Chapter 20: Carbohydrate Biosynthesis in Plants and Bacteria

Question 1

• Anabolic pathways are generally _____ rather than _____
• Catabolism and anabolism proceed _____ in a dynamic steady state, so the energy yielding degradation of cellular components is counterbalanced by biosynthetic processes

Answer 1

• reductive, oxidative
• simultaneously

Question 2

Photosynthetic Carbohydrate Synthesis

• The synthesis of carbohydrates in animal cells always employs precursors having at least _____ carbons, all of which are less oxidized than the carbon in _____.
• Plants and photosynthetic microorganisms, by contrast, can synthesize carbohydrates from _____ and _____, reducing _____ at the expense of the energy and reducing power furnished by _____ and _____ generated by the light-dependent reactions of photosynthesis

Answer 2

• three, CO₂
• CO₂, water, CO₂,
• ATP, NADPH

Question 3

CO₂ assimilation

Answer 3

the conversion of CO₂ to simple (reduced) organic compounds

Question 4

CO₂ fixation, carbon fixation

Answer 4

• the specific reaction in which CO₂ is incorporated (fixed) into a three-carbon organic compound, the triose phosphate 3-phosphoglycerate
  
  • the precursor of more complex biomolecules, including sugars, polysaccharides, and the metabolites
  • all carbon-containing compounds of the plant cell are synthesized from triose
• The light-driven synthesis of ATP and NADPH provides energy and reducing power for the fixation of CO₂ into trioses

Question 5

plastids

Answer 5

• where most of the biosynthetic activities in plants (including CO2 assimilation) occur
• a family of self-reproducing organelles bounded by a double membrane
• has a small genome that encodes some of their proteins
• most other proteins are imported into plastids
• reproduce by binary fission
• genome is a single circular DNA molecule
• they use their own enzymes and ribosomes to synthesize the proteins encoded by that genome

Question 6

plastids: chloroplasts

Answer 6

• sites of CO₂ assimilation
  
  • enzymes for this process are in the strom

Question 7

plastids: Amyloplasts

Answer 7

• colorless plastids
• have no internal membranes
  
  • analogous to the photosynthetic membranes (thylakoids) of chloroplasts
• packed with starch granules

Question 8

plastids: proplastids

Answer 8

• has not internal membrane
• colourless and small
• It is from where plastids are derived

Question 9

interconversions of plastids

Answer 9

• A mature chloroplast has extensive internal membranes
• The internal membranes can be lost, converting a mature chloroplast into a proplastid
• Proplastid gives rise to a pre-granal plastid and then a mature chloroplast
• Proplastids in nonphotosynthetic tissues (such as root) give rise to amyloplasts

Question 10

Calvin cycle, photosynthetic carbon reduction cycle

summary

Answer 10

• Anabolic
• Occurs in the stroma
• Carbon enters the Calvin cycle in the form of CO₂, and it’s reduced to carbohydrates, leaveing in the form of sugar
  
  • CO₂ is reduced w/the H from NADPH acquired in light reactions
  • conversion requires ATP provided by light reactions
• Referred to as the dark reaction or light-independent reaction, because it does not require light
• The carb produced directly from Calvin cycle is G3P
• For the net synthesis of one G3P, the cycle must take place 3 times
  
  • Fixing three molecules of CO₂ in total
  • one per turn of the cycle
• Has 3 stages
• cyclical, with the continuous conversion of CO2 to triose and hexose phosphates

Question 11

Calvin cycle, photosynthetic carbon reduction cycle

stage 1

Answer 11

carbon-fixation reaction

• condensation of CO2 with a five-carbon acceptor, ribulose 1,5-bisphosphate (RuBP), to form two molecules of 3-phosphoglycerate
  
  • ​3-phosphoglycerate is an intermediate in photosynthesis
  • plants w/this first intermediate are called C₃ plants
• this reaction is catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase, rubisco
  
  • it covalently attaches CO₂ to the five-carbon sugar RuBP
  • then cleaves the unstable six-carbon intermediate to form two molecules of 3-phosphoglycerate
    
    • one has the carbon introduced as CO2 in its carboxyl group
  • two forms of rubisco
    
    • Form I
      
      • found in vascular plants, algae, and cyanobacteria
      • has a complex form I structure
      • eight identical large subunits w/catalytic sites
      • eight identical small subunits
    • From II
      
      • photosynthetic bacteria
      • simpler in structure
      • has 2 subunits that resemble the large subunits of the plant enzyme
  • low turnover number
    
    • three molecules of CO2 are fixed per second at 25°C
    • large qty of enzyme needed
    • makes up almost 50% of soluble protein in chloroplast
    • one of the most abundant enzymes in the biosphere
  • a carbamoylated Lys side chain with a bound Mg²⁺ ion is key to the enzyme
    
    • Mg²⁺ brings together and orients the reactants at the active site
    • It polarizes the CO₂, opening it to nucleophilic attack by the five-carbon enediolate reaction intermediate formed on the enzyme
    • resulting six-carbon intermediate breaks down to yield two molecules of 3-phosphoglycerate
  • Regulation
    
    • is inactive until carbamoylated on the ε amino group of Lys²⁰¹
      
      • Ribulose 1,5-bisphosphate inhibits carbamoylation by binding tightly to the active site and locking the enzyme in the “closed” conformation, imaking Lys²⁰¹ inaccessible
    • Rubisco activase
      
      • activated by light
      • overcomes inhibition by promoting ATP-dependent release of the ribulose 1,5-bisphosphate
      • this exposes Lys²⁰¹ to nonenzymatic carbamoylation by CO₂ followed by Mg²⁺ binding, which activates the rubisco
    • 2-carboxyarabinitol 1-phosphate
      
      • “nocturnal inhibitor”
      • synthesized in the dark
      • naturally occurring transition-state analog
      • inhibitor of carbamoylated rubisco
      • broken down when light returns or is expelled by rubisco activase, activating the rubisco.

Question 12

Calvin cycle, photosynthetic carbon reduction cycle

stage 2

Answer 12

Reduction

Conversion of 3-Phosphoglycerate to Glyceraldehyde 3-Phosphate (G3P)

• 3-phosphoglycerate is converted to G3P in two steps
  
  • Each 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1-3-phosphoglycerate
    
    • catalyzed by 3-phosphoglycerate kinase
  • A pair of electrons donated from NADPH, reduces 1-3-phosphoglycerate which loses a phosphate group (Pi) and becomes G3P, a sugar
    
    • catalyzed by glyceraldehyde 3-phosphate dehydrogenase
• the process is the reversal of glycolysis, with one exception: the nucleotide cofactor for the reduction is NADPH rather than NADH
• fates of G3P
  
  • For 3 CO₂, 6 G3P are formed
  • One G3P is counted as a net gain of carbs, and exits the cycle to be used by the plant cell
    
    • small fraction used immediately as a source of energy
    • most is converted to sucrose for transport or stored in the chloroplast as starch
    • some condenses with dihydroxyacetone phosphate in the stroma to form fructose 1,6-bisphosphate, a precursor of starch
    • some is converted to dihydroxyacetone phosphate, leaves the chloroplast and, in the cytosol, can be degraded glycolytically to provide energy or used to form fructose 6-phosphate and hence sucrose.
  • Five G3P are recycled to regenerate the 3 molecules of ribulose 1,5-bisphosphate (RuBP)

Question 13

Calvin cycle, photosynthetic carbon reduction cycle

stage 3

Answer 13

Regeneration of Ribulose 1,5-Bisphosphate (RuBP) from Triose Phosphates (G3P)

• Regeneration of the CO2 acceptor (RuBP)
• Pathway
  
  1. aldolase catalyzes the reversible condensation of glyceraldehyde 3-phosphate with dihydroxyacetone phosphate, yielding fructose 1,6-bisphosphate
  2. fructose 1,6-bisphosphatase cleaves fructose 1,6-bisphosphate to fructose 6-phosphate and Pi
     
     • exergonic and irreversible
  3. transketolase transfers a 2-carbon ketol group (CH₂OH—CO—) from fructose 6-phosphate (ketose phosphate donor) to glyceraldehyde 3-phosphate (aldose phosphate acceptor) forming pentose xylulose 5-phosphate and the tetrose erythrose 4-phosphate
     
     • reversible transfer
     • transketolase contains thiamine pyrophosphate (TPP) as its prosthetic group and requires Mg²⁺
  4. aldolase combines erythrose 4-phosphate with dihydroxyacetone phosphate to form the seven-carbon sedoheptulose 1,7-bisphosphate
  5. sedoheptulose 1,7-bisphosphatase, converts bisphosphate to sedoheptulose 7-phosphate
     
     • exergonic and irreversible
  6. Transketolase converts sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate to two pentose phosphates: ribose 5-phosphate and xylulose 5-phosphate
  7. ribose 5-phosphate isomerase converts ribose 5-phosphate to ribulose 5-phosphate
  8. ribulose 5-phosphate epimerase converts xylulose 5-phosphate to ribulose 5-phosphate
  9. ribulose 5-phosphate phosphorylates kinase ribulose 5-phosphate to ribulose 1,5-bisphosphate
     
     • very exergonic and irreversible
  10. ​PDF pg 837

Question 14

Stoichiometry of CO2 assimilation in the Calvin cycle.

The net result of three turns of the Calvin cycle the following is produced:

Answer 14

• 1 triose phosphate (glyceraldehyde 3-phosphate, or G3P)
• 9 ATP
• 6 NADPH are consumed

Question 15

• NADPH and ATP are produced in the light-dependent reactions of photosynthesis in about the same ratio (_____) as they are consumed in the Calvin cycle.
• Nine ATP molecules are converted to _____ and _____ in the generation of a molecule of triose phosphate
• Eight of the phosphates are released as ______ and combined with eight _____ to regenerate ATP.
• The ninth phosphate is incorporated into the _____ _____ .
• To convert the ninth ADP to ATP, a molecule of _____ must be imported from the cytosol

Answer 15

• 2:3
• ADP, phosphate
• P_i, ADP
• triose phosphate
• Pi

Question 16

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by CO₂ assimilation (_____ _____ and ______ ______) to starch, which is temporarily stored in the _____ as insoluble granules

Answer 16

• glyceraldehyde 3-phosphate (G3P)
• dihydroxyacetone phosphate
• chloroplast

Question 17

All the reactions of the Calvin cycle except those catalyzed by _____, _____ _____, and _____ _____ _____ also take place in animal tissues. Lacking these three enzymes, animals cannot carry out net conversion of CO2 to _____

Answer 17

• rubisco
• sedoheptulose 1,7-bisphosphatase
• ribulose 5-phosphate kinase
• glucose

Question 18

P_i–triose phosphate antiporter

• It is located in the inner _____ _____
• has two functions:

Answer 18

• chloroplast membrane
• functions
  
  • facilitates the exchange of cytosolic Pi for stromal dihydroxyacetone phosphate
  • transport of ATP and reducing equivalents

Question 19

Pi–triose phosphate antiporter

exchange of cytosolic Pi for stromal dihydroxyacetone phosphate pathway

Answer 19

• catalyzes the one-for-one exchange of Pi with two types of triose phosphate:
  
  • dihydroxyacetone phosphate
  • 3-phosphoglycerate
• moves triose phosphate into the cytosol for sucrose synthesis
  
  • Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is “harvested.”
• for every molecule of triose phosphate removed from the chloroplast, one P_i is transported into the chloroplast
• the transported P_i becomes the ninth P_i used in regenerating ATP

Question 20

Pi–triose phosphate antiporter

transport of ATP and reducing equivalents

Answer 20

• neither ATP nor NADPH can cross the chloroplast membrane
• moves ATP equivalents and reducing equivalents from the chloroplast to the cytosol
• Dihydroxyacetone phosphate formed in the stroma is transported to the cytosol
• triose phosphate isomerase converts it to glyceraldehyde 3-phosphate
• glyceraldehyde 3-phosphate dehydrogenase converts it to 1,3-Bisphosphoglycerate producing NADH
• phosphoglycerate kinase converts it to 3-phosphoglycerate generating ATP
• 3-Phosphoglycerate reenters the chloroplast and is reduced in several steps back to and is reduced to dihydroxyacetone phosphate

Question 21

• when chloroplasts are illuminated light-induced transport of protons across the thylakoid membrane increases the stromal _____ from about 7 to about 8 accompanied by a flow of _____ from the thylakoid compartment into the stroma, which signal the availability of _____ and _____
• two enzymes that are more active in an alkaline environment and at high [Mg²⁺]
• the following 4 Calvin cycle are activated by light-driven reduction of disulfide bonds between two Cys residues critical to their catalytic activities:
• When these Cys residues are disulfide-bonded the enzymes are _____, which is normal in the dark
• With illumination, electrons flow from photosystem I to ferredoxin, which passes electrons to a small, soluble, disulfide-containing protein called _____, which is reduced by by ferredoxin-thioredoxin reductase.
• Reduced ______ donates electrons for the reducive cleavage reactions accompanied by conformational changes that increase enzyme activities.
• At nightfall, the Cys residues in the four enzymes are reoxidized to their disulfide forms, the enzymes are _____, and _____ is not expended in CO₂ assimilation.
• Instead, ______ synthesized and stored during the daytime is degraded to fuel _____ at night

Answer 21

• pH, Mg²⁺, ATP, NADPH
• rubisco and Fructose 1,6-bisphosphatase
• Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and glyceraldehyde 3-phosphate dehydrogenase
• inactive
• thioredoxin
• thioredoxin
• inactivated, ATP
• starch, glycolysis

Question 22

• Rubisco is not absolutely specific for _____ as a substrate.
• _____ competes with _____ at the active site, and about once in every three or four turnovers, rubisco catalyzes the condensation of _____ with ribulose 1,5-bisphosphate to form an unstable intermediate.
• The unstable intermediate thus formed splits into _____ (recycled) and _____, which can reenter the Calvin cycle.

Answer 22

• CO₂
• O₂, CO₂, O₂
• 2-phosphoglycolate, 3-phosphoglycerate

Question 23

glycolate pathway

Answer 23

• converts two molecules of 2-phosphoglycolate to a molecule of serine (three carbons) and a molecule of CO₂

Pathway

• in the chloroplast
  
  • phosphatase converts 2-phosphoglycolate to glycolate
  • glycolate is exported to the peroxisome
• in the peroxisome
  
  • glycolate is oxidized by O₂, into glyoxylate
  • glyoxylate undergoes transamination to glycine
  • hydrogen peroxide formed as a side product of glycolate oxidation is rendered harmless by peroxidases
  • Glycine passes from the peroxisome to the mitochondrial matrix
• in the mitochondrial matrinx
  
  • Glycine undergoes oxidative decarboxylation by the glycine decarboxylase complex
    
    • enzyme similar to pyruvate dehydrogenase complex and the α-ketoglutarate dehydrogenase complex
    • oxidizes glycine to CO₂ and NH₃,
    • reduces NAD⁺ to NADH
    • transfers remaining carbon from glycine to the cofactor tetrahydrofolate
    • tetrahydrofolate transfers to a second glycine by serine hydroxymethyltransferase
  • serine hydroxymethyltransferase produces serine
  • serine enters peroxisomes
• in the peroxisome
  
  • serine is converted to hydroxypyruvate
  • hydroxypyruvate is converted to glycerate
  • glycerate reenters the chloroplasts
• in the chloroplasts
  
  • glycerate is phosphorylated and converted to 3-phosphoglycerate, which is used to regenerate ribulose 1,5-bisphosphate, rejoining the Calvin cycle
  • Oxygen (shaded blue) is consumed at two steps during photorespiration
• PDF pgs 844 - 845

Question 24

• In bright sunlight the glycolate salvage pathway produces about five times more _____ than is typically produced by all the oxidations of the citric acid cycle
• As a result mitochondria contain prodigious amounts of the _____ _____ _____; in In nonphotosynthetic parts of a plant it is in low concentrations

Answer 24

• CO₂
• glycine decarboxylase complex

Question 25

• The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes O₂ and produces CO₂—hence the name _____
• Unlike mitochondrial respiration, it does not _____ _____ and may actually inhibit net biomass formation as much as 50%

Answer 25

• photorespiration, oxidative photosynthetic carbon cycle or C₂ cycle
• conserve energy

Question 26

C₄ plants

Answer 26

• evolved a mechanism to circumvent the problem of wasteful photorespiration
• The step in which CO₂ is fixed into a three-carbon product, 3-phosphoglycerate, is preceded by several steps, one of which is temporary fixation of CO2 into a four-carbon compound
• the assimilation process is known as C₄ metabolism or the C₄ pathway
• typically grow at high light intensity and high temperatures
• high photosynthetic rates, high growth rates
• low photorespiration rates, low rates of water loss
• a specialized leaf structure
• Photosynthesis in the leaves involves two cell types: mesophyll and bundle-sheath cells
• Pathway: PDF pg 846 - 849

Question 27

CAM plants

Answer 27

• Succulent plants
• have another variation on photosynthetic CO2 fixation
• reduces loss of water vapor through the pores by which CO₂ and O₂ must enter leaf tissue.
• seperate initial trapping of CO₂ and fixation by rubisco
• Pathway
  
  • At night, when the air is cooler and moister
    
    • the stomata open to allow entry of CO₂
    • CO₂ is fixed into oxaloacetate by PEP carboxylase
    • oxaloacetate is reduced to malate and stored in the vacuoles
      
      • to protect cytosolic and plastid enzymes from the low pH produced by malic acid dissociation
  • During the day
    
    • the stomata close, preventing water loss
    • CO₂ trapped overnight in malate is released as CO₂ by the NADP-linked malic enzyme
    • This CO₂ is now assimilated by the action of rubisco and the Calvin cycle enzymes

Question 28

• During active photosynthesis in bright light, a plant leaf produces more carbohydrate (as _____ _____) than it needs for generating energy or synthesizing precursors.
• The excess is converted to ______ and transported to other parts of the plant, to be used as _____ or _____.
• In most plants, _____ is the main storage form, but in a few plants, such as sugar beet and sugarcane, _____ is the primary storage form.
• The synthesis of sucrose and starch occurs in different cellular compartments: ______ and _____, respectively

Answer 28

• triose phosphates
• sucrose, fuel, stored
• starch, sucrose
• cytosol, plastids

Question 29

• Starch, like glycogen, is a high molecular weight polymer of D-glucose in (_____) linkage.
• It is synthesized in _____ for temporary storage as one of the stable end products of photosynthesis, and for long-term storage it is synthesized in the _____ of the nonphotosynthetic parts of plants

Answer 29

• α1→4
• chloroplasts, amyloplasts

Question 30

• The mechanism of glucose activation in starch synthesis is similar to that in glycogen synthesis. An activated nucleotide sugar, in this case _____, is formed by condensation of glucose 1-phosphate with ATP in a reaction made essentially irreversible by the presence of inorganic pyrophosphatase in plastids
• Starch synthase then transfers glucose residues from _____ to preexisting _____ molecules.
• The monomeric units are almost certainly added to the _____ end of the growing polymer, as they are in glycogen synthesis

Answer 30

• ADP-glucose
• ADP-glucose, starch
• nonreducing

Question 31

Most of the triose phosphate generated by CO₂ fixation in plants is converted to ____ or _____

Answer 31

• sucrose
• starch

Question 32

Sucrose synthesis

Answer 32

• Sucrose is synthesized in the cytosol
• dihydroxyacetone phosphate and glyceraldehyde 3-phosphate are exported from the chloroplast.
• Aldolase condenses two triose phosphates to form fructose 1,6-bisphosphate
• fructose 1,6-bisphosphatase hydrolysis fructose 1,6-bisphosphate to fructose 6-phosphate.
• Sucrose 6-phosphate synthase then catalyzes the reaction of fructose 6-phosphate with UDP-glucose to sucrose 6-phosphate (Fig. 20–24)
• sucrose 6-phosphate phosphatase removes the phosphate group, making sucrose available for export to other tissues.
• reaction catalyzed by sucrose 6-phosphate synthase is a low-energy process (ΔG’° = -5.7 kJ/mol)
• the hydrolysis of sucrose 6-phosphate to sucrose is sufficiently exergonic (ΔG’° = -16.5 kJ/mol) to make the overall synthesis of sucrose essentially irreversible
• One difference between plant and animal cells is the absence in the plant cell cytosol of the enzyme inorganic pyrophosphatase, which catalyzes the reaction
  
  • makes the process more favorable energetically, tending to make these reactions irreversible
  • In plants, this enzyme is present in plastids but absent from the cytosol

Question 33

• Five-sixths of the triose phosphate formed in the Calvin cycle must be recycled to _____ _____.
• If more than one-sixth of the triose phosphate is drawn out of the cycle to make ______ and _____, the cycle will slow or stop.
• However, insufficient conversion of triose phosphate to starch or sucrose would tie up _____, leaving a chloroplast deficient in _____, which is also essential for operation of the Calvin cycle

Answer 33

• ribulose 1,5-bisphosphate
• sucrose, starch
• phosphate, Pi

Question 34

• The flow of triose phosphates into sucrose is regulated by the activity of _____ _____ (FBPase-1) and the enzyme that effectively reverses its action, ______ ______ (PPPFK-1)
• Both enzymes are regulated by _____ _____ (F26BP), which inhibits FBPase-1 and stimulates PP-PFK-1
• Phosphofructokinase-2, responsible for F26BP synthesis, is _____ by dihydroxyacetone phosphate or 3-phosphoglycerate and ______ by fructose 6-phosphate and Pi.
• During active photosynthesis, dihydroxyacetone phosphate is produced and Pi is consumed, resulting in inhibition of PFK-2 and lowered concentrations of F26BP.
• This favors greater flux of triose phosphate into fructose 6-phosphate formation and sucrose synthesis.
• With this regulatory system, sucrose synthesis occurs when the level of triose phosphate produced by the Calvin cycle exceeds that needed to maintain the operation of the cycle

Answer 34

• fructose 1,6-bisphosphatase
• PPi-dependent phosphofructokinase
• fructose 2,6-bisphosphate
• inhibited, stimulated

Question 35

• Sucrose synthesis is also regulated at the level of sucrose 6-phosphate synthase, which is allosterically _____ by glucose 6-phosphate and ______ by Pi.
• It’s also regulated by
• A protein kinase phosphorylates the enzyme on a specific Ser residue, making it less _____, and a phosphatase reverses this inactivation by removing the phosphate
• Inhibition of the kinase by glucose 6-phosphate, and of the phosphatase by Pi, ______ the effects of these two compounds on sucrose synthesis.
• When hexose phosphates are abundant, sucrose 6-phosphate synthase is activated by glucose 6-phosphate; when Pi is elevated (as when photosynthesis is slow), sucrose synthesis is ______
• During active photosynthesis, triose phosphates are converted to fructose 6-phosphate, which is rapidly equilibrated with glucose 6-phosphate by phosphohexose isomerase. Because the equilibrium lies far toward glucose 6-phosphate, as soon as fructose 6-phosphate accumulates, the level of glucose 6-phosphate rises and sucrose synthesis is ______

Answer 35

• activated, inhibited
• phosphorylation and dephosphorylation
• active
• enhances
• slowed
• stimulated

Question 36

• key regulatory enzyme in starch synthesis is
• it is ______ by 3-phosphoglycerate (which accumulates during active photosynthesis) and ______ by Pi (which accumulates when light-driven condensation of ADP and Pi slows)
• When sucrose synthesis slows, 3-phosphoglycerate formed by CO2 fixation accumulates, _____ this enzyme and ______ the synthesis of starch

Answer 36

• ADP-glucose pyrophosphorylase
• activated, inhibited
• activating, stimulating

Question 37

The structure of cellulose is simple:

Answer 37

• linear polymers of thousands of (β 1 → 4)-linked D-glucose units
• assembled into bundles of about 36 chains
• bundles aggregate side by side to form a microfibril

Question 38

cellulose must be synthesized from ______ precursors but deposited and assembled outside the _____ _____

Answer 38

• intracellular
• plasma membrane

Question 39

The complex enzymatic machinery that assembles cellulose chains spans the plasma membrane, with one part positioned to bind the substrate, ______, in the cytosol and another part extending to the outside, responsible for elongating and crystallizing cellulose molecules in the extracellular space

Answer 39

UDP-glucose

Question 40

terminal complexes, rosettes

Answer 40

• composed of six large particles arranged in a regular hexagon with a diameter of about 30 nm
• Several proteins, including the catalytic subunit of cellulose synthase, make up the terminal complex

Question 41

synthesis of cellulose in a vascular plant

Answer 41

• Glucose is attached to sitosterol to form a lipid-linked glucan forming Sitosterol β-glucoside on the inner face of the plasma membrane
• intracellular cellulose synthase adds several more glucose residues to the first one, in (β1→4) linkage, forming a short oligosaccharide chain attached to the sitosterol forming sitosterol dextrin
• sitosterol dextrin flips across to the outer face of the plasma membrane, and associates with Sucrose synthase
• Sucrose synthase generates UDPglucose
• used for cellulose synthesis
• generated from sucrose produced during photosynthesis
• cytosollic UDP-glucose is used by cellulose synthase (which spans the plasma membrane) as the precursor for extracellular cellulose synthesis
• A membrane-bound form of sucrose synthase forms a complex with cellulose synthase
• UDP-glucose is transferred from sucrose directly into cell wall synthesis
• the complex moves along the plasma membrane, following the course of microtubules in the cortex (the cytoplasmic layer just under the membrane)
• a second form of cellulose synthase extends the polymer to 500 to 15,000 glucose units, extruding it onto the outer surface of the cell
• one enzyme molecule adds many glucose units before releasing the growing cellulose chain
• a rosette synthesizes six cellulose chains
• The finished cellulose is in the form of crystalline microfibrils, each consisting of 36 separate cellulose chains lying side by side, all with the same (parallel) orientation of nonreducing and reducing ends
• When the 36 polymers reach some critical length, their synthesis is terminated by an unknown mechanism; crystallization into a microfibril follows

In the activated precursor of cellulose (UDP-glucose), the glucose is α-linked to the nucleotide, but in the product (cellulose), glucose residues are (β1→4)-linked, so there is an inversion of configuration at the anomeric carbon (C-1) as the glycosidic bond forms

Question 42

• The peptidoglycan that gives bacterial envelopes their strength and rigidity is an alternating linear copolymer of _____ (GlcNAc) and _____ _____ (Mur2Ac), linked by (β1→4) glycosidic bonds and cross-linked by short peptides attached to the Mur2Ac (Fig. 20–30)
• See PDF pg. 854-855 for assembly of the polysaccharide

Answer 42

• N-acetylglucosamine
• N-acetylmuramic acid