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Xylans

Xiao Liu

10/22/2010

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Big picture

Plan

tcellwall

Proteins

Polysaccharides

Otherproteins

Structuralproteins

GRPs

HRGPs

PRPsExtensinGPs!ellulose

Pectin

He"icellulose

Xylo#lucan

Xylans$annans%Gluco"annans&

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How do xylans affect our well-being?

Xylans can help reduce some diseases in humans

Xylans are important functional ingredients inbaked products.

Xylans impact brewing properties of grains.

Xylans can be converted to xylitol, a natural food sweetener.

Xylans are maor constituents in the nonnutritional constituent of feed in monogastric animals.

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!ntroduction of Xylans

ylans are a family of structurally diverse plant polysaccharides with a backbone

composed of ",#-linked $-%-xylosyl resid

ues.

http'//www(scienti)cpsychic(co"/)tness/car*ohydrates2(ht"l

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&aor types of xylans

!n almost all cases, the backbone is substituted with monosaccharide or disaccharide side chains, to varying degrees.

glucuronic acid and #-'-methyl glucuronic acid (glucuronoxylan, )X*,

arabinose (arabinoxylan, +X*,

a combination of acidic and neutral sugars (glucuronoarabinoxylan, )+X*.

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+rabinoxylan

+rabinoxylan is the predominant hemicellulose of grasses.

-arabinofuranose attached randomly by "

/ and0or "1 linkages to the xylose units throughout the chain.

2ide chains containing arabinosyl, galactosyl, glucosyluronic acid, and #-'-methyl glucosyluronic acid residues have been identified.

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+rabinoxylan

http'//www(*tinternet(co"/+"artin(chaplin/hyara(ht"l

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)lucuronoxylan

&aor components of the secondary cell walls ofdicots ("34-154*6

(",/*-linked %-glucuronyl ()lc+*

#-'-methyl-)lc+ (&e)lc+* residues attached to7-/ 8 every "5 Xyl residues

8 954 contain one '-acetyl group at 7-/ or 7-1

. 7ontain a distinct :glycosyl se;uence< at the re

ducing end.

%evoid of +ra units

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)lucuronoxylan

,illia" S -or. and $alcol" Oeill% ioche"ical control o xylan *iosynthesis3 which end is up4

5wo6do"ain architecture' a typicalpoly"er do"ain and a distinct7#lycosyl se8uence9 do"ain'

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)lucuronoarabinoxylan ()+X*

ra units are added to the O6: position o the xylosyl units o the*ac.*one

;eruloyl #roups are esteri)ed to the O6< position o the ra units

in a*out e=ery

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)+X in type !! walls

5he "aAor cross6lin.in# #lycans othe pri"ary cell walls oco""elinoid "onocots(5ype @@ walls 'cellulose "icro)*rils B cross6

lin.in# GXs!haracteristics'Pectin6poor "atrixLittle structural proteinGlc units contri*ute to char#e

density >interconnectin#?GXs'1( ranched GXs' cross6lin.in# is

*loc.ed >durin# elon#ation?2( Cn*ranched GXs' hydro#en6

*ond to cellulose or to eachother

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Xylan biosynthesis

2ynthesi=ed in )olgi +pparatus.

Backbone synthesis6

!>X, !>X"5, !>X"5, !>X"#

2ynthesis of side chain primer or terminator oligosaccharide6

!>X9, !>X@, A+>C2

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)X biosynthesis

2erveral glycosyltransferases may be involved in the initiation and elongation of the polymer backboneD other en=ymes for the add

ition and 0or modification of the side chain.

Eive genes (E>+@, !>X@, !>X?, A+>C2, !>X"

#* in +rabidopsis have been identified to beinvolved in )X synthesis. Fhey encode putative )Fs that may have a role in forming red

ucing end.

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Fwo models of )X biosynthesis mechanism

$odel >a?' GX is synthesiDed *y transer o xylosyl residues to thereducin# end o the chain($odel >*?' the 7#lycosyl se8uence9 acts as a pri"er xylosyl residues are

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)+X biosynthesis Gewly synthesi=sed )+X polymers have re

gular structurester treat"ent oendoxylanase @@@% only three.inds o oli#osaccharides arereleased

Possi*le explanation o theor"ation o GX ra#"ents'two un*ranched Xyl residues atthe reducin# end and one or

two un*ranced Xyl residue atthe nonreducin# end(

5his type o re#ular structure isusually the result o acooperati=e "echanis"

*etween enDy"es'Xyl5% ra5% Glc5(

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+X biosynthesis

Xylosyltransferase and arabinosyl transferase activities have been detected in microsomal fractions isolated from wheat and

barley.

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hy are we desired to know how Xylansare synthesi=ed ?

7ontribution to the recalcitrance in biofuelproduction.

!n paper manufacture, decrease in brightne

ss of final product

&e)lc+ Hexenuronic +cid

!n animal feed, loss of nutrition

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+bsence of branches from xyla

n in +rabidopsisguxmutants reveals potential for simplificatio

n of lignocellulosic biomass Iennifer 7. &ortimer, )odfrey A. &iles, %avid &. Brown,

Jhinong Jhang, &arcelo A. 2egura, Fhilo eimar, Xiaolan Ku, Leith +. 2effeen, Mlaine 2tephen, 2imon >. Furner,and Aaul %upree

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!dentification of candidate secondary cell wall )Fs

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)F@ family

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2ubcellular locali=ation of )CX" and )CX/

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Lnockout6 gux", gux/, gux" gux/

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&orphology and Ahenotype

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%ouble mutant is slightly weaker than F

5E$'secondary cell wall o

xyle" )*ers

;our6point *endin# test' ' ,5 ste"s ' gux1 gux2ste"s

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Xylan structure and ;uantity in stem of F andguxmutant plants.

@R was characteriDedwith P!E usin# GH'

Fecrease in intensity o$eGlc>Xyl?I

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Nuantification of Xyl residues substituted and Xyl backbone

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&onosaccharide analysis of F andgux"andgux/ stem

Xyl *ac.*one unaltered Glc

reduced(

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&+%!-F'E &2 analysis of xylan structure

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)uxF activity in F andgux" gux/

Gux5 acti=ity is stron#ly reduced

in the dou*le "utant

5he acti=ity o Xyl5 was

unaJected in the dou*le "utant

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)uxF activity in F andgux" gux/

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Aroperties of F andgux" gux/ xylan

Fou*le "utant shows i"pro=ed

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Aroperties of F andgux" gux/ xylan

Glc/$eGlc contri*ute tosolu*ility

gux1 gux2 could *e hydrolyDed to

"onosaccharide in the presence o6

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7onclusions

!dentification of two )olgi-locali=ed putativeglycosyltransferases, )CX" and )CX/, that are re;uired for the addition of both glucuro

nic acid and #-'-methylglucuronic acid branches to xylan in +rabidopsis stem cell wall.

gux" gux/ double mutants show loss of xyla

n glucuronyltansferase activity and lack almost all detectable xylan substitution, but nochange in xylan backbone ;uantity.

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Fhe xylan ingux" gux/ shows improved extractibility

Xylan chain extension and substitution ar

e not obligatorily coupled during synthesis.

Eermentable sugar release from lignocell

ulose can be increased by reducing xylanbranching.

+lterations in crop xylan structure could be a feasible goal for the bioprocessing industr .

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>eference

Alant 7ell alls, Aeter +lbersheim, etc,

Biochemistry O &olecular Biology of Alants, B. Buchanan, . )ruissem, >. Iones,Mds.

Biochemical control of xylan biosynthesis- which end isup? illiam 2 Kork, etc.

Xylan biosynthesis6 Gews from the grass. +hmed Eaik.