5
Biosynthesis of Bacterial Cellulose
freeze-fracture.analysis.allowed.the.identification.of.a.linear.array.of.pores.
in.the.cell.envelope.of.G. xylinus.(Brown.et.al..1976;.Zaar.1979)..The.pores.represent.the.cellulose-synthesizing.sites.and.a.number.of.glucan.chains.arise.
from. a. single. pore. forming. a. subelementary. fibril.. These. subelementary.
fibrils. assemble. with. adjoining. subelementary. fibrils. giving. rise. to. microfibrils. that. assemble. into. 20–50. nm. wide. ribbons. in. G. xylinus.. The. rather.
flat.and.twisted.cellulose.ribbon.formed.by.G. xylinus.cells.is.a.fascinating.
structure.that.is.made.up.of.hundreds.of.glucan.chains.that.associate.with.
each.other.in.a.specific.manner,.giving.rise.to.crystalline.cellulose.I.
The.assembly.of.the.cellulose.microfibrils.is.visualized.as.a.two-step.process. of. polymerization. and. crystallization,. with. polymerization. of. glucose.
residues. to. form. a. glucan. chain. taking. place. in. the. membrane. and. final.
(. second.stage).crystallization.of.the.glucan.chains.into.cellulose.I.occurring.
in.the.extracellular.space..This.coupled.process.of.polymerization.and.crystallization.can.be.disturbed.in.the.presence.of.certain.compounds,.such.as.
calcofluor,.and.studies.using.these.compounds.have.provided.evidence.that.
crystallization.is.the.rate-limiting.step.in.the.synthesis.of.cellulose.(Benziman.
et. al.. 1980).. More. importantly,. assembly. of. the. cellulose. I. crystallite. is. proposed.to.proceed.in.stages,.with.the.formation.of.glucan.chain.sheets.by.van.
der. Waals. bonding. in. the. first. stage. followed. by. stacking. of. the. sheets. by.
H-bonding.to.give.rise.to.the.crystalline.structure.(Cousins.and.Brown.1995,.
1997a,.1997b).
A.single.G. xylinus.cell.produces.a.ribbon.of.cellulose.with.10–100.microfibrils.. Each. microfibril. assembles. at. a. linear. terminal. complex. (TC). that.
consists. of. three. subunits. (cellulose-synthesizing. sites). and. each. subunit.
contains.at.least.16.cellulose.synthase.catalytic.subunits.(Brown.1996)..Each.
catalytic.subunit.produces.a.single.β-1,4-glucan.chain,.and.16.glucan.chains.
in.a.single.TC.subunit.form.a.minicrystal..Minicrystals.from.three.adjacent.
TC.subunits.assemble.to.form.a.crystalline.microfibril,.and.microfibrils.originating.from.adjacent.TCs.assemble.to.form.a.cellulose.ribbon..Assembly.of.
cellulose.synthase.catalytic.subunits.with.associated.proteins.to.form.a.TC.
subunit.(cellulose-synthesizing.complex).is.discussed.in.a.later.section.
Cellulose Synthesis in Bacteria is Catalyzed by the
Membrane-Associated Cellulose Synthase
Soon. after. the. identification. of. UDP-glucose. as. a. sugar. donor. in. glycosyl.
transfer.reactions,.it.was.shown.to.be.the.substrate.for.in.vitro.synthesis.of.
cellulose. in. cell-free. extracts. of. G. xylinus. (Glaser. 1958).. However,. a. breakthrough.in.determining.cellulose.synthase.activity.in.cell-free.extracts.from.
G. xylinus.was.not.observed.until.the.1980s,.when.it.was.shown.that.a.protein.
Biosynthesis of Bacterial Cellulose
freeze-fracture.analysis.allowed.the.identification.of.a.linear.array.of.pores.
in.the.cell.envelope.of.G. xylinus.(Brown.et.al..1976;.Zaar.1979)..The.pores.represent.the.cellulose-synthesizing.sites.and.a.number.of.glucan.chains.arise.
from. a. single. pore. forming. a. subelementary. fibril.. These. subelementary.
fibrils. assemble. with. adjoining. subelementary. fibrils. giving. rise. to. microfibrils. that. assemble. into. 20–50. nm. wide. ribbons. in. G. xylinus.. The. rather.
flat.and.twisted.cellulose.ribbon.formed.by.G. xylinus.cells.is.a.fascinating.
structure.that.is.made.up.of.hundreds.of.glucan.chains.that.associate.with.
each.other.in.a.specific.manner,.giving.rise.to.crystalline.cellulose.I.
The.assembly.of.the.cellulose.microfibrils.is.visualized.as.a.two-step.process. of. polymerization. and. crystallization,. with. polymerization. of. glucose.
residues. to. form. a. glucan. chain. taking. place. in. the. membrane. and. final.
(. second.stage).crystallization.of.the.glucan.chains.into.cellulose.I.occurring.
in.the.extracellular.space..This.coupled.process.of.polymerization.and.crystallization.can.be.disturbed.in.the.presence.of.certain.compounds,.such.as.
calcofluor,.and.studies.using.these.compounds.have.provided.evidence.that.
crystallization.is.the.rate-limiting.step.in.the.synthesis.of.cellulose.(Benziman.
et. al.. 1980).. More. importantly,. assembly. of. the. cellulose. I. crystallite. is. proposed.to.proceed.in.stages,.with.the.formation.of.glucan.chain.sheets.by.van.
der. Waals. bonding. in. the. first. stage. followed. by. stacking. of. the. sheets. by.
H-bonding.to.give.rise.to.the.crystalline.structure.(Cousins.and.Brown.1995,.
1997a,.1997b).
A.single.G. xylinus.cell.produces.a.ribbon.of.cellulose.with.10–100.microfibrils.. Each. microfibril. assembles. at. a. linear. terminal. complex. (TC). that.
consists. of. three. subunits. (cellulose-synthesizing. sites). and. each. subunit.
contains.at.least.16.cellulose.synthase.catalytic.subunits.(Brown.1996)..Each.
catalytic.subunit.produces.a.single.β-1,4-glucan.chain,.and.16.glucan.chains.
in.a.single.TC.subunit.form.a.minicrystal..Minicrystals.from.three.adjacent.
TC.subunits.assemble.to.form.a.crystalline.microfibril,.and.microfibrils.originating.from.adjacent.TCs.assemble.to.form.a.cellulose.ribbon..Assembly.of.
cellulose.synthase.catalytic.subunits.with.associated.proteins.to.form.a.TC.
subunit.(cellulose-synthesizing.complex).is.discussed.in.a.later.section.
Cellulose Synthesis in Bacteria is Catalyzed by the
Membrane-Associated Cellulose Synthase
Soon. after. the. identification. of. UDP-glucose. as. a. sugar. donor. in. glycosyl.
transfer.reactions,.it.was.shown.to.be.the.substrate.for.in.vitro.synthesis.of.
cellulose. in. cell-free. extracts. of. G. xylinus. (Glaser. 1958).. However,. a. breakthrough.in.determining.cellulose.synthase.activity.in.cell-free.extracts.from.
G. xylinus.was.not.observed.until.the.1980s,.when.it.was.shown.that.a.protein.