Comparison of the structures of representative ribonuclease H proteins from each enzyme kinetic characterization of protein tyrosine phosphatases. H1 and H2 subtypes is shown in red. RNases H2 are larger than H1 and usually have additional helices. RNases H1 have been extensively studied to explore the relationships between structure and enzymatic activity. RNase H1 homologs and absent in the HIV RNase H domain, but examples of retroviral domains with C-helices do exist.
In prokaryotes, RNase H2 is enzymatically active as a monomeric protein. In eukaryotes, it is an obligate heterotrimer composed of a catalytic subunit A and structural subunits B and C. Both prokaryotic and eukaryotic H2 enzymes can cleave single ribonucleotides in a strand. H2 is the dominant source of RNase H activity there and is important for maintaining genome stability. Some prokaryotes possess an additional H2-type gene designated RNase HIII in the Roman-numeral nomenclature used for the prokaryotic genes. RNase H domain as an example. In addition, it is possible that one of the substrate’s oxygen atoms participates directly in the reaction as a base. RNase H domains, with a complex evolutionary history. The structure of the trimeric human H2 complex, with the catalytic A subunit in blue, the structural B subunit in brown, and the structural C subunit in pink.