House cleaning, a part of good housekeeping

Cellular metabolism constantly generates by-products that are wasteful or even harmful. Such compounds are excreted from the cell or are removed through hydrolysis to normal cellular metabolites by various 'house-cleaning' enzymes. Some of the most important contaminants are non-canonical nucleoside triphosphates (NTPs) whose incorporation into the nascent DNA leads to increased mutagenesis and DNA damage. Enzymes intercepting abnormal NTPs from incorporation by DNA polymerases work in parallel with DNA repair enzymes that remove lesions produced by modified nucleotides. House-cleaning NTP pyrophosphatases targeting non-canonical NTPs belong to at least four structural superfamilies: MutT-related (Nudix) hydrolases, dUTPase, ITPase (Maf/HAM1) and all-alpha NTP pyrophosphatases (MazG). These enzymes have high affinity (Km's in the micromolar range) for their natural substrates (8-oxo-dGTP, dUTP, dITP, 2-oxo-dATP), which allows them to select these substrates from a mixture containing a approximately 1000-fold excess of canonical NTPs. To date, many house-cleaning NTPases have been identified only on the basis of their side activity towards canonical NTPs and NDP derivatives. Integration of growing structural and biochemical data on these superfamilies suggests that their new family members cleanse the nucleotide pool of the products of oxidative damage and inappropriate methylation. House-cleaning enzymes, such as 6-phosphogluconolactonase, are also part of normal intermediary metabolism. Genomic data suggest that house-cleaning systems are more abundant than previously thought and include numerous analogous enzymes with overlapping functions. We discuss the structural diversity of these enzymes, their phylogenetic distribution, substrate specificity and the problem of identifying their true substrates.