DNA Cytosine Methylation: Structural and Thermodynamic Characterization of the Epigenetic Marking Mechanism

DNA cytosine methyltransferases regulate the expression of the genome through the precise epigenetic marking of certain cytosines with a methyl group, and aberrant methylation is a hallmark of human diseases including cancer. Targeting these enzymes for drug design is currently a high priority. We have utilized ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations to investigate extensively the reaction mechanism of the representative DNA methyltransferase <i>Hha</i>I (M.<i>Hha</i>I) from prokaryotes, whose overall mechanism is shared with the mammalian enzymes. We obtain for the first time full free energy profiles for the complete reaction, together with reaction dynamics in atomistic detail. Our results show an energetically preferred mechanism in which nucleophilic attack of cytosine C5 on the <i>S</i>-adenosyl-l-methionine (AdoMet) methyl group is concerted with formation of the Michael adduct between a conserved Cys in the active site with cytosine C6. Spontaneous and reversible proton transfer between a conserved Glu in the active site and cytosine N3 at the transition state was observed in our simulations, revealing the chemical participation of this Glu residue in the catalytic mechanism. Subsequently, the β-elimination of the C5 proton utilizes as base an OH^– derived from a conserved crystal water that is part of a proton wire water channel, and this <i>syn</i> β-elimination reaction is the rate-limiting step. Design of novel cytosine methylation inhibitors would be advanced by our structural and thermodynamic characterization of the reaction mechanism

DNA Cytosine Methylation: Structural and Thermodynamic Characterization of the Epigenetic Marking Mechanism

DNA cytosine methyltransferases regulate the expression of the genome through the precise epigenetic marking of certain cytosines with a methyl group, and aberrant methylation is a hallmark of human diseases including cancer. Targeting these enzymes for drug design is currently a high priority. We have utilized ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations to investigate extensively the reaction mechanism of the representative DNA methyltransferase <i>Hha</i>I (M.<i>Hha</i>I) from prokaryotes, whose overall mechanism is shared with the mammalian enzymes. We obtain for the first time full free energy profiles for the complete reaction, together with reaction dynamics in atomistic detail. Our results show an energetically preferred mechanism in which nucleophilic attack of cytosine C5 on the <i>S</i>-adenosyl-l-methionine (AdoMet) methyl group is concerted with formation of the Michael adduct between a conserved Cys in the active site with cytosine C6. Spontaneous and reversible proton transfer between a conserved Glu in the active site and cytosine N3 at the transition state was observed in our simulations, revealing the chemical participation of this Glu residue in the catalytic mechanism. Subsequently, the β-elimination of the C5 proton utilizes as base an OH^– derived from a conserved crystal water that is part of a proton wire water channel, and this <i>syn</i> β-elimination reaction is the rate-limiting step. Design of novel cytosine methylation inhibitors would be advanced by our structural and thermodynamic characterization of the reaction mechanism

Structural and Dynamic Characterization of Polymerase κ’s Minor Groove Lesion Processing Reveals How Adduct Topology Impacts Fidelity

DNA lesion bypass polymerases process different lesions with varying fidelities, but the structural, dynamic, and mechanistic origins of this phenomenon remain poorly understood. Human DNA polymerase κ (Polκ), a member of the Y family of lesion bypass polymerases, is specialized to bypass bulky DNA minor groove lesions in a predominantly error-free manner, by housing them in its unique gap. We have investigated the role of the unique Polκ gap and N-clasp structural features in the fidelity of minor groove lesion processing with extensive molecular modeling and molecular dynamics simulations to pinpoint their functioning in lesion bypass. Here we consider the <i>N</i>²-dG covalent adduct derived from the carcinogenic aromatic amine, 2-acetylaminofluorene (dG-<i>N</i>²-AAF), that is produced via the combustion of kerosene and diesel fuel. Our simulations reveal how the spacious gap directionally accommodates the lesion aromatic ring system as it transits through the stages of incorporation of the predominant correct partner dCTP opposite the damaged guanine, with preservation of local active site organization for nucleotidyl transfer. Furthermore, flexibility in Polκ’s N-clasp facilitates the significant misincorporation of dTTP opposite dG-<i>N</i>²-AAF via wobble pairing. Notably, we show that N-clasp flexibility depends on lesion topology, being markedly reduced in the case of the benzo­[<i>a</i>]­pyrene-derived major adduct to <i>N</i>²-dG, whose bypass by Polκ is nearly error-free. Thus, our studies reveal how Polκ’s unique structural and dynamic properties can regulate its bypass fidelity of polycyclic aromatic lesions and how the fidelity is impacted by lesion structures

Aging Mechanism of Soman Inhibited Acetylcholinesterase

Acetylcholinesterase (AChE) is a crucial enzyme in the cholinergic nervous system that hydrolyzes neurotransmitter acetylcholine (ACh) and terminates synaptic signals. The catalytic serine of AChE can be phosphonylated by soman, one of the most potent nerve agents, and subsequently undergo an aging reaction. This phosphonylation and aging process leads to irreversible AChE inhibition, results in accumulation of excess ACh at the synaptic clefts, and causes neuromuscular paralysis. By employing Born–Oppenheimer <i>ab initio</i> QM/MM molecular dynamics simulations with umbrella sampling, a state-of-the-art approach to simulate enzyme reactions, we have characterized the aging mechanism of soman phosphonylated AChE and determined its free energy profile. This aging reaction starts with the scission of the O2–Cα bond, which is followed by methyl migration, and results in a tertiary carbenium intermediate. At the transition state, the scissile O2–Cα bond is already cleaved with an average O–C distance of 3.2 ± 0.3 Å and the migrating methyl group is shared between Cα and Cβ carbons with C–C distances of 1.9 ± 0.1 and 1.8 ± 0.1 Å, respectively. The negatively charged phosphonate group is stabilized by a salt bridge with the imidazole ring of the catalytic histidine. A major product of aging, 2,3-dimethyl-2-butanol can be formed swiftly by the reaction of a water molecule. Our characterized mechanism and simulation results provide new detailed insights into this important biochemical process