Monocupin

Monocupin is a term used in protein science to denote a protein that belongs to the cupin superfamily and consists of a single cupin domain. Cupins are a large and diverse family of beta-barrel proteins characterized by conserved sequence motifs and a wide range of functions, including enzymatic activity, metal binding, and seed storage.

A monocupin typically contains a single cupin domain of about 110 to 140 amino acids. The two canonical cupin motifs, G(X)5HXH(X)3,4E and G(X)5PXG, are present within the domain and coordinate a metal at the active site in many family members. Monocupins can function as enzymes, oxalate oxidases and other oxidoreductases among them, or serve as non-enzymatic structural or storage proteins. They may occur as solitary polypeptides or as homodimers; their oligomeric state can vary by protein.

Distribution: monocupins are found across plants, bacteria, fungi, and some algae. In plants, monocupins include germin-like proteins that participate in defense responses and cell-wall remodeling; in microbes, monocupins contribute to metabolism and stress responses.

Classification: within the cupin superfamily, monocupin refers specifically to a single-domain member, whereas bicupin describes proteins with two cupin domains in tandem. The distinction informs predictions about structure and potential function but many monocupins have diverse roles that are not limited to a single activity.

See also: cupin superfamily; germin-like proteins; seed storage proteins.