Sandbox Reserved 333: Difference between revisions

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:Mevalonate diphosphate decarboxylase (MDD) is an important enzyme required for the biosynthesis of cholesterol and other isoprenoids in mammals, bacteria, yeast and fungi <ref name = "Byres"> 17583736 </ref>. MDD is a member of the GHMP (Galactokinase, Homoserine kinase, mevalonate kinase and phosphomevalonate kinase) enzyme family, and is responsible for the conversion of mevalonate diphosphate to isopentenyl pyrophosphate with the help of 1 ATP molecule<ref name = "Byres"/> <ref name = "Voynova"> 18823933 </ref>. Even though the kinases in the GHMP family differ in quaternary structure and ability to bind a wide variety of substrates, they share a characteristic alpha/beta fold and similar sequences <ref name = "Byres"/> <ref name = "ByresMartin"> 16511101 </ref>. Some GHMP kinases exist as dimers, some as tetramers and some as monomers <ref name = "Byres"/>. The amino acid residues in MDD are highly conserved across all species, indicating the specific important activity of the enzyme <ref name = "Byres"/>.

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:Mevalonate diphosphate decarboxylase exists as a symmetrical dimer<ref name = "Byres"/> <ref name = "Voynova"/> <ref name ="ByresMartin"/> . The C-terminal domains of each monomer are symmetrically oriented towards one another around a solvent-filled channel <ref name = "Byres"/>. The dimer is stabilized between alpha helices 6 and 10 on the monomers, and also through salt bridge interactions, tyrosine and proline stacking, and hydrophobic interactions <ref name = "Byres"/>. The interface between the monomers is very small, with only 7% of the total surface area of the monomer engaged in the interface interaction <ref name = "Voynova"/>. This small interface between monomers is a characteristic of GHMP kinases <ref name = "Voynova"/>. Each monomer consists of a single polypeptide chain with 331 amino acid residues. Each polypeptide chain has 13 alpha helices and 15 beta chains. The active site on each monomer is a deep, highly charged cleft made up seven segments of polypeptide chain, which is located away from the other monomer, and is unaffected by dimerization <ref name = "Byres"/>. An ATP binding polypeptide segment called the P loop is also located near the active site <ref name = "Byres"/>.  A total of 19 amino acid residue side chains are involved with substrate binding in the active site <ref name = "Byres"/>.

:The mevalonate pathway encompasses 3 different enzymes that convert mevalonate to isopentenyl pyrophosphate, which is an important building block for all isoprenoids <ref name = "Andreassi"> 19485344 <ref/>. Mevalonate diphosphate decarboxylase is the last enzyme in this pathway, and it converts mevalonate diphosphate to IPP <ref name = "Andreassi"/>. The conversion of mevalonate diphosphate to isopentenyl pyrophosphate is a two-stage reaction <ref name = "Byres">. First, MDD binds an ATP molecule to the P loop near the active site, and the mevalonate diphosphate in the active site <ref name = "Byres"/>. Specifically, the Asp293 residue in the active site of MDD abstracts a proton from the C3 hydroxyl group of mevalonate diphosphate, creating a nucleophile that attacks the γ-phosphoryl group of ATP <ref name = "Byres"/>. The phosphorylation of the C3 carbon creates an unstable intermediate and a good leaving group on C3 <ref name = "Byres"/>. The second stage of the reaction is when MDD dephosphorylates and decarboxylates the substrate, releasing isopentenyl pyrophosphate, inorganic phosphate, ADP and a CO2 molecule <ref name = "Byres"/><ref name = "Voynova"/>. The IPP molecules can be joined together to make cholesterol or other isoprenoids.

Mevalonate diphosphate decarboxylase is a necessary enzyme in the cholesterol and isoprenoid biosynthesis pathway <ref name = "Byres"/><ref name = "Krepkiy"> 15169949 <ref/> <ref name = "Voynova"/> <ref name = "ByresMartin"/>. Without this enzyme, the cholesterol synthesis production decreases <ref name = "Krepkiy"/>, which can be detrimental to many organisms that rely on the formation of IPP for cholesterol, electron transport, membrane structures and anchors, and signaling pathways <ref name = "ByresMartin"/>.  One such organism that requires MDD, is the Trypanosoma bruceii, a parasite that is transmitted to the human bloodstream through the bite of the tsetse fly, that causes African Sleeping sickness <ref name = "ByresMartin"/>. MDD was thought to be a potential target enzyme for an inhibitor that would disable the catalytic activity of MDD, thereby stopping IPP production and effectively killing the parasite <ref name = "ByresMartin"/>. It is believed now that the MDD found in Trypanosoma bruceii resembles human MDD too closely, and so it would be difficult to make a species specific inhibitor for MDD <ref name = "Byres"/>.