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Pt-barrel
The PT-barrel, is a novel protein fold that was discovered in the crystal structure of the prenyltransferase, Orf2 from Streptomyces sp. strain CL190. == Structure ==
The PT-barrel consists of a closed β-sheet comprising ten anti-parallel β-strands arranged around a central β-barrel core, itself surrounded by a ring of α-helices forming the outer, solvent exposed surface of the barrel. The secondary connectivity nearly conforms to an (ααββ)5 classification, but is more specifically described using the (ααββ)4-(αββ)−α nomenclature, where helices 6 and 8, both involved in inter-protein contacts in the crystal lattice, display a helical “kink”. The most hydrophobic section of the PT-barrel is the region residing between the outer surface of the cylindrical β-barrel and the belt of surrounding α-helices. Additionally, a number of hydrophobic residues located inside the barrel accommodate the prenyl tail of the Geranyl-diPhosphate (GPP) and GSPP molecules, while the diphosphate or the thio-diphosphate head groups of substrate and substrate analogs, respectively, point toward the “upper”, more polar end of the barrel where a Mg2+ ion is coordinated. Finally, the bottom of the barrel is capped by a short C-terminal helix (α11).
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