Threonine Deaminase from Salmonella typhimurium

Abstract

Purified threonine deaminase of Salmonella typhimurium was found to be monodisperse and to have a molecular weight of 194,000.The protein, in 6 M guanidine HCI and 0.1 M P-mercaptoethanol, dissociates into polypeptide chains of molecular weight 48,500.Tryptic peptide analysis implies that the four component chains of the native enzyme are of identical amino acid sequence.L-Isoleucine and L-valine, which are allosteric effecters of this enzyme, have no effect on the sedimentation properties and do not produce detectable changes in the optical rotatory dispersion curve of the native enzyme.The precise mechanism by which "allosteric effecters" control the catalytic properties of enzymes is unknown.A number of regulatory enzymes have been shown to be composed of subunits.In one such enzyme, aspartate transcarbamylase of Escherichia coli, the component subunits are of two distinct types: a regulatory subunit, which binds the effector, and a catalytic subunit, which contains the active site (1).Conformational changes associated with effector molecule binding have been inferred from sedimentation velocity measurements on aspartate transcarbamylase (2).More dramatic effects have been observed with the homoserine dehydrogenase of Rhodospirillum rubrurn.L-Threonine, an inhibitor of this enzyme, promotes its aggregation, whereas L-isoleucine and L-methionine reverse the L-threonine inhibition and cause the enzyme to dissociate (3).It is widely believed that the mechanism underlying the modification of regulatory enzymes by allosteric effecters involves conformational changes associated with effector binding.In enzymes composed of subunits, these changes may primarily involve rearrangement of subunits relative to one another, with only minor changes in the internal conformation of the individual subunits.Evidence bearing on these points is for the most part