Structure-Function Studies of the Large Subunit of Ribonucleotide Reductase from Homo sapiens and Saccharomyces cerevisiae

Sufficient pools of deoxyribonucleotide triphophates (dNTPs) are essential for the high fidelity replication and repair of DNA, the hereditary material for a majority of living organisms. Ribonucleotide reductase (Rnr) catalyzes the rate-limiting step of de-novo DNA synthesis, the reduction of ribonucleosides to deoxyribonucleosides. Since the cell relies primarily upon ribonucleotide reductase for its dNTPs, both the cellular levels and activity of Rnr are heavily regulated, especially when DNA damage occurs or during replication blocks in the cell cycle. If dNTP pools become too high, too low, or imbalanced, genomic instability results, leading to either the formation of cancerous cells or cell death. High levels of dNTPs are required by actively propagating cells for the replication of new DNA molecules. Therefore, Rnr makes an excellent target for anti-cancer, anti-microbial, and anti-fungal chemotherapeutic agents.

Deficiencies in the cellular mismatch repair (MMR) machinery have been linked to genetic instability and carcinogenesis. Two alleles of Rnr1 were recently discovered, Rnr1^S269P and Rnr1^S610F, which have a mismatch repair synthetic lethal (msl) phenotype in Saccharomyces cerevisiae cells with missing or defective MMR genes. To uncover the molecular mechanism of the msl phenotype in these two mutants, recombinant Rnr1p-S269P and Rnr1p-S610F were subjected to in vitro activity assays, X-ray crystallography, and in vitro nucleoside-binding assays (Chapter 3). The Rnr1^S269P allele was shown to dysregulate specificity cross talk by X-ray crystallography experiments, leading to reduced levels of dATP in the cell. A 2-fold reduction in binding of ADP substrate was observed in the Rnr1^S610F allele, however reduction of the k_cat is believed to cause the observed msl phenotype in this mutant.