{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Recent studies have revealed that, in addition to the canonical dihydrofolate reductase (DHFR) enzyme, human cells express a second DHFR enzyme—commonly designated DHFR2 or DHFR‐like protein 1 (DHFRL1)—which appears to have evolved from a “pseudogene” (also known as DHFRP4) and exhibits distinct biochemical and cellular properties. Unlike the classical DHFR, DHFR2 has a lower specific activity and a higher Km for dihydrofolate, suggesting a reduced substrate affinity that may be optimally tuned for specific cellular environments. In human cells, DHFRL1 is localized primarily to mitochondria, where it plays a critical role in the de novo synthesis of thymidylate, thereby contributing to the maintenance of mitochondrial DNA integrity (see."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " Molecular dynamics investigations further indicate that steric effects, such as those mediated by an extended arginine side chain, compromise dihydrofolate binding to DHFRL1 relative to its conventional counterpart, underscoring unique structural adaptations (see."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " In rodents, however, the mitochondrial DHFR activity appears to stem solely from the canonical enzyme, as functional expression of a second DHFR is absent despite the presence of related pseudogenes (see."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": " Moreover, in certain archaea such as Haloferax volcanii, a distinct second DHFR (encoded by hdrB) has been identified; this enzyme displays markedly different biochemical characteristics from hdrA and is capable of sustaining cellular growth without thymidine supplementation, suggesting that evolutionary divergence has yielded DHFR isoforms tailored to specific metabolic niches (see."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": "\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Gráinne McEntee, Stefano Minguzzi, Kirsty O'Brien, et al. "}, {"type": "b", "children": [{"type": "t", "text": "The former annotated human pseudogene dihydrofolate reductase-like 1 (DHFRL1) is expressed and functional."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Proc Natl Acad Sci U S A (2011)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1073/pnas.1103605108"}], "href": "https://doi.org/10.1073/pnas.1103605108"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "21876184"}], "href": "https://pubmed.ncbi.nlm.nih.gov/21876184"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Donald D Anderson, Cynthia M Quintero, Patrick J Stover "}, {"type": "b", "children": [{"type": "t", "text": "Identification of a de novo thymidylate biosynthesis pathway in mammalian mitochondria."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Proc Natl Acad Sci U S A (2011)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1073/pnas.1103623108"}], "href": "https://doi.org/10.1073/pnas.1103623108"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "21876188"}], "href": "https://pubmed.ncbi.nlm.nih.gov/21876188"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Jian Gao, Wei Cui, Yuguo Du, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Insight into the molecular mechanism about lowered dihydrofolate binding affinity to dihydrofolate reductase-like 1 (DHFRL1)."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Mol Model (2013)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1007/s00894-013-2018-2"}], "href": "https://doi.org/10.1007/s00894-013-2018-2"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "24122410"}], "href": "https://pubmed.ncbi.nlm.nih.gov/24122410"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Linda Hughes, Robert Carton, Stefano Minguzzi, et al. "}, {"type": "b", "children": [{"type": "t", "text": "An active second dihydrofolate reductase enzyme is not a feature of rat and mouse, but they do have activity in their mitochondria."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "FEBS Lett (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.febslet.2015.05.017"}], "href": "https://doi.org/10.1016/j.febslet.2015.05.017"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "25980602"}], "href": "https://pubmed.ncbi.nlm.nih.gov/25980602"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "R Ortenberg, O Rozenblatt-Rosen, M Mevarech "}, {"type": "b", "children": [{"type": "t", "text": "The extremely halophilic archaeon Haloferax volcanii has two very different dihydrofolate reductases."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Microbiol (2000)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1046/j.1365-2958.2000.01815.x"}], "href": "https://doi.org/10.1046/j.1365-2958.2000.01815.x"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "10760149"}], "href": "https://pubmed.ncbi.nlm.nih.gov/10760149"}]}]}]}