Antibiotic resistance is being recognized as a world-wide crisis in modern medicine, rapidly outpacing available treatment options. The serine-beta-lactamase OXA-48 was first identified from a carbapenem- and multidrug- resistant Klebsiella pneumoniae isolate in Turkey in 2001, and is now one of the most geographically widespread members of the class D beta-lactamases. OXA-48 like beta-lactamases differ only by a few amino acid substitutions and/or deletions, and the resulting effects were not always evident. Thermal stability (DSC) and protein crystallization were investigated for OXA-48, OXA-181, OXA-245, OXA-163 and OXA-436 to gain a greater insight on the subject. Three OXA-48 mutants affecting the dimer interface (R189A, R206A and R189A/R206A) were investigated by DSC, dimer affinity determination, and enzyme activity assays. Based on these results it was concluded that the OXA-48 dimer is strong and held together by many salt-bridges at the dimer interface. Destabilization of the dimer does not seem to affect the enzyme activity significantly, but appears to be important for the thermal stability of the enzyme. The nature of the conserved active site motif (S70, T71, X, K73) were also studied, using crystal structures of OXA-48 mutants S70T and S70T/T71S, and measuring the enzyme activity. The obtained results suggested that both residues are crucial for the enzyme since the mutations greatly reduced the enzyme activity. This was explained by an increased steric strain, and unfavorable orientation of the active site residues. Finally, the OXA-48 mutants P68A and P68A/Y211S (discovered in previous evolution studies selecting for OXA-48 ceftazidime and ceftazidime-avibactam resistance) were investigated by enzyme kinetics. The results suggested increased flexibility in the active site due to lack of P68, allowing for hydrolysis of larger substrates such as ceftazidime, to which OXA-48 is inactive. However, this seems to come at the cost of reduced ability to hydrolyse other substrates, compared to OXA-48.