Lysine 258 in aspartate aminotransferase: Enforcer of the circe effect for amino acid substrates and general-base catalyst for the 1,3-prototropic shift

The replacement of Lys258 by alanine (K258A) in aspartate aminotransferase reduces the rate constant for the central, 1,3-prototropic shift by 106-10'-fold, confirming the role of Lys258 as the general-base catalyst for this step. The rate constant for the 1,3-prototropic shift interconverting K258A aldimine and ketimine intermediates is pH-independent, like that of the wild-type enzyme (WT-AATase). K258A binds amino acid substrates in external aldimine intermediates 10 5 -fold more tightly than does WT-AATase