... activity of mutants L160T, L160P, K13 3A, K13 3L, K13 3S and K13 3T dropped to less than 5%. When guanine wasused as the substrate, the activity of L160T, K13 3A, K13 3T, K13 3V fell to 18%, 14%, 13.8% and ... and crystals. The N-terminus of each subunit of the tetra-mer forms crystal contacts with the active site of asubunit of the neighboring tetramer. As a result, onetetramer links to four other ... the active site of HGPRT [ 3–6 ]. The crystal structure of wild-type HGPRT from Thermoanaerobacter tengcongensis was solved recently[7]. In the present study, we sought to further ourunderstanding...