liminfoAmino Acid Reference
Free reference guide: Amino Acid Reference
About Amino Acid Reference
The Amino Acid Reference is a comprehensive biochemistry resource covering all 20 standard amino acids plus selenocysteine (the 21st amino acid). Each entry includes the one-letter code, three-letter abbreviation, molecular weight in Daltons, isoelectric point (pI), side chain structure, and key biochemical roles such as phosphorylation sites, disulfide bond formation, and UV absorbance properties.
Beyond individual amino acids, this reference provides essential classification data including the Kyte-Doolittle hydrophobicity scale, pKa values for all ionizable groups, conservative substitution groups used in sequence alignment (BLOSUM62), and the complete list of 9 essential amino acids. These are the core concepts tested in biochemistry, molecular biology, and bioinformatics coursework.
All data is organized into six categories: nonpolar, polar uncharged, acidic (negatively charged), basic (positively charged), amino acid classification, and special amino acids. The searchable interface lets you quickly find any amino acid by name, code, property, or category, making it ideal for exam preparation, protein analysis, and laboratory reference.
Key Features
- Complete data for all 20 standard amino acids with MW, pI, side chain, and one/three-letter codes
- Kyte-Doolittle hydrophobicity index for each residue from Ile (4.5) to Arg (-4.5)
- pKa values for alpha-COOH, alpha-NH3+, and all ionizable side chains (Asp, Glu, His, Cys, Tyr, Lys, Arg)
- Conservative substitution groups for sequence alignment and BLOSUM62 scoring interpretation
- UV absorbance data for aromatic amino acids (Trp 280nm, Tyr 274nm, Phe 257nm) with extinction coefficients
- Essential amino acid identification with mnemonic aid and conditionally essential amino acids
- Selenocysteine (Sec/U) with UGA codon and SECIS element requirements
- Bilingual Korean/English interface with category-based filtering and instant search
Frequently Asked Questions
What are the 9 essential amino acids?
The 9 essential amino acids that must be obtained from diet are: Histidine (H), Isoleucine (I), Leucine (L), Lysine (K), Methionine (M), Phenylalanine (F), Threonine (T), Tryptophan (W), and Valine (V). Additionally, Arginine, Cysteine, Glutamine, Tyrosine, Proline, and Glycine are considered conditionally essential.
How is the isoelectric point (pI) of an amino acid determined?
The pI is the pH at which the amino acid has no net electrical charge. For simple amino acids, pI is the average of the two pKa values (alpha-COOH and alpha-NH3+). For amino acids with ionizable side chains, pI is the average of the two pKa values that flank the zwitterionic form. For example, Asp has pI = (2.09 + 3.65)/2 = 2.77.
Which amino acids absorb UV light at 280nm?
Tryptophan (280nm, extinction coefficient 5,500 M-1cm-1), Tyrosine (274nm, 1,490 M-1cm-1), and Phenylalanine (257nm, weak absorption) are the aromatic amino acids that absorb UV. Protein concentration is commonly estimated at A280 using the formula: e280 = nTrp*5500 + nTyr*1490 + nCys*125.
What is the Kyte-Doolittle hydrophobicity scale?
The Kyte-Doolittle scale assigns a hydrophobicity index to each amino acid, ranging from Isoleucine (4.5, most hydrophobic) to Arginine (-4.5, most hydrophilic). This scale is widely used for transmembrane domain prediction and protein folding analysis. A sliding window average above 1.8 typically indicates a transmembrane region.
What are conservative amino acid substitutions?
Conservative substitutions replace one amino acid with another of similar physicochemical properties: hydrophobic (A, V, I, L, M), aromatic (F, Y, W), positively charged (K, R, H), negatively charged (D, E), and polar (S, T, N, Q). These substitutions score positively in the BLOSUM62 matrix and typically preserve protein function.
Why is Glycine unique among amino acids?
Glycine is the smallest amino acid with only a hydrogen atom as its side chain (-H), making it the only achiral (non-chiral) standard amino acid. Its small size provides exceptional backbone flexibility, and it is critical in collagen structure where it occupies every third position in the Gly-X-Y repeat motif.
What makes Cysteine important for protein structure?
Cysteine contains a thiol (-SH) side chain with pKa 8.3 that can form disulfide bonds (S-S) with another cysteine residue. These covalent crosslinks stabilize protein tertiary and quaternary structure, especially in secreted proteins. Cysteine also coordinates zinc in zinc finger motifs and acts as a nucleophile in enzyme active sites.
What is selenocysteine and how is it encoded?
Selenocysteine (Sec, U) is the 21st amino acid, with a selenol (-SeH) group instead of the thiol in cysteine. It is encoded by the UGA codon, which normally signals stop, but is read as Sec when a SECIS (Selenocysteine Insertion Sequence) element is present in the mRNA. It is found in selenoproteins like glutathione peroxidase.