Basis for the equilibrium constant in the interconversion of l-lysine and l-beta-lysine by lysine 2,3-aminomutase.

l-beta-lysine and beta-glutamate are produced by the actions of lysine 2,3-aminomutase and glutamate 2,3-aminomutase, respectively. The pK(a) values have been titrimetrically measured and are for l-beta-lysine: pK(1)=3.25 (carboxyl), pK(2)=9.30 (beta-aminium), and pK(3)=10.5 (epsilon-aminium). For beta-glutamate the values are pK(1)=3.13 (carboxyl), pK(2)=3.73 (carboxyl), and pK(3)=10.1 (beta-aminium). The equilibrium constants for reactions of 2,3-aminomutases favor the beta-isomers. The pH and temperature dependencies of K(eq) have been measured for the reaction of lysine 2,3-aminomutase to determine the basis for preferential formation of beta-lysine. The value of K(eq) (8.5 at 37 degrees C) is independent of pH between pH 6 and pH 11; ruling out differences in pK-values as the basis for the equilibrium constant. The K(eq)-value is temperature-dependent and ranges from 10.9 at 4 degrees C to 6.8 at 65 degrees C. The linear van't Hoff plot shows the reaction to be enthalpy-driven, with DeltaH degrees =-1.4 kcal mol(-1) and DeltaS degrees =-0.25 cal deg(-1) mol(-1). Exothermicity is attributed to the greater strength of the bond C(beta)-N(beta) in l-beta-lysine than C(alpha)-N(alpha) in l-lysine, and this should hold for other amino acids.