The degradation of L-threonine to glycine in both prokaryotes and eukaryotes takes place through a two-step biochemical pathway. In the first step, L-threonine (L-Thr) is oxidised to 2-amino-3-oxobutanoate (2A-3OBU) using NAD+ as acceptor. This reaction is catalysed by mitochondrial L-threonine 3-dehydrogenase (TDH) (Edgar 2002). The human activity is inferred from the characterised porcine Tdh (Edgar 2002b, Kao & Davis 1994). TDH is thought to exist as a tetramer on the mitochondrial inner membrane in complex with dimeric 2-amino-3-ketobutyrate coenzyme A ligase (GCAT), the second enzyme in this pathway (Tressel et al. 1986). With these two enzymes located together, it stops the rapid and spontaneous decarboxylation of 2A-3OBU to aminoacetone and carbon dioxide and instead, results in glycine formation.