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| This Sandbox is Reserved from January 10, 2010, through April 10, 2011 for use in BCMB 307-Proteins course taught by Andrea Gorrell at the University of Northern British Columbia, Prince George, BC, Canada. |
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Mevalonate Diphosphate Decarboxylase
Introduction
- Mevalonate diphosphate decarboxylase (MDD) is an important enzyme required for the biosynthesis of cholesterol and other isoprenoids in mammals, bacteria, yeast and fungi [1]. 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[1] [2]. 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 [1] [3]. Some GHMP kinases exist as dimers, some as tetramers and some as monomers [1]. The amino acid residues in MDD are highly conserved across all species, indicating the specific important activity of the enzyme [1].
Structure
- Mevalonate diphosphate decarboxylase exists as a symmetrical dimer[1] [2] [3] . The C-terminal domains of each monomer are symmetrically oriented towards one another around a solvent-filled channel [1]. 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 [1]. The interface between the monomers is very small, with only 7% of the total surface area of the monomer engaged in the interface interaction [2]. This small interface between monomers is a characteristic of GHMP kinases [2]. 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 [1]. An ATP binding polypeptide segment called the P loop is also located near the active site [1]. A total of 19 amino acid residue side chains are involved with substrate binding in the active site [1].
Reaction
- The mevalonate pathway encompasses 3 different enzymes that convert mevalonate to isopentenyl pyrophosphate, which is an important building block for all isoprenoids [4]. Mevalonate diphosphate decarboxylase is the last enzyme in this pathway, and it converts mevalonate diphosphate to IPP [4]. The conversion of mevalonate diphosphate to isopentenyl pyrophosphate is a two-stage reaction [1]. First, MDD binds an ATP molecule to the P loop near the active site, and the mevalonate diphosphate in the active site [1]. 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 [1]. The phosphorylation of the C3 carbon creates an unstable intermediate and a good leaving group on C3 [1]. The second stage of the reaction is when MDD dephosphorylates and decarboxylates the substrate, releasing isopentenyl pyrophosphate, inorganic phosphate, ADP and a CO2 molecule [1][2]. The IPP molecules can be joined together to make cholesterol or other isoprenoids.
Significance
Mevalonate diphosphate decarboxylase is a necessary enzyme in the cholesterol and isoprenoid biosynthesis pathway [1][5] [2] [3]. Without this enzyme, the cholesterol synthesis production decreases [5], 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 [3]. 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 [3]. 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 [3]. 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 [1].
