A research team led by biochemist Scott Garman at the University ofMassachusetts Amherst has discovered a key interaction at the heartof a promising new treatment for a rare childhood metabolicdisorder known as Fabry disease. The discovery will helpunderstanding of other protein-folding disorders such asAlzheimer's, Parkinson's and Huntington's diseases, as well.Findings are featured as the cover story in the current issue of Chemistry & Biology . People born with Fabry disease have a faulty copy of a single genethat codes for the alpha-galactosidase ( -GAL) enzyme, one of thecell's "recycling" machines. When it performs normally, -GALbreaks down an oily lipid known as GB3 in the cell's recyclingcenter, or lysosome. |
But when it underperforms or fails, Fabrysymptoms result. Patients may survive to adulthood, but thedisorder leads to toxic lipid build-up in blood vessels and organsthat compromise kidney function or lead to heart disease , for example. The faulty gene causes its damage by producing a misfolded protein,yielding an unstable, poorly functioning -GAL enzyme. Likeorigami papers, these proteins are unfolded to start and onlybecome active when folded into precise shapes. At present, enzymereplacement therapy (ERT) is the only FDA-approved treatment forsuch lysosomal storage disorders as Fabry, Pompe and Gaucherdiseases, but ERT requires a complicated and expensive process topurify and replace the damaged -GAL enzyme, and it must beadministered by a physician.
Instead of replacing the damaged enzyme, an alternative routecalled pharmacological chaperone (PC) therapy is currently in PhaseIII clinical trials for Fabry disease. It relies on using smaller,"chaperone" molecules to keep proteins on the right track towardproper folding, but their biochemical mechanism is not wellunderstood, says Garman. Now, he and colleagues report results of a thorough exploration atthe atomic level of the biochemical and biophysical basis of twosmall molecules for potentially stabilizing the -GAL enzyme. Hesays their use in PC therapy could one day be far less expensivethan the current standard, ERT, and can be taken orally. This work, which improves knowledge of a whole class of molecularchaperones, represents the centerpiece of UMass Amherst studentAbigail Guce's doctoral thesis and was supported by the NationalInstitutes of Health.
Other members of the team are graduatestudents Nat Clark and Jerome Rogich. "The interactions we looked at are exactly the things occurring inthe clinical trial right now," Garman says. Further, "the sameconcept is now being applied to other protein-folding diseases suchas Parkinson's and Alzheimer's disease . Many medical researchers are trying to keep proteins frommisfolding by using small chaperone molecules. Our studies havedefinitely advanced the understanding of how to do that." In their current paper, Garman and colleagues compare the abilityof two small chaperone molecules, galactose and1-deoxygalactononjirimycin (DGJ) to stabilize the -GAL protein,to help it resist unfolding in different conditions such as hightemperature and different pH levels.
They found that each chaperone has very different affinities: DGJbinds tightly and galactose binds loosely to the -GAL, yet theydiffer in only two atomic positions. "Tight is better, because youcan use less drug for treatment," Garman says. "We now can explainDGJ's high potency, its tight binding, down to individual atoms." In earlier studies as in the current work, the UMass Amherst teamused their special expertise in X-ray crystallography to createthree-dimensional images of all atoms in the protein to understandhow it carries out its metabolic mission. They also found a newbinding site for small molecules on human -GAL that had neverbeen observed before. Crystallography on the two chaperones bound to the -GAL enzymeshowed that a single interaction between the enzyme and DGJ wasresponsible for DGJ's high affinity for the enzyme.
Otherexperiments also showed the ability of the 11- and 12-atomchaperones to protect the large, 6,600-atom -GAL from unfoldingand degradation. For the first time, by making a single change in one amino acid inprotein, they forced the DGJ to bind weakly, indicating that oneatomic interaction is responsible for DGJ's high affinity. "It was surprising to find these two small molecules that look verymuch the same have very different affinities for this enzyme," saysGarman, "and we now understand why. The iminosugar DGJ has highpotency due to a single ionic interaction with -GAL. Overall, ourstudies show that this small molecule keeps the enzyme fromunfolding, or when it unfolds, the process happens more slowly, allof which you need in treating disease." The UMass Amherst team plans to next use the principles, assays andexperiments they developed here on enzymes defective in other humandiseases to examine new therapies for them and related disorders.
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