Dihydroorotate dehydrogenases (DHODs) are flavoenzymes catalyzing the oxidation of (S)-dihydroorotate to orotate in the biosynthesis of UMP, the precursor of all other pyrimidine nucleotides. On the basis of sequence DHODs can be divided into two classes, class 1, further divided in subclass 1A and 1B, and class 2. This division corresponds to the differences in cellular location and the nature of the electron acceptor. In this paper we report a study of Lactococcus lactis DHODA, a representative of the class 1A enzymes. Based on the DHODA structure we selected seven residues highly conserved among both main classes of DHODs as well as three residues representing surface charges close to the active site for site directed mutagenesis. The availability of both kinetic and structural data on the mutant enzymes allowed us to define the role individual structural segments plays in catalysis. For the first time we have also structurally proven the presence of an open active site loop in DHODA and obtained information about the interactions that control the movements of loops around the active site. Furthermore, in one of the mutant structures we observe differences between the two monomers of the dimer, confirming an apparent asymmetry between the two substrate binding sites of the dimer that was indicated by the kinetic results.