The term C cap (C-cap, Ccap) describes an amino acid in a particular position within a protein or polypeptide.[1][2][3][4][5][6] The C cap residue of an alpha helix is the last amino acid residue at the C terminus of the helix. More precisely, it is defined as the last residue (i) whose NH group is hydrogen-bonded to the CO group of residue i-4 (or sometimes residue i-3). Because of this it is sometimes also described as the residue following the helix.
Certain motifs occur commonly at or around the C cap, notably the Schellman loop and the niche (protein structural motif).
The N cap is the corresponding amino acid residue at the other end of the helix
References
edit- ^ Richardson, JM; Richardson DC (1988). "Amino acid preferences for specific locations at the ends of alpha-helices". Science. 240 (4859): 1648–1652. Bibcode:1988Sci...240.1648R. doi:10.1126/science.3381086. PMID 3381086. S2CID 38467101.
- ^ Presta, LG; Rose GD (1988). "Helix Caps". Science. 240 (4859): 1632–1641. Bibcode:1988Sci...240.1632P. doi:10.1126/science.2837824. PMID 2837824.
- ^ Doig, AJ; MacArthur MW (1997). "Structures of N-termini of helices in proteins". Protein Science. 6 (1): 147–155. doi:10.1002/pro.5560060117. PMC 2143508. PMID 9007987.
- ^ Aurora, R; Rose GD (1998). "Helix Capping". Protein Science. 7 (1): 21–38. doi:10.1002/pro.5560070103. PMC 2143812. PMID 9514257.
- ^ Gunasekaran, K; Nagarajam HA (1998). "Stereochemical punctuation marks in protein structure". Journal of Molecular Biology. 275 (5): 917–932. doi:10.1006/jmbi.1997.1505. PMID 9480777. S2CID 35919397.
- ^ Leader, DP; Milner-White EJ (2011). "The structure of the ends of helices in globular proteins". Proteins. 79 (3): 1010–1019. doi:10.1002/prot.22942. PMID 21287629. S2CID 22240314.