Scientists discover how mutation in the copper-regulating protein ...

Tag : lead,copper
Using a combination of computer simulations and cutting-edge labexperiments, physical biochemists at Rice University havediscovered how a small genetic mutation -- which is known to causeWilson disease -- subtly changes the structure of a large, complexprotein that the body uses to keep copper from building up to toxiclevels.
"The protein we study is like a big puzzle," said lead authorAgustina Rodriguez-Granillo, the Rice doctoral student inbiochemistry and cell biology who carried out the mathematicalsimulations and laboratory research. "The mutation that causes mostcases of Wilson disease is well-known, but our study looks at theoverall puzzle to see how such a small mutation can alter the shapeand function of such a large and complex protein."
The protein in question is called ATP7B, which is a multidomainprotein that sits in an internal membrane and regulates themovement of copper atoms inside human cells. Though largequantities of copper can be toxic, our bodies need a small amountfor key enzymes involved in, for example, respiration and brainfunctions. ATP7B acts something like a warehouse manager, lockingup bulk quantities of copper and handing it out for use in theseproteins.
Wilson disease is a genetic disorder that alters the ATP7Bprotein's ability to work, causing copper to build up to toxiclevels in the liver, brain, eyes and other organs. Over time thedisease can cause life-threatening organ damage. Wilson diseaseaffects as many as 150,000 people worldwide.
The new study is available online from the Journal of MolecularBiology. It focused on the genetic flaw that causes most cases ofWilson disease. That flaw, known as H1069Q, is caused when just oneout of the more than 1,400 amino acids in ATP7B is changed. Thatamino acid is a histidine located at position 1069. In thedisease-causing form of the protein, this histidine is replacedwith a glutamic acid.
"This mutation occurs at a crucial location where the proteintypically binds with a molecule called ATP that provides the energythe protein needs to move copper from place to place," said studyco-author Pernilla Wittung-Stafshede, an adjunct professor ofbiochemistry and cell biology at Rice and Rodriguez-Granillo'sadviser. Wittung-Stafshede, professor in chemistry at UmeaUniversity in Sweden, said, "Past studies have compared thebehavior of the mutant protein with that of the nonmutant and foundvery little difference, so it was unclear how this small change ledto the devastating effects that are seen in Wilson disease."
Using a combination of experimental data and computer simulationsthat looked specifically at a portion of the protein called theN-domain, where the H1069Q mutation occurs, Wittung-Stafshede,Rodriguez-Granillo and postdoctoral researcher Erik Sedlak (now atthe University of Texas at San Antonio) confirmed that ATP'sfunction was significantly reduced in the mutant form of theprotein. They also found that the mutation caused structuralchanges in other sections of the protein that were far away fromthe mutation site. For example, the healthy form of the protein iscapped with a large, flexible loop. The purpose of the loop isunknown, but its shape is altered and more compact in the diseasedform of the protein.
"This implies that the loop has some importance, perhaps inregulation of ATP7B's activities, and we intend to follow up onthis in our future studies," Rodriguez-Granillo said.
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