SUBUNIT STRUCTURE OF B COMPONENT OF ESCHERICHIA COLI TRYPTOPHAN SYNTHETASE

Sedimentation equilibrium studies demonstrate that the B subunit of tryptophan synthetase dissociates into two polypeptides upon treatment of the protein with concentrations of urea greater than 4 m or with performic acid. Some dissociation is observed even in dilute solution in phosphate buffer. C-Terminal amino acid analysis and gel electrophoresis experiments support the hypothesis that the native enzyme is a dimer composed of two identical or very similar subunits whose molecular weight is 45,000 g/mole. The ureadissociated enzyme can be reconstituted with 80% of the original activity if thiol concentrations are maintained at the proper level during removal of urea. A hybrid enzyme, one of whose subunits contains pyridoxal phosphate reduced onto the enzyme with sodium borohydride, exhibits one-half the specific activity of the native enzyme in catalyzing the conversion of indole into tryptophan. © 1969, American Chemical Society. All rights reserved.