This domain is found at the C-terminal end of the large alpha/beta domain making up various pyrimidine nucleoside phosphorylases [1,2]. It has slightly different conformations in different members of this family. For example, in pyrimidine nucleoside phosphorylase (PYNP, Swiss:P77826) there is an added three-stranded anti-parallel beta sheet as compared to other members of the family, such as E. coli thymidine phosphorylase (TP, Swiss:P07650) [1]. The domain contains an alpha/ beta hammerhead fold and residues in this domain seem to be important in formation of the homodimer [1].

1: The crystal structure of pyrimidine nucleoside phosphorylase in a closed conformation. Pugmire MJ, Ealick SE; Structure 1998;6:1467-1479. PMID:9817849

2: Three-dimensional structure of thymidine phosphorylase from Escherichia coli at 2.8 A resolution. Walter MR, Cook WJ, Cole LB, Short SA, Koszalka GW, Krenitsky TA, Ealick SE; J Biol Chem 1990;265:14016-14022. PMID:2199449