The mammalian protein prostaglandin H2 synthase (PGHS) is a bifunctional enzyme that acts upstream of several important cellular processes including blood vessel dilation, platelet segregation, and inflammation. The crucial role of this protein motivated research in more primitive bacterial PGHS orthologs to understand its evolutionary origins. One such ortholog was identified in the cyanobacterium Nostoc punctiforme, a 10S-dioxygenase (Np-diox) which is predicted to fold in a structure similar to the mammalian PGHS structure, despite lacking the peroxidase activity exhibited by its relative. Homology modeling identified amino acids in Np-diox that are believed to be important for catalysis yet deviate from mammalian PGHS. Using site-directed mutagenesis, we mutated the hypothesized critical amino acids so that kinetic assays could be used to investigate the catalytic role of each residue. Mutant and wild-type proteins were expressed in BL21(DE3) cells and purified to homogeneity using Ni-NTA and Sephacryl S-200 chromatography. Kinetic studies are in progress which will determine the impact of each mutation on dioxygenase and peroxidase activity using linoleic and oleic acid substrates. Results from the kinetic analyses will enhance both our understanding of the Np-diox mechanism and the evolutionary development of mammalian prostaglandin synthase.