Genes, Molecules
and Cells
Select an amino acid from the drop-down list. Then hit ‘Show’ to display its side-chain (R group).
- Why only side-chains?
The α-amino and α-carboxyl groups of free amino acids are condensed into peptide bonds (CO-NH) in proteins. So it is the side chains that determine the properties of proteins. - Why the dimmed C-N in proline?
Proline is technically an imino acid, because its three-carbon side-chain is bonded to the α-nitrogen, leaving it with only an α-NH to form a peptide bond (CO-N) in proteins. To show the proline ring it is necessary to include the α-C and the α-N. However, they have been dimmed for consistency with the structures of the other amino-acid side-chains, where they are not shown. - Why the charged form of certain amino acids?
Most often we are interested in proteins at physiological pH (ca. 7.4), where aspartic acid, glutamic acid, arginine and lysine are almost completely ionized. The weakly basic histidine side-chain is generally not completely protonated at this pH, but the protonated form is shown because of its functional importance.