Niche

Niche (/ˈniːʃ/ or US /ˈnɪtʃ/) may refer to:

Niche (architecture)

A niche in classical architecture is an exedra or an apse that has been reduced in size, retaining the half-dome heading usual for an apse. Nero's Domus Aurea (AD 64–69) was the first semi-private dwelling that possessed rooms that were given richly varied floor plans, shaped with niches and exedras; sheathed in dazzling polished white marble, such curved surfaces concentrated or dispersed the daylight.

The word derives from the Latin nidus or nest, via the French niche. The Italian nicchio for a sea-shell may also be involved, as the traditional decoration for the top of a niche is a scallop shell, as in the illustration, hence also the alternative term of "conch" for a semi-dome, usually reserved for larger exedra.

Niche (protein structural motif)

In the area of protein structural motifs, niches are three or four amino acid residue features in which main-chain CO groups are bridged by positively charged or δ+ groups. The δ+ groups include groups with two hydrogen bond donor atoms such as NH₂ groups and water molecules. In typical proteins, 7% of amino acid residues belong to niches bound to a δ+ group, while another 7% have the conformation but no single cationic bridging group is detected. Two websites are available for examining niches in proteins, Motivated Proteins: ; or PDBeMotif: .

Niches are of two kinds, distinguished as niche3 (3 residues, i to i+2) and niche4 (4 residues, i to i+3). In a niche3 motif the δ+-binding carbonyl group is from residues i and i+2 while in a niche4 motif it is from residues i and i+3.

A niche3 has the α conformation for residue i+1 and the β conformation for residue i+2; a niche4 has the α conformation for residues i+1 and i+2 and the β conformation for residue i+3.