Last modified September 9, 2013
This subsection of the ‘General annotation (Comments)’ section provides information relevant to cofactors. A cofactor is any non-protein substance required for an enzyme to be catalytically active. Some cofactors are inorganic, such as metal ions in various oxidation states. Others, such as most vitamins, are organic molecules. Cofactors are either bound tightly to active sites or may be loosely associated with the enzyme. They may also be important for structural integrity, i.e. in their absence the enzyme does not fold properly or becomes unstable.
Substrates for a reaction (NAD, FAD, ATP, ATP) are listed in the ‘Catalytic activity’ subsection. The exceptions are the rare cases when an enzyme binds the same type of molecule as a substrate at one site and as a cofactor at other sites.
When two or more similar molecules can act as cofactor at the same site, they are listed in the ‘Cofactor’ subsection.
When an enzyme binds different cofactors at different sites, there are listed on separate lines in the ‘Cofactor’ subsections for each different cofactor.
Ubiquitous substances, such as water, are not considered as cofactors and thus not annotated.
In the case of multimeric enzymatic complex, the cofactor is annotated only in the entries of subunits that directly binds it.