Enzymes containing the cyclodeaminase domain function in channeling one-carbon units to the folate pool. In most cases, this domain catalyses the cyclisation of formimidoyltetrahydrofolate to methenyltetrahydrofolate as shown in reaction (1). In the methylotrophic bacterium Methylobacterium extorquens, however, it catalyses the interconversion of formyltetrahydrofolate and methylenetetrahydrofolate ,as shown in reaction (2)
(1) 5-formimidoyltetrahydrofolate = 5,10-methenyltetrahydrofolate + NH(3)
(2) 10- formyltetrahydrofolate = 5,10-methenyltetrahydrofolate + H(2)O
In prokaryotes, this domain mostly occurs on its own, while in eukaryotes it is fused to a glutamate formiminotransferase domain (which catalyses the previous step in the pathway) to form the bifunctional enzyme formiminotransferase-cyclodeaminase . The eukaryotic enzyme is a circular tetramer of homodimers , while the prokaryotic enzyme is a dimer [10215859, 15651027].
The crystal structure of the cyclodeaminase enzyme from Thermaotogoa maritima has been studied . It is a homodimer, where each monomer is composed of six alpha helices arranged in an up and down helical bundle, forming a novel fold. The location of the active site is not known, but sequence alignments revealed two clusters of conserved residues located in a deep pocket within the dimmer interface. This pocket was large enough to accommodate the reaction product and it was postulated that this is the active site.