sentences:  The N-terminal has a pair of antiparallel strands, with residues that interact with bound orotate and Lys 26 that extends to the active site and forms a bond with the flexible loop in its closed form.Orotic acid and PRPP are stabilized in the active site mostly by hydrogen bonding with stabilizing interactions from Lys 26, Phe 34 and Phe 35 to orotic acid, as well as Thr 128, Ala 129, Gly 130, Ala 132, Asp 124, Lys 26 and Lys 73 to PRPP.When Lys 26 is mutated, orotate phosphoribosyltransferase often exhibits reduced activity and specificity.