The primary goal of the research is the tertiary structural determination of the phosphoribosyltransferases (PTRase), porcine liver quinolinate PRTase and human erythrocyte hypoxanthine-guanine PRTase, by means of X-ray diffraction. Since the PRTase family of enzymes found in organisms ranging from bacteria to man show many striking similarities in their physical and chemical properties as well as in their critical function of neucleotide and aromatic residue biosynthesis, a knowledge of the three-dimensional architecture of two of these enzymes should yield a general understanding of PRTase catalytic action and evolutionary history. Furthermore, a high incidence of structural gene mutation with resultant metabolic disorders as Lesch-Nyhan syndrome have been seen in human PRTases, especially in hypoxanthine-guanine PRTase. A clear comprehension of the PRTase structure-function relationship will hopefully facilitate an alleviation of suffering from such diseases and perhaps even their elimination through proper genetic manipulation.