Browsing by Subject "Homocysteine"

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Dietary and genetic influences on neural tube defects
(2014-08) Fathe, Kristin Renee; Finnell, Richard H.
Neural tube defects (NTDs) are a world health issue, affecting approximately 1 in every 1000 live births. These congenital defects arise from the improper closure of the neural tube during development, resulting in significant, life-threatening malformations of the central nervous system. Although it has been observed that supplementing women of child-bearing age with folates greatly decreases the chances of having an NTD affected baby, unfortunately these defects still occur. It is accepted that these complex disorders arise from a combination of genetic, environmental, and dietary influences. One such dietary influence is the one-carbon metabolism metabolite, homocysteine. Homocysteine is a byproduct of methylation reactions in the cell that exists in an inverse homeostasis with folate. Homocysteine can also undergo a transformation that allows it to then react with exposed lysine or cysteine residues on proteins, in a process known as N-homocysteinylation or S-homocysteinylation respectively. High levels of homocysteine have been long correlated with many disease states, including NTDs. One potential mechanism by which homocysteine confers its negative effects is through protein N-homocysteinylation. Here, a novel and high-throughput assay for N-homocysteinylation determination is described. This assay is shown to be accurate with mass spectrometry then shown to be biologically relevant using known hyperhomocysteinemia mouse models. This assay was then applied to a cohort of neural tube closure staged mouse embryos with two different genetic mutations that have previously been shown to predispose mice to NTDs. The genotypes explored here are mutations to the LRP6 gene and the Folr1 gene, both of which have been described as folate-responsive NTD mouse models. It was seen that maternal diet and embryonic genotype had the largest influence on the developmental outcome of these embryos; however, the inverse relationship between folate and homocysteine seemed to be established at this early time point, emphasizing the importance of the balance in one-carbon metabolism. One of these genes, LRP6, was then explored in a human cohort of spina bifida cases. Four novel mutations to the LRP6 gene were found and compared to the mouse model used in the previous study. One of the mutations found in the human population was seen to mimic that of the LRP6 mouse model, therefore expanding the potential of this NTD model.
Structural analysis and discovery of lead compounds for the fungal methionine synthase enzyme
(2013-12) Ubhi, Devinder Kaur; Robertus, Jon D.
Methionine synthases catalyze methyl transfer from 5-methyl-tetrahydrofolate (5-methyl-THF) to L-homocysteine (Hcy) in order to generate methionine (Met). Mammals, including humans, use a cobalamin dependent form, while fungi use a cobalamin independent protein called Met6p. The large structural differences between them make Met6p a potential anti-fungal drug target. Met6p is a 90 kDa protein with the active site located between two (βα)₈ barrels. The active site has a catalytic Zn²+ and binding sites for the two substrates, Hcy and folate. I present the crystal structures of three engineered variants of the Met6p enzyme from Candida albicans. I also solved Met6p in complex with several substrate and product analogs, including Hcy, Met, Gln, 5-methyl-THF-Glu₃ and Methotrexate-Glu₃ (MTX-Glu₃), and the bi-dentate ligand S-adenosyl homocysteine. Also described is a new fluorescence-based activity assay monitoring Hcy. Lastly, a high-throughput Differential Scanning Fluorimetry (DSF) assay was used to screen thousands of compounds in order to identify ligands which bind Met6p. My work details the mode of interaction of Hcy and folate with the Met6p protein. Several residues important to activity were discovered, like Asn 126 and Tyr 660, and proven to be important by site directed mutagenesis. Structural analysis revealed an important aspect of the mechanism. When Hcy binds to its pocket it makes strong ion pairs with the enzyme. In particular, 614 moves toward the substrate amine and triggers a rearrangement of active site loops; this draws the catalytic Zn²+ toward the Hcy thiol where a new ligand bond is formed, activating the thiol for methyl transfer. The work presented here lays the groundwork for structure based drug design and makes the development of Met6p specific bi-dentate ligands feasible. The fluorescence based activity assay I developed was successfully used to test the folate analog MTX-Glu₃, which inhibits with an IC₅₀ of ~4 mM. I also discovered our first bi-dentate ligand in the form of S-adenosyl homocysteine.