Its primary function is to catalyze the reversible interconversion of serine and glycine with tetrahydrofolate (THF) serving as the one-carbon carrier. This reaction serves as the major source of one-carbon groups required for the biosynthesis of purines, thymidylate, methionine, and other important biomolecules. Also exhibits THF-independent aldolase activity toward beta-hydroxyamino acids, producing glycine and aldehydes, via a retro-aldol mechanism. Thus, is able to catalyze the cleavage of L-threonine, L-allo-threonine, L-threo-beta-phenylserine and L-erythro-beta-phenylserine. This second activity is likely to be physiological in H.thermophilus, which is an organism that lacks the ortholog gene for the 'real' threonine aldolase characterized in mesophilic bacteria (LtaE), yeast and plants. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O = (6S)-5,6,7,8-tetrahydrofolate + L-serine L-threonine = acetaldehyde + glycine L-allo-threonine = acetaldehyde + glycine kcat is 18.7 sec(-1) for the L-serine hydroxymethyltransferase reaction, 2.3 sec(-1) for the L-threonine cleavage, and 18.0 sec(-1) for the L-allo-threonine cleavage, at 70 degrees Celsius. In the THF-independent aldolase reaction, the rate constants for the erythro form substrates and for the substrates with beta-phenyl groups are larger than those for the threo form substrates and for the substrates with beta-methyl groups, respectively. Optimum pH is 7.4 for the THF-independent L-allothreonine aldolase activity. More than 95% of full activity remains at pH 8.4, although the activities are reduced to about 90% at pH 6.9 and about 75% at pH 5.9. Highly thermostable. Retains almost complete THF-independent L-allo-threonine aldolase activity after being incubated at 75 degrees Celsius for 10 minutes, and retains about 77% residual activity after being treated at 87 degrees Celsius for the same time. One-carbon metabolism; tetrahydrofolate interconversion. Amino-acid biosynthesis; glycine biosynthesis; glycine from L-serine: step 1/1. Homodimer. Belongs to the SHMT family.