Assessing the Effects of the Primary Coordination Sphere on the Rate and Mechanism of Zinc Thiolate Alkylation
Additional Funding Sources
The project described was supported by Institutional Development Awards (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under Grant Nos. P20GM10 3408 and P20GM10909 5. We also acknowledge support from The Biomolecular Research Center at Boise State with funding from the National Science Foundation, Grant Nos. 0619793 and 0923535, the MJ Murdock Charitable Trust and the Idaho State Board of Education. This project is supported by the American Chemical Society Project SEED Program.
Abstract
The Ada protein of Escherichia coli is an enzyme that functions to repair DNA damage induced by alkylating agents. The mutagenic effects of DNA alkylation include transcription errors that affect the expression of genetic sequences. The N-terminus, an active site on the enzyme, of prokaryotes consists of a zinc metalloprotein center coordinated to four cysteine residues. It is proposed that the alkyl group from the alkylated DNA transfers to Cys-38, repairing the previously damaged DNA structure. The mechanism, however, is controversial with dissociative and associative pathways being suggested. To explore the mechanism through dissociation, a zinc thiolate compound with two pyridine rings and a thiophene ring was synthesized and reacted with methyl iodide to measure reaction rate. The presence of a sulfur-rich thiophene ring is thought to encourage dissociation. Further experiments involved the synthesis of a zinc thiolate complex series that included electron donating and withdrawing groups to determine their effects on the thiolate’s dissociating capabilities and limitations. Crossover experiments and methyl iodide reactions on each complex in the series have revealed that the addition of electron withdrawing groups tend to encourage dissociation while electron donating groups keep the thiolate bound. These experimental conclusions have increased the understanding of zinc thiolate function and mechanism as well as the factors that affect them.
Assessing the Effects of the Primary Coordination Sphere on the Rate and Mechanism of Zinc Thiolate Alkylation
The Ada protein of Escherichia coli is an enzyme that functions to repair DNA damage induced by alkylating agents. The mutagenic effects of DNA alkylation include transcription errors that affect the expression of genetic sequences. The N-terminus, an active site on the enzyme, of prokaryotes consists of a zinc metalloprotein center coordinated to four cysteine residues. It is proposed that the alkyl group from the alkylated DNA transfers to Cys-38, repairing the previously damaged DNA structure. The mechanism, however, is controversial with dissociative and associative pathways being suggested. To explore the mechanism through dissociation, a zinc thiolate compound with two pyridine rings and a thiophene ring was synthesized and reacted with methyl iodide to measure reaction rate. The presence of a sulfur-rich thiophene ring is thought to encourage dissociation. Further experiments involved the synthesis of a zinc thiolate complex series that included electron donating and withdrawing groups to determine their effects on the thiolate’s dissociating capabilities and limitations. Crossover experiments and methyl iodide reactions on each complex in the series have revealed that the addition of electron withdrawing groups tend to encourage dissociation while electron donating groups keep the thiolate bound. These experimental conclusions have increased the understanding of zinc thiolate function and mechanism as well as the factors that affect them.
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