Substrate Specificity Diversity of Human Terminal Deoxynucleotidyltransferase May Be a Naturally Programmed Feature Facilitating Its Biological Function

Terminal 2'-deoxynucleotidyl transferase (TdT) is a unique enzyme capable of catalysing template-independent elongation of DNA 3' ends during V(D)J recombination. The mechanism controlling the enzyme's substrate specificity, which is necessary for its biological function, remains unknown. Accordingly, in this work, kinetic and mutational analyses of human TdT were performed and allowed to determine quantitative characteristics of individual stages of the enzyme-substrate interaction, which overall may ensure the enzyme's operation either in the distributive or processive mode of primer extension. It was found that conformational dynamics of TdT play an important role in the formation of the catalytic complex. Meanwhile, the nature of the nitrogenous base significantly affected both the dNTP-binding and catalytic-reaction efficiency. The results indicated that neutralisation of the charge and an increase in the internal volume of the active site caused a substantial increase in the activity of the enzyme and induced a transition to the processive mode in the presence of Mg2+ ions. Surrogate metal ions Co2+ or Mn2+ also may regulate the switching of the enzymatic process to the processive mode. Thus, the totality of individual factors affecting the activity of TdT ensures effective execution of its biological function.

The Kinetic Mechanism of 3′-5′ Exonucleolytic Activity of AP Endonuclease Nfo from E. coli

Escherichia coli apurinic/apyrimidinic (AP) endonuclease Nfo is one of the key participants in DNA repair. The principal biological role of this enzyme is the recognition and hydrolysis of AP sites, which arise in DNA either as a result of the spontaneous hydrolysis of an N-glycosidic bond with intact nitrogenous bases or under the action of DNA glycosylases, which eliminate various damaged bases during base excision repair. Nfo also removes 3′-terminal blocking groups resulting from AP lyase activity of DNA glycosylases. Additionally, Nfo can hydrolyze the phosphodiester linkage on the 5′ side of some damaged nucleotides on the nucleotide incision repair pathway. The function of 3′-5′-exonuclease activity of Nfo remains unclear and probably consists of participation (together with the nucleotide incision repair activity) in the repair of cluster lesions. In this work, using polyacrylamide gel electrophoresis and the stopped-flow method, we analyzed the kinetics of the interaction of Nfo with various model DNA substrates containing a 5′ single-stranded region. These data helped to describe the mechanism of nucleotide cleavage and to determine the rates of the corresponding stages. It was revealed that the rate-limiting stage of the enzymatic process is a dissociation of the reaction product from the enzyme active site. The stability of the terminal pair of nucleotides in the substrate did not affect the enzymatic-reaction rate. Finally, it was found that 2′-deoxynucleoside monophosphates can effectively inhibit the 3′-5′-exonuclease activity of Nfo.

Common Kinetic Mechanism of Abasic Site Recognition by Structurally Different Apurinic/Apyrimidinic Endonucleases

Apurinic/apyrimidinic (AP) endonucleases Nfo (Escherichia coli) and APE1 (human) represent two conserved structural families of enzymes that cleave AP-site-containing DNA in base excision repair. Nfo and APE1 have completely different structures of the DNA-binding site, catalytically active amino acid residues and catalytic metal ions. Nonetheless, both enzymes induce DNA bending, AP-site backbone eversion into the active-site pocket and extrusion of the nucleotide located opposite the damage. All these stages may depend on local stability of the DNA duplex near the lesion. Here, we analysed effects of natural nucleotides located opposite a lesion on catalytic-complex formation stages and DNA cleavage efficacy. Several model DNA substrates that contain an AP-site analogue [F-site, i.e., (2R,3S)-2-(hydroxymethyl)-3-hydroxytetrahydrofuran] opposite G, A, T or C were used to monitor real-time conformational changes of the tested enzymes during interaction with DNA using changes in the enzymes' intrinsic fluorescence intensity mainly caused by Trp fluorescence. The extrusion of the nucleotide located opposite F-site was recorded via fluorescence intensity changes of two base analogues. The catalytic rate constant slightly depended on the opposite-nucleotide nature. Thus, structurally different AP endonucleases Nfo and APE1 utilise a common strategy of damage recognition controlled by enzyme conformational transitions after initial DNA binding.

Lesion Recognition and Cleavage of Damage-Containing Quadruplexes and Bulged Structures by DNA Glycosylases

Human telomeres as well as more than 40% of human genes near the promoter regions have been found to contain the sequence that may form a G-quadruplex structure. Other non-canonical DNA structures comprising bulges, hairpins, or bubbles may have a functionally important role during transcription, replication, or recombination. The guanine-rich regions of DNA are hotspots of oxidation that forms 7,8-dihydro-8-oxoguanine, thymine glycol, and abasic sites: the lesions that are handled by the base excision repair pathway. Nonetheless, the features of DNA repair processes in non-canonical DNA structures are still poorly understood. Therefore, in this work, a comparative analysis of the efficiency of the removal of a damaged nucleotide from various G-quadruplexes and bulged structures was performed using endonuclease VIII-like 1 (NEIL1), human 8-oxoguanine-DNA glycosylase (OGG1), endonuclease III (NTH1), and prokaryotic formamidopyrimidine-DNA glycosylase (Fpg), and endonuclease VIII (Nei). All the tested enzymes were able to cleave damage-containing bulged DNA structures, indicating their important role in the repair process when single-stranded DNA and intermediate non–B-form structures such as bubbles and bulges are formed. Nevertheless, our results suggest that the ability to cleave damaged quadruplexes is an intrinsic feature of members of the H2tH structural family, suggesting that these enzymes can participate in the modulation of processes controlled by the formation of quadruplex structures in genomic DNA.