Catalyzes the condensation of formaldehyde with tetrahydromethanopterin (H(4)MPT) to 5,10-methylenetetrahydromethanopterin. Catalyzes the reversible formation of ribulose-5-phosphate and formaldehyde from 3-hexulose-6-phosphate. 5,6,7,8-tetrahydromethanopterin + formaldehyde = 5,10-methylenetetrahydromethanopterin + H2O D-ribulose 5-phosphate + formaldehyde = D-arabino-hex-3-ulose 6-phosphate Carbohydrate biosynthesis; D-ribose 5-phosphate biosynthesis. In the N-terminal section; belongs to the formaldehyde-activating enzyme family. In the C-terminal section; belongs to the HPS/KGPDC family. HPS subfamily.