structures are shown at pH 7.4, with side chains colored by class.
0 / 0streak 0best —
The odd one-letter codesF = "Fenylalanine", W = "tWyptophan" (two rings, double-u), Y = "tYrosine", R = "aRginine", K = the letter before L (Lysine), D = "asparDic", E = "glutEmic", N = "asparagiNe", Q = "Q-tamine".
The two acids and their amidesAspartate (D) and glutamate (E) are the negative ones. Add NH₂ in place of O⁻ and you get asparagine (N) and glutamine (Q). Glu is one carbon longer than Asp.
Three bases"His, Lys, Arg go up the pKa ladder": His ≈ 6, Lys ≈ 10.5, Arg ≈ 12.5. Only His is partly protonated near pH 7, which makes it the classic active-site acid–base catalyst.
Sulfur pairCysteine has an S–H that forms disulfide bridges; methionine has a thioether (S–CH₃) and starts every protein (AUG).
Aromatic trioPhe, Tyr, Trp. Tyr = Phe + OH. Trp has the double ring and the biggest side chain. Trp and Tyr absorb UV at 280 nm (how proteins are measured).
The hydroxylsSer, Thr, Tyr can be phosphorylated by kinases. Ser is the nucleophile in serine proteases (chymotrypsin, trypsin).
Special shapesGlycine: no chiral center, very flexible. Proline: ring back to the backbone N, rigid, breaks helices. Both are common in turns and in collagen (Gly–Pro–Hyp).
Branched-chain trioVal, Leu, Ile (BCAAs) are metabolized in muscle; maple syrup urine disease can't break them down.
Essential amino acids"PVT TIM HaLL": Phe, Val, Thr, Trp, Ile, Met, His, Leu, Lys. The body can't make them, so they must come from food.
Charge at a pHIf pH < pKa, the group is protonated (COOH neutral, NH₃⁺ positive). If pH > pKa, it is deprotonated (COO⁻ negative, NH₂ neutral).
Isoelectric point (pI)Neutral side chain: average the two backbone pKas (≈ 6). Acidic: average the two lowest pKas. Basic: average the two highest.
ChiralityAll proteinogenic amino acids except glycine are L. All L-amino acids are S except cysteine, which is R because sulfur outranks the carboxyl.