Ty1/Copia family of RNase HI in long-term repeat retroelements. Ribonuclease H (RNase H) enzymes are divided into two major families, Type 1 and Type 2, based on amino acid sequence similarities and biochemical properties. RNase H is an endonuclease that cleaves the RNA strand of an RNA/DNA hybrid in a sequence non-specific manner in the presence of divalent cations. RNase H is widely present in various organisms, including bacteria, archaea and eukaryotes. RNase HI has also been observed as adjunct domains to the reverse transcriptase gene in retroviruses, in long-term repeat (LTR)-bearing and non-LTR retrotransposons. RNase HI in LTR retrotransposons perform degradation of the original RNA template, generation of a polypurine tract (the primer for plus-strand DNA synthesis), and final removal of RNA primers from newly synthesized minus and plus strands. The catalytic residues for RNase H enzymatic activity, three aspartatic acids and one glutamatic acid residue (DEDD) are unvaried across all RNase H domains. Phylogenetic patterns of RNase HI of LTR retroelements is classified into five major families, Ty3/Gypsy, Ty1/Copia, Bel/Pao, DIRS1 and the vertebrate retroviruses. Ty1/Copia family is widely distributed among the genomes of plants, fungi and animals. RNase H inhibitors have been explored as an anti-HIV drug target because RNase H inactivation inhibits reverse transcription.

Alpha-N-acetylglucosaminidase (NAGLU) tim-barrel domain. Alpha-N-acetylglucosaminidase, a lysosomal enzyme required for the stepwise degradation of heparan sulfate. Mutations on the alpha-N-acetylglucosaminidase (NAGLU) gene can lead to Mucopolysaccharidosis type IIIB (MPS IIIB; or Sanfilippo syndrome type B) characterized by neurological dysfunction but relatively mild somatic manifestations. The structure shows that the enzyme is composed of three domains. This central domain has a tim barrel fold.

Alpha-N-acetylglucosaminidase (NAGLU) C-terminal domain. Alpha-N-acetylglucosaminidase, a lysosomal enzyme required for the stepwise degradation of heparan sulfate. Mutations on the alpha-N-acetylglucosaminidase (NAGLU) gene can lead to Mucopolysaccharidosis type IIIB (MPS IIIB; or Sanfilippo syndrome type B) characterized by neurological dysfunction but relatively mild somatic manifestations. The structure shows that the enzyme is composed of three domains. This C-terminal domain has an all alpha helical fold.