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Embargo by Science
Wire servicesand broadcastnews,
October5, 1995,3:00p.m. Pacifictime
Print media, October 6, 1995
For information:
Robin B. Goldsmith
(619)554-8134
#100695
Scientistsat The Scripps ResearchInstitute and Duke University DetermineStructure
of an Enryme Responsiblefor EnvironmentalCyclingof Sulfur and Nitrogenby Plants,
F'ungiand Bacteria
La Jolla, CA. October6, t995 - Scientistsat The ScrippsResearch
Institute(TSRI) andDuke
Universityhavedetermineda seriesof atomicstructuresfor an enzymethat givesresearchers
an
increasedunderstanding
of the fundamentalchemistryfor nitrogenandsulfur cycleson earth.
The closelyrelatedsulfiteandnitrite reductases
areproteinscalledenzymesthat catalyzethe
reductionsof sulfiteto sulfideandnitrite to ammonia.Thesearethe only knownreactionsin
biology in which an enzymedeliverssix electronsto a singleatom.
Accordingto TSRI'sElizabethD. Getzoff,Ph.D.,andher graduatestudentBrian Crane,"This
chemistryis critical for assimilationof sulfurandnitrogeninto the biosphereandalsois involved
in dissimilatoryprocesses
that restorereducedformsof theseelementsto the environment."
Sulfur andnitrogenaretwo of theprimaryelementsthat composeorganicmatter.Humansand
all otheranimalsrely indirectlyon this chemistrybecause
they mustobtainreducedsulfur and
nitrogenin their diet. Similarenzymesalsoareusedby somebacteriato "breathe"sulfur and
nitrogeninsteadof oxygen.This processproducessulfidein the environmentthat is a severe
contaminationproblemfor the oil andsewagetreatmentindustry.
-more-
Page2--
TSRI/Duke University ScientistsDetermineStructure of an Enryme
Responsiblefor Environmental Cycling of Sulfur and Nitrogen
Scientistsbelievethat the sulfitereductionis likely the remnantof ancientmetabolicprocesses
that allowedearlybacteriato generateenergyfrom sulfur,a billion yearsbeforesignificant
amountsof oxygenexistedon the planet.Biologicalreductionof oxidizedsulfurwasoneof the
first contributionsof living organismsto the complexcyclingof elementson earth.
The work waspublishedin the October6 issueof Sciencemagazinein an articleentitled,
"Sulfite ReductaseStructureatl.6 A: EvolutionandCatalysisfor Reductionof Inorganic
Anions,"by Brian R. Crane,Lewis M. Siegel,Ph.D.,andElizabethD. Getzoff,Ph.D.
To understandhow theseenzymescan catalyzereactionsthat areotherwiseunfavorableat
ambienttemperaturesandpressures,Craneand Getzoffhavedeterminedthe shapeand structure
of bacterialsulfitereductase
hemoprotein(SiRHP)at the level of atomicdetail;in complexwith
an inhibitor andits substratesulfite.Thehomologybetweenthe two symmetrichalvesof SiRHP
hasrevealedpatternsof amino acidsthat arekey to determiningthe protein'sform and function.
Getzoffcommented,"We havefoundtracesof thesepatternsin the genesthat codefor other
proteins,suggestingthat this may be a commonmotif usedby enzymesthat performother
oxidativeor reductivechemistryin virtually all living organisms,includingman.Thus,sulfite
reductaseseemsto be a progenitormoleculeandsomeof its featureshavebeenmaintainedand
adaptedthroughoutthe diversityof life."
The techniqueof x-ray crystallographywasusedto determinethe structureof SiRHP.In this
procedure,x-raysgenerated
projected
by a largeenergystoragering-called a synchrotron--are
through crystalsof pure proteinto createa distinctivepatternthat canbe analyzedwith the help
of high-poweredcomputersto measurethe exactpositionof eachatom.
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