20 Amino Acids

Alanine [A] Ala

• Aliphatic R-group (CH₃)

• Hydrophobic

• Ambivalent (overall neutral)

Arginine [R] Arg

• guanidino group

• designed to bind with phosphate

• important in nitrogen metabolism

• basic

• polar

guanidino group

responsible for Sakaguchi reaction

Sakaguchi reaction

designed to bind with phosphate

Aspartic Acid [D] Asp

• one of the acidic amino acids

• acts as general acid in enzyme active centers

Asparagine [N] Asn

• amide of Aspartic acid

• common site for attachment of carbohydrates in glycoproteins

Cysteine [C] Cys

• sulfur-containing

• Plays a key role in stabilizing extracellular proteins

• Can react with itself to form an oxidized dimer by the formation of a disulfide bond (primary structure S-S)

Glutamic Acid [E] Glu

• one additional CH₃

• interconvertible with α-ketoglutarate

• important in the biosynthesis of proline

Glycine [G] Gly

• smallest and simplest amino acid

• low MW

• not chiral

• ambivalent

• exists mainly as zwitterion

Proline [P] Pro

• responsible for the formation of the beta-turn/bends

• imino acid

• when found in an α helix, the helix will have a slight bend due to the lack of the hydrogen bonds

• found at the end of α helix or in turns or loops

histidine [H] His

• has imidazole

• basic

Isoleucine [I] Ile

Side chain is not reactive and therefore not involved in any covalent chemistry in enzyme active centers.

Leucine [L] Leu

• has one additional methylene group in its side chain compared with valine

• hydrophobic (terminal left)

• buried in folded proteins

Lysine [K] Lys

• positively charged ε-amino group

• hydrophillic

Methionine [M] Met

• has thiol ether

• plays role in metabolism forming S-adenosylmethionine

S-adenosylmethionine

acts as methyl donor during methylation (drug metabolism)

Phenylalanine [F] Phe

• derivative of alanine with a phenyl substituent on the β carbon

• hydrophobic

Serine [S] Ser

• OH-containing

• Polar

• differs from alanine in that one of the methylenic hydrogens is replaced by a hydroxyl group

• hydrophilic

Threonine [T] Thr

• non-aromatic OH

• α and β carbons are optically active

optically active

capable of rotating plane-polarized light either to the left or right

Tryptophan [W] Trp

has an indole ring

Tyrosine [Y] Tyr

• phenolic side chain

• OH containing

• derived from phenylalanine by hydroxylation in the para-position

Valine [V] Val

• hydrophobic

• found in the interior of proteins