title: Stereochemical course of the 3'----5'-exonuclease activity of DNA polymerase I.
abstract: (Sp)-2'-Deoxyadenosine 5'-O-[1-17O,1-18O,1,2-18O]triphosphate has been synthesized by desulfurization of (Sp)-2'-deoxyadenosine 5'-O-(1-thio[1,1-18O2]diphosphate) with N-bromosuccinimide in [17O]water, followed by phosphorylation with phosphoenolpyruvate-pyruvate kinase. A careful characterization of the product using high-resolution 31P NMR revealed that the desulfurization reaction proceeded with approximately 88% direct in-line attack at the alpha-phosphorus and 12% participation by the beta-phosphate to form a cyclic alpha,beta-diphosphate. The latter intermediate underwent hydrolysis by a predominant nucleophilic attack on the beta-phosphate. This complexity of the desulfurization reaction, however, does not affect the stereochemical integrity of the product but rather causes a minor dilution with nonchiral species. The usefulness of the (Sp)-2'-deoxyadenosine 5'-O-[1-17O,1-18O,1,2-18O]triphosphate in determining the stereochemical course of deoxyribonucleotidyl-transfer enzymes is demonstrated by using it to delineate the stereochemical course of the 3'----5'-exonuclease activity of DNA polymerase I. Upon incubation of this oxygen-chiral substrate with Klenow fragment of DNA polymerase I in the presence of poly[d(A-T)] and Mg2+, a quantitative conversion into 2'-deoxyadenosine 5'-O-[16O,17O,18O]monophosphate was observed. The stereochemistry of this product was determined to be Rp. Since the overall template-primer-dependent conversion of a deoxynucleoside triphosphate into the deoxynucleoside monophosphate involves incorporation into the polymer followed by excision by the 3'----5'-exonuclease activity and since the stereochemical course of the incorporation reaction is known to be inversion, it can be concluded that the stereochemical course of the 3'----5'-exonuclease is also inversion.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D004256:DNA Polymerase I / Q000378:metabolism*; D003838:Deoxyadenine Nucleotides / Q000138:chemical synthesis / Q000378:metabolism; D005092:Exonucleases / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D011067:Poly dA-dT / Q000494:pharmacology; D013237:Stereoisomerism; D013455:Sulfur
chemical_list: D003838:Deoxyadenine Nucleotides; D011067:Poly dA-dT; C012873:2'-deoxy-5'-adenosine monophosphate; D013455:Sulfur; D004256:DNA Polymerase I; D005092:Exonucleases; C026600:2'-deoxyadenosine triphosphate