Threonine Deaminase from a Mutant Requiring Isoleucine or Pyridoxine: Inability to Form a Tetrameric State

Abstract

Threonine deaminase from a mutant of Escherichia coli growing alternatively with isoleucine or pyridoxine is a dimer, whereas the wild-type enzyme is a tetramer.

Threonine deaminase from a nonsense mutant of Escherichia coli requiring isoleucine or pyridoxine: evidence for half-of-the-sites reactivity

“…The mutant threonine deaminase was purified to homogeneity by conventional procedures. The enzyme is a dimer of identical subunits of an approximate molecular weight of 43,000 (Grimminger and Feldner, 1974), whereas the wild-type enzyme is a tetramer of 50,000-dalton subunits (Calhoun et al, 1973;Grimminger et al, 1973). The mutant enzyme is not inhibited by isoleucine and does not bind isoleucine, as shown by equilibrium dialysis experiments.…”

“…Cell extracts of the mutant IP7 requiring isoleucine or vitamin B6 contain a low threonine deaminase activity (8). The catalytic activity of the mutant enzyme is not affected by isoleucine, in contrast to the feedback inhibi-tion of the wild-type enzyme by isoleucine (24).…”

“…coli BL3 (15) and of mutant IP7 (8) was purified as described previously. The assay of biosynthetic threonine deaminase has been described (8). Crude extracts containing biodegradative threonine deaminase were prepared by ultrasonic treatment of cells in 0.2 M potassium phosphate (pH 8) containing 1 mM glutathione and 1 mM adenosine 5monophosphate.…”