Catalyzes the anaerobic formation of alpha-ketobutyrate and ammonia from threonine in a two-step reaction. The first step involved a dehydration of threonine and a production of enamine intermediates (aminocrotonate), which tautomerizes to its imine form (iminobutyrate). Both intermediates are unstable and short-lived. The second step is the nonenzymatic hydrolysis of the enamine/imine intermediates to form 2-ketobutyrate and free ammonia. In the low water environment of the cell, the second step is accelerated by RidA. TdcB also dehydrates serine to yield pyruvate via analogous enamine/imine intermediates. L-threonine = 2-oxobutanoate + NH4(+) L-serine = NH4(+) + pyruvate Each protein molecule can bind up to four molecules of AMP, which act as an allosteric activator to the enzyme. The enzyme is also inhibited by alpha-keto acids and other catabolites. Amino-acid degradation; L-threonine degradation via propanoate pathway; propanoate from L-threonine: step 1/4. In the native structure, TdcB is in a dimeric form, whereas in the TdcB-AMP complex, it exists in a tetrameric form (dimer of dimers). In the absence of glucose and oxygen. Belongs to the serine/threonine dehydratase family.