@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 = {<p>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.</p>
},
	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 {1164,
	title = {Thiol trapping and metabolic redistribution of sulfur metabolites enable cells to overcome cysteine overload.},
	journal = {Microb Cell},
	volume = {4},
	year = {2017},
	month = {2017 Mar 27},
	pages = {112-126},
	abstract = {<p>Cysteine is an essential requirement in living organisms. However, due to its reactive thiol side chain, elevated levels of intracellular cysteine can be toxic and therefore need to be rapidly eliminated from the cellular milieu. In mammals and many other organisms, excess cysteine is believed to be primarily eliminated by the cysteine dioxygenase dependent oxidative degradation of cysteine, followed by the removal of the oxidative products. However, other mechanisms of tackling excess cysteine are also likely to exist, but have not thus far been explored. In this study, we use , which naturally lacks a cysteine dioxygenase, to investigate mechanisms for tackling cysteine overload. Overexpressing the high affinity cysteine transporter, , enabled yeast cells to rapidly accumulate high levels of intracellular cysteine. Using targeted metabolite analysis, we observe that cysteine is initially rapidly interconverted to non-reactive cystine . A time course revealed that cells systematically convert excess cysteine to inert thiol forms; initially to cystine, and subsequently to cystathionine, S-Adenosyl-L-homocysteine (SAH) and S-Adenosyl L-methionine (SAM), in addition to eventually accumulating glutathione (GSH) and polyamines. Microarray based gene expression studies revealed the upregulation of arginine/ornithine biosynthesis a few hours after the cysteine overload, and suggest that the non-toxic, non-reactive thiol based metabolic products are eventually utilized for amino acid and polyamine biogenesis, thereby enabling cell growth. Thus, cells can handle potentially toxic amounts of cysteine by a combination of thiol trapping, metabolic redistribution to non-reactive thiols and subsequent consumption for anabolism.</p>
},
	issn = {2311-2638},
	doi = {10.15698/mic2017.04.567},
	author = {Deshpande, Anup Arunrao and Bhatia, Muskan and Laxman, Sunil and Bachhawat, Anand Kumar}
}
