This subsection of the ‘Function’ section describes the interaction between a single amino acid and another chemical entity. Priority is given to the annotation of physiological ligands.
A group of ligand-binding residues that are separated by no more than three residues is shown as a ‘Region‘
The general nature of the ligand (substrate, carbohydrate etc.) is indicated in the ‘Description field. If defined, the exact chemical composition is also provided.
By default, the interaction involves the amino acid side chain. For interactions mediated via the nitrogen atoms of the histidine side chain, we indicate, when known, which nitrogen is involved: Pros is the term used for the nitrogen atom that is closest to the alpha-carbon, and tele for the other.
Example: Q57I19, P73925
Interactions which occur via the backbone oxygen or nitrogen are also specified.
Interactions are considered to be non-covalent by default. When a substance is covalently bound with a cofactor or a prosthetic group, this is indicated by using the qualifier ‘covalent’.
For residues of enzymes that bind both to the substrate and the product of the reaction we have chosen to indicate binding to the substrate.
This subsection complements the information on covalent binding described in the ‘Modified residue’, ‘Glycosylation’ and ‘Lipidation’ subsections as well as the information in the ‘Active site’ subsection concerning unstable reaction intermediates.