Amino acids commonly found in proteins


Amino acid
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Description of R group
alanine ala   A non-polar
arginine arg   R basic, hydrophobic
asparagine asn   N amide version of below - polar
aspartic acid asp   D acidic - polar when ionised, otherwise can form hydrogen bonds
cysteine cys   C polar; contains -SH (thiol) group (can form disulphide bridges with other cysteine residues); also very reactive with metal ions
glutamine gln   Q amide version of below - polar
glutamic acid glu   E acidic- polar when ionised, otherwise can form hydrogen bonds
glycine gly   G simplest amino acid, allows full rotation of polypeptide chain: polar
histidine his   H basic
isoleucine ile   I non-polar
leucine leu   L non-polar
lysine lys   K basic
methionine met   M non-polar, contains S
phenylalanine phe   F non-polar, hydrophobic, aromatic ring
proline pro   P (not a true amino acid: imino group >NH, cyclical shape so less possibility of chain rotation)  non-polar
serine ser   S polar
threonine thr   T polar
tryptophan trp   W non-polar, aromatic ring: quite reactive
tyrosine tyr   Y polar, aromatic ring
valine val   V non-polar

Notes about side chains
n.b. Each amino acid  has both amino and carboxylic acid groups but these are are "used up" to form covalent peptide bonds, so do not confuse these with amino and carboxylic acid groups mentioned below.
Exception : the first amino and last carboxylic acid groups in the sequence (at either end of the polypeptide chain).

Acidic side chains can ionise and become negatively charged. Generally due to -COOH (carboxylic acid) groups in the R groups.
Basic side chains can ionise and become positively charged. Generally due to R groups containing extra nitrogen-containing groups (not always -NH2:  amino)

Polar side chains attract other polar side chains, and allow hydrogen bonding with other polar side chains. Hydroxyl (-OH)  groups make them attracted to water (hydrophilic), so they often face outwards from molecule.
Non-polar side chains attract other non-polar side chains, forming hydrophobic bonds. Presence of  hydrogen and alkyl groups, and lack of -OH groups make them repelled by water (hydrophobic), so they may face to centre of molecule.

Aromatic rings make bulky side chains which may affect coiling of the polypeptide chain.

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Mnemonics to help you remember 1-letter codes

Amino acid explorer

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