Crystallization of the bifunctional methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase domain of the human trifunctional enzyme

Methylenetetrahydrofolate([H 4 ] folate) dehydrogenase (D) and methenyl[H 4 ] folate cyclohydrolase (C) coexist as a bifunctional enzyme (DC) or as the amino‐terminal domain of a trifunctional enzyme (DCS) where the third activity is 10‐formyl[H 4 ]lfolate synthetase (S). Two crystal forms of the DC domain of the human cytosolic DCS enzyme have been grown from polyethyleneglycol solution. The monoclinic P2 1 crystals diffract to 2.8 Å with a = 72.5 Å, b = 68.5 Å, c = 125.2 Å, and β = 91.8° but were found to be twinned. The orthorhombic P2 1 2 1 2 1 crystals diffract to 2.5 Å with a = 67.7 Å, b = 135.9 Å, c = 61.6 Å, and contain two molecules per asymmetric unit. Proteins 26:479–480 © 1996 Wiley‐Liss, Inc.