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Abstract

Formaldehyde activating enzyme (Fae) was first discovered in
methylotrophic bacteria, where it is involved in the oxidation of
methanol to CO2 and in formaldehyde detoxification. The 18 kDa protein
catalyzes the condensation of formaldehyde with tetrahydromethanopterin
(H4MPT) to methylene-H4MPT. We describe here that Fae is also present
and functional in the methanogenic archaeon Methanosarcina barkeri. The
faeA homologue in the genome of M. barkeri was heterologously expressed
in Escherichia coli and the overproduced purified protein shown to
actively catalyze the condensation reaction: apparent V-max = 13 U/mg
protein (1 U = mu mol/min); apparent Km for H4MPT = 30 mu M; apparent Km
for formaldehyde = 0.1 mM. By Western blot analysis the concentration of
Fae in cell extracts of M. barkeri was determined to be in the order of
0.1% of the soluble cell proteins. Besides the faeA gene the genome of
M. barkeri harbors a second gene, faeB-hpsB, which is shown to code for
a 42 kDa protein with both Fae activity (3.6 U/mg) and
hexulose-6-phosphate synthase (Hps) activity (4.4 U/mg). The results
support the recent proposal that in methanogenic archaea Fae and Hps
could have a function in ribose phosphate synthesis.