@article {2248, title = {Allosteric inhibition of MTHFR prevents futile SAM cycling and maintains nucleotide pools in one-carbon metabolism.}, journal = {J Biol Chem}, volume = {295}, year = {2020}, month = {2020 11 20}, pages = {16037-16057}, abstract = {

Methylenetetrahydrofolate reductase (MTHFR) links the folate cycle to the methionine cycle in one-carbon metabolism. The enzyme is known to be allosterically inhibited by SAM for decades, but the importance of this regulatory control to one-carbon metabolism has never been adequately understood. To shed light on this issue, we exchanged selected amino acid residues in a highly conserved stretch within the regulatory region of yeast MTHFR to create a series of feedback-insensitive, deregulated mutants. These were exploited to investigate the impact of defective allosteric regulation on one-carbon metabolism. We observed a strong growth defect in the presence of methionine. Biochemical and metabolite analysis revealed that both the folate and methionine cycles were affected in these mutants, as was the transsulfuration pathway, leading also to a disruption in redox homeostasis. The major consequences, however, appeared to be in the depletion of nucleotides. C isotope labeling and metabolic studies revealed that the deregulated MTHFR cells undergo continuous transmethylation of homocysteine by methyltetrahydrofolate (CHTHF) to form methionine. This reaction also drives SAM formation and further depletes ATP reserves. SAM was then cycled back to methionine, leading to futile cycles of SAM synthesis and recycling and explaining the necessity for MTHFR to be regulated by SAM. The study has yielded valuable new insights into the regulation of one-carbon metabolism, and the mutants appear as powerful new tools to further dissect out the intersection of one-carbon metabolism with various pathways both in yeasts and in humans.

}, keywords = {Adenosine Triphosphate, Allosteric Regulation, Humans, Methylation, Methylenetetrahydrofolate Reductase (NADPH2), S-Adenosylmethionine, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins}, issn = {1083-351X}, doi = {10.1074/jbc.RA120.015129}, author = {Bhatia, Muskan and Thakur, Jyotika and Suyal, Shradha and Oniel, Ruchika and Chakraborty, Rahul and Pradhan, Shalini and Sharma, Monika and Sengupta, Shantanu and Laxman, Sunil and Masakapalli, Shyam Kumar and Bachhawat, Anand Kumar} } @article {1985, title = {Anabolic SIRT4 Exerts Retrograde Control over TORC1 Signaling by Glutamine Sparing in the Mitochondria.}, journal = {Mol Cell Biol}, volume = {40}, year = {2020}, month = {2020 Jan 03}, abstract = {

Anabolic and catabolic signaling mediated via mTOR and AMPK (AMP-activated kinase) have to be intrinsically coupled to mitochondrial functions for maintaining homeostasis and mitigate cellular/organismal stress. Although glutamine is known to activate mTOR, whether and how differential mitochondrial utilization of glutamine impinges on mTOR signaling has been less explored. Mitochondrial SIRT4, which unlike other sirtuins is induced in a fed state, is known to inhibit catabolic signaling/pathways through the AMPK-PGC1α/SIRT1-peroxisome proliferator-activated receptor α (PPARα) axis and negatively regulate glutamine metabolism via the tricarboxylic acid cycle. However, physiological significance of SIRT4 functions during a fed state is still unknown. Here, we establish SIRT4 as key anabolic factor that activates TORC1 signaling and regulates lipogenesis, autophagy, and cell proliferation. Mechanistically, we demonstrate that the ability of SIRT4 to inhibit anaplerotic conversion of glutamine to α-ketoglutarate potentiates TORC1. Interestingly, we also show that mitochondrial glutamine sparing or utilization is critical for differentially regulating TORC1 under fed and fasted conditions. Moreover, we conclusively show that differential expression of SIRT4 during fed and fasted states is vital for coupling mitochondrial energetics and glutamine utilization with anabolic pathways. These significant findings also illustrate that SIRT4 integrates nutrient inputs with mitochondrial retrograde signals to maintain a balance between anabolic and catabolic pathways.

}, issn = {1098-5549}, doi = {10.1128/MCB.00212-19}, author = {Shaw, Eisha and Talwadekar, Manasi and Rashida, Zeenat and Mohan, Nitya and Acharya, Aishwarya and Khatri, Subhash and Laxman, Sunil and Kolthur-Seetharam, Ullas} }