Decarboxylates L-threonine-O-3-phosphate to yield (R)-1-amino-2-propanol O-2-phosphate, the precursor for the linkage between the nucleotide loop and the corrin ring in cobalamin. H(+) + O-phospho-L-threonine = (R)-1-aminopropan-2-yl phosphate + CO2 Cofactor biosynthesis; adenosylcobalamin biosynthesis. Homodimer. Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.