New insights into DNA polymerase mechanisms provided by time-lapse crystallography

Structural basis for sirtuin 2 activity and modulation: current state and opportunities

Sirtuin 2 (SIRT2) is a ubiquitously expressed cellular enzyme that deacylates protein lysine residues using NAD+ as a cofactor. SIRT2-mediated post-translational modifications on a plethora of protein targets position the enzyme to exert a wide-ranging regulatory role in many physiological and pathological processes. More than 39 SIRT2 crystal structures in complex with substrates, products, mimetics of substrates and products, and modulators, have been reported. The Rossmann fold of the catalytic core presents inducible acyl and cofactor binding cavities that accommodate acyl chains of diverse lengths. These structures have provided information for the design of mechanism- and substrate-based inhibitors. Indeed, a specific SIRT2 selectivity pocket has been described and can be targeted by different chemotypes. Despite the determination of many crystal structures, numerous open questions remain, especially relating to the development of small molecule modulators, full or partial activation or inhibition, and relating these effects to different therapeutic applications. Additional questions include understanding the role of the disordered termini, and the role of potential quaternary states (monomer, dimer, and trimer). Deeper insight into these issues may facilitate the development of SIRT2 selective modulators that can be tailored to different pathological scenarios, such as viral infections and cancers, in which either activation or inhibition of SIRT2 may be of therapeutic benefit. This review covers the following topics: (1) primary to quaternary and catalytic structural biology; (2) structural insights into molecular modulation of SIRT2 (inhibition and selectivity by mechanism-based inhibitors, substrate-mimicking inhibitors, C pocket-binding inhibitors, and selectivity pocket binding inhibitors, including insights to activation; and (3) the impact of structural variations (mutations, post-translational modifications, polymorphs, protein interactions). Despite considerable progress, key knowledge gaps remain regarding the design of optimized SIRT2 modulators. Addressing these uncertainties, particularly within the realms of full/partial activation/inhibition, off-target effects, and tailoring modulators to specific pathologies, will require further investigation into the roles of the SIRT2 disordered termini, quaternary states, and post-translational modifications. Ultimately, unraveling these intricacies holds the key to unlocking the therapeutic potential of SIRT2 modulation.

Non-canonical amino acids uncover the significant impact of Tyr671 on Taq DNA polymerase catalytic activity

Responsible for synthesizing the complementary strand of the DNA template, DNA polymerase is a crucial enzyme in DNA replication, recombination and repair. A highly conserved tyrosine (Tyr), located at the C-terminus of the O-helix in family A DNA polymerases, plays a critical role in enzyme activity and fidelity. Here, we combined the technology of genetic code extension to incorporate non-canonical amino acids and molecular dynamics (MD) simulations to uncover the mechanisms by which Tyr671 impacts substrate binding and conformation transitions in a DNA polymerase from Thermus aquaticus. Five non-canonical amino acids, namely l-3,4-dihydroxyphenylalanine (l-DOPA), p-aminophenylalanine (pAF), p-acetylphenylalanine (pAcF), p-cyanophenylalanine (pCNF) and p-nitrophenylalanine (pNTF), were individually incorporated at position 671. Strikingly, Y671pAF and Y671DOPA were active, but with lower activity compared to Y671F and wild-type. Y671pAF showed a higher fidelity than the Y671F, despite both possessing lower fidelity than the wild-type. Metadynamics and long-timescale MD simulations were carried out to probe the role of mutations in affecting protein structure, including open conformation, open-to-closed conformation transition, closed conformation, and closed-to-open conformation transition. The MD simulations clearly revealed that the size of the 671 amino acid residue and interactions with substrate or nearby residues were critical for Tyr671 to determine enzyme activity and fidelity.

For the Better or for the Worse? The Effect of Manganese on the Activity of Eukaryotic DNA Polymerases

DNA polymerases constitute a versatile group of enzymes that not only perform the essential task of genome duplication but also participate in various genome maintenance pathways, such as base and nucleotide excision repair, non-homologous end-joining, homologous recombination, and translesion synthesis. Polymerases catalyze DNA synthesis via the stepwise addition of deoxynucleoside monophosphates to the 3' primer end in a partially double-stranded DNA. They require divalent metal cations coordinated by active site residues of the polymerase. Mg2+ is considered the likely physiological activator because of its high cellular concentration and ability to activate DNA polymerases universally. Mn2+ can also activate the known DNA polymerases, but in most cases, it causes a significant decrease in fidelity and/or processivity. Hence, Mn2+ has been considered mutagenic and irrelevant during normal cellular function. Intriguingly, a growing body of evidence indicates that Mn2+ can positively influence some DNA polymerases by conferring translesion synthesis activity or altering the substrate specificity. Here, we review the relevant literature focusing on the impact of Mn2+ on the biochemical activity of a selected set of polymerases, namely, Polβ, Polλ, and Polµ, of the X family, as well as Polι and Polη of the Y family of polymerases, where congruous data implicate the physiological relevance of Mn2+ in the cellular function of these enzymes.