The structure determination of apotransketolase and the comparison of its three-dimensional structure with that of the holoenzyme has revealed that no large conformational changes are associated with cofactor binding. Two loops at the active site are flexible in the apoenzyme which enables ThDP to reach its binding site. Binding of the cofactor induces defined conformations for these two loops at the active site. One of these loops is directly involving in binding of the cofactors, Ca2+ and ThDP. This loop acts like a flap which closes off the diphosphate binding site. After binding of the cofactor, residues of this loop form interactions to residues of loop 383-398 from the second subunit. These interactions stabilize the conformation of the two loops from a flexible to a 'closed' conformation.