2015 biokimia biosynthesis carbohydrate 7

8/15/2019 2015 Biokimia Biosynthesis Carbohydrate 7

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LEHNINGER

PRINCIPLES OF BIOCHEMISTRY

Fifth Edition

David L. Nelson and Michael M. Cox

© 2008 W. H. Freeman and Company

CHA!"# 20

$ioener%e&icsCarbohydrate Biosynthesis in Plants

and Bacteria


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1. Catabolic (oxidative) pathway anabolic (reductive) pathway: use chemical

energy in the form of ATP and NADH or NADPH to synthesize cellular

components from simple precursor molecules

2. In contrast to animal (require three carbons), plants and photosynthetic bacteria can

synthesize carbohydrate from CO2 and H2O, reducing CO2 at the expense of ATPand NADPH that are generated by the light-dependent reaction.

3. CO2 assimilation, CO2 fixation: CO2 3-phophoglycerate by Calvin cycle

4. Key enzymes are

regulated by 1) reductiondisulfide bond 2) pH and

Mg2+ 3) allosteric

regulation

4) covalent modification


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Plastids are organelles unique to plant cells and algae

1. Plastids: a family of self-reproducing

organelles bounded by a double membrane.

1)Chloroplasts : sites of CO2 assimilation instroma

2)Proplastid: loss of internal membrane and

chlorophyll

3)Amyloplast: no thylakoid, rich in starch,

4)Interconvertible each other


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!he &hree s&a%es o' C(2 assimila&ion in pho&osyn&he&ic

or%anisms )CAL*+N C,CL"-


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Stage I: Fixation of CO2 into 3-phophoglycerate

1. Condensation of CO2 with a riblose-1!"-bisphosphate to for# two #olecles of 3-phosphoglycerate

2. Cataly$ed by riblose 1!"-bisphosphatecarboxylase%oxygenase &rbisco' : co(alentattach#ent of CO2 and clea(age of the nstable

six carbon inter#ediate3. Central to the proposed #echanis# for rbisco is

a carba#oylated )ys side chain with a bond *g2+

ion.


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Stage 2: con(ersion of 3-phosphoglycerate toglyceraldyhye-3 phosphate


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&oichiome&ry o' C(2 assimila&ion in &he Calvin cycle.

1. Con(ersion of three CO2 and

one phosphate to triosephosphate

2. Six ,/0 and / arereired ,/0 and /are prodced in the light-

dependent reaction in abotthe sa#e ration&2:3' as theyare cons#ed in Cal(in cycle

3. 4ight /i are released in Cal(in

cycle.5. In order to regenerate /!

one /i #st be i#ported fro#cytosol.

". 6bisco! septlose 1!7-bisphosphatase! riblose "-phosphate 8inase is absencein ani#als.


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A transport system exports triosephosphates from the chloroplast and importphosphate1. /i-triose phosphate antiporter si#ltaneosly #o(es / i into

the chloroplast and #o(es triphosphate into the cytosol.2. Scrose synthesis release /i.

3. If this exchange were bloc8ed! triose phosphate synthesiswold ic8ly deplete the a(ailable /i in chloroplast.


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1. he en$y#es are #ore acti(e in an al8aline en(iron#ent andat high *g2+

2. cti(ation of rbisco by for#ation of the carba#oyllysine is

faster at al8line p0 and high *g2+

3. Frctose 1.9-bisphosphatse acti(ity is increase when p0 and*g2+ rise


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1. Ribulase 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-

bisphosphatase, and glycerladehyde 3-phosphate dehydrogenase are activated light-

driven reduction of between two Cys residues.


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1. In dark, mitochondrial respiration + photorespiration (consumed O2 and produce

CO2)

2. Photorespiration result from rubisco’s oxygenase activity and produce 2-

phosphoglycolate, a metabolically useless product.


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/lycola&e pa&hay. : glycolate

pathway converts two molecules

of 2-phosphoglycolate to oneserine and CO2The serine is converted to

hydroxypyruvate and then to

glycerate in peroxisomesglycerate reenters the

chloroplasts to be

phosphorylated! re"oining the

Calvin cycle# Oxygen $shaded

blue% is consumed at two steps

during photorespiration#


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Car1on assimila&ion in C plan&s.

&n order to evade the wasteful

photophorespiration! CO2fixation and rubisco activity are

spatially separated in C' plant


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A model 'or s&arch syn&hesis. 1. During active photosynthesis, the excess is converted to sucrose and starch, main

storage form.

2. ADP-glucose is the activated nucleotide sugar.

3. Strach synthase: two active site that alternate in inserting a glucosyl residue onto thereducing end.


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3crose syn&hesis


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2015 biokimia biosynthesis carbohydrate 7

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