title: The tryptophan synthase alpha 2 beta 2 complex: a comparison of the reactivity of amino groups in the alpha and beta 2 subunits and in the complex by differential labeling studies.
abstract: The interaction of the alpha and beta 2 subunits of tryptophan synthase of Escherichia coli to form an alpha 2 beta 2 complex has been probed by differential labeling studies. In the first step the separate alpha or beta 2 subunit or the alpha 2 beta 2 complex was labeled by reductive methylation with trace amounts of [3H]HCHO in the presence of NaCNBH3. In the second step the 3H-labeled preparation was fully labeled under denaturing conditions with [14C]HCHO and NaCNBH3. Peptides containing labeled monomethyl or dimethyl amino groups were isolated after thermolytic digestion or after cyanogen bromide treatment. The 3H/14C ratio of each peptide is a measure of the relative reactivity of the amino group or groups in each peptide. The most reactive amino group in the alpha subunit, lysine-109, is strongly shielded from modification in the alpha 2 beta 2 complex. The most reactive amino group in the beta 2 subunit, the amino-terminal threonine, is not shielded from modification in the alpha 2 beta 2 complex.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D002250:Carbon Radioisotopes; D055598:Chemical Phenomena; D002621:Chemistry; D004926:Escherichia coli / Q000201:enzymology; D046911:Macromolecular Substances; D008745:Methylation; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000032:analysis; D014316:Tritium; D014367:Tryptophan Synthase / Q000302:isolation & purification*
chemical_list: D000596:Amino Acids; D002250:Carbon Radioisotopes; D046911:Macromolecular Substances; D010446:Peptide Fragments; D014316:Tritium; D014367:Tryptophan Synthase