Biochemistry and medicinal chemistry · BEGINNER LESSON
Phosphorothioate
Phosphorothioates are phosphorus-containing compounds in which sulfur replaces an oxygen in a phosphate group. In DNA and oligonucleotides, this change can create a sulfur-containing backbone linkage that alters stability and protein interactions.
What it is
A phosphorothioate is a phosphate-related structure in which sulfur takes the place of a nonbridging oxygen. In biology, this can occur in the DNA sugar-phosphate backbone; in chemistry and drug design, it also appears in oligonucleotides, which are short chains of DNA- or RNA-like units. Sources describe both natural bacterial DNA phosphorothioate systems and synthetic phosphorothioate linkages used to make oligonucleotides more resistant to nuclease degradation, where nucleases are enzymes that cut nucleic acids.
[1] Occurrence, evolution, and functions of DNA phosphorothioate ...pmc.ncbi.nlm.nih.gov › articles › PMC5879708 [2] 1 Introduction [3] New Frontiers in phosphorothioate formation: harnessing inorganic phosphorus sources - PubMed
How it acts in bacteria
In bacteria, DNA phosphorothioate modification is added by DndABCDE proteins. Together with DndFGH, these proteins can form a restriction-modification system, a defense system that marks self DNA and helps protect cells from invading foreign DNA. The sources also report that some bacterial genomes contain only the modification side, without the restriction side.
How it acts in oligonucleotide drugs
In antisense oligonucleotides, or ASOs, phosphorothioate (PS) linkages help the molecule resist nuclease breakdown and support RNase H activity. RNase H is an enzyme that cuts RNA when it is paired with a complementary DNA-like strand, which can help reduce a target RNA message. The PS group also increases binding to many proteins, which can improve uptake into cells.
Limitations and trade-offs
The same protein binding that helps uptake can also cause problems. The sources report nonspecific protein binding, pro-inflammatory effects, complement activation, thrombocytopenia, and other toxicity concerns for PS oligonucleotides. In bacteria, PT systems are not always complete: many identified systems lack DndFGH, so the observed modification is not always paired with the full restriction module. The literature also notes that synthetic and natural phosphorothioate chemistry has multiple possible starting materials and methods, and current synthesis routes still have limitations.
[1] Occurrence, evolution, and functions of DNA phosphorothioate ...pmc.ncbi.nlm.nih.gov › articles › PMC5879708 [2] 1 Introduction [3] New Frontiers in phosphorothioate formation: harnessing inorganic phosphorus sources - PubMed
SEE THE IDEA
Phosphorothioate: one chemical change, several biological effects
A nonbridging oxygen in a phosphate group is replaced by sulfur.
The changed linkage sits in a nucleic-acid chain.
The sulfur-containing linkage can bind proteins differently.
This can produce bacterial defense, drug stability, or toxicity.
Cause-and-effect chain: the sulfur substitution changes the backbone, which changes protein interactions and therefore the observed biological behavior.
The same chemical substitution can support bacterial defense or improve oligonucleotide stability, but it can also increase unwanted protein binding.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.