Eukaryotic NAD+ synthetase Qns1 contains an essential, obligate intramolecular thiol glutamine amidotransferase domain related to nitrilase.
Level 5 - mechanism / opinion, no new human data
Bench biochemistry and yeast genetics study with no human data.
PubMed 12771147 · doi:10.1074/jbc.M302257200
What was done
Investigated the mechanism of yeast glutamine-dependent NAD+ synthetase (Qns1) by generating active-site mutations in either its N-terminal nitrilase-related domain or its synthetase domain. The authors tested these mutants for their ability to complement lethal qns1 gene disruption in yeast in vivo, evaluated trans-complementation between co-expressed mutant classes that form heteromultimers, and measured in vitro enzymatic activities (ammonia-dependent NAD+ synthesis and basal glutaminase activity).
What was found
The abstract reports qualitative genetic and biochemical mechanisms without numerical values: - Both the nitrilase-related and synthetase active sites are essential in vivo; neither single mutant rescued the lethal phenotype of qns1 disruption. - Nitrilase-domain mutants retained ammonia-dependent NAD+ synthetase activity in vitro, while synthetase-domain mutants retained basal glutaminase activity. - Nitrilase and synthetase mutant classes failed to trans-complement each other despite forming stable heteromultimers in vivo. - Ammonia transfer from the glutaminase active site to the synthetase active site is obligately intramolecular over a predicted distance of 46 Å within the monomer.
Why it matters
Identifies the nitrilase-related domain of eukaryotic NAD+ synthetase as the fourth independently evolved glutamine amidotransferase family in nature and defines its requirement for intramolecular ammonia channeling during NAD+ biosynthesis.
Limits
Work was performed strictly in yeast models and in vitro cell-free assays; findings were not verified in mammalian systems. The abstract reports no quantitative kinetic rates, binding affinities, or numeric statistical data.
Cited by
- supports Eukaryotic NAD synthetase in organisms such as yeast, humans, and fruit flies contains a glutaminase domain that converts glutamine to glutamate to liberate an ammonia group for NAD synthesis.