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Metabolic Regulation > Mansfield > Flashcards

Mansfield Flashcards

0
Q

Destinations for nuclear encoded proteins

A

Synthesised on cytosolic ribosomes
Plastid transit peptide
Chaperone proteins regulate folding
TOC and TIC proteins

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1
Q

Ultra structure of the chloroplast

A

Envelope- outer and inner membrane
Inner membrane- selectively permeable, outer is freely.

Stroma- all enzymes for carbon assimilation

Internal lamellae- appressed= granal thylakoids
non appressed= stroma thylakoids
Contain chlorophylls and pigments for light dependent.

Lumen- wat oxidation. Reservoir of protons for e transport. A continuous system.

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2
Q

The Hill reaction

light dependent

A

2H2O + 2A –> 2AH2 + O2
Where A is the electron acceptor DCPIP
NADP+ the electron accepter in chloroplasts

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3
Q

Translocation across the chloroplast membrane

A

Protein bound to cytosolic chaperone
Has a lumenal and stromal transit peptide
Importe into stroma via TOC and TIC, peptide cleaved.
Second cleaved after entry into thylakoids lumen.

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4
Q

Light absorption by chlorophyll

A

Raises low energy e to generate NADPH
Antenna- 3 light harvesting complex polypeptides and associated pigments.
Outer have higher a:b ratio.
Channel energy captured by chlorophylls to reaction centre

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5
Q

Chlorophyll wavelengths

A

440-480
550-700
P700 and P680 max excitation wavelength

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6
Q

Charge separation by light

A

Electron in excited Chl molecule promoted to higher level.
Special pair of α- chlorphylls at reaction centre
Passes electron to acceptor (NADP+)
Electron hole filled by donor

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7
Q

The electron transfer chain

Electron transfer between 2 reaction centres

A

PSI reaction centre- P700 loss of e –> ferredoxin -> NADPH
P700 re reduced by plastocycanin (e from P680 event)
P680 re reduced by water splitting complex. O2 released.
Protons created by water splitting used to create ATP
2:3 NADH ATP produced

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8
Q

Structure of the PSI core complex

A

Excitation of P700 of PSI -> loss of electron
Electron transferred -> FeS -> Ferredoxin
Transferred from ferredoxin to NADP+ by ferredoxin-NADP reductase
NADPH formed
P700 re reduced by plastocyanin

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9
Q

The Z scheme

A

Shows electron transfer in noncylic photosynthesis
Shows reduction potential
Each electron must be lifted twice by photons in PSII and PSI
H+ across thylakoid membrane via Cytb6f

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10
Q

Structure of PSII

A 
Water split by Mn ions
2e transferred to P680
Light then raises P680 electrons
QA (D2) --> QB (D1)
2e and 2H transferred to plastiquinone
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11
Q

Water splitting complex

A

Cluster of 4 Mn2+ ions
Lumenal side of thylakoid membrane, bound to D1 and D2
Evolution of O2
2H2O –> O2 + 4H+ + 4e

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12
Q

ATP synthase complex

A

Protons pumped into lumen by Cytb6f
Re enter stroma by CFo subunit of ATP synthase
Induces change in CF1 –> ATP formed
CFo I and III - encoded by chloroplast genome
CFo II encoded by the nuclear genome

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13
Q

Cyclic electron transport (alternative)

A

Varying degree depending on light conditions
Only involves PSI
P700 -> Ferredoxin -> back to Cytb6f -> PC
PC redonates electrons to P700
No NADPH is formed, but still ATP synthesis
Allows plant to control ATP levels/ ATP:NADH ratio

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14
Q

Lateral heterogeneity of photo systems

A

Non-appressed- PSI and ATP synthases
Appressed- PSII
Complexes in the stromal thylakoid are more hydrophilic (PSI, ATP)
Cytb6f uniformly distributed.
Connected by mobile carriers PQ, PC and ferredoxin

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15
Q

How do herbicides kill plants?

A

Derivatives of urea and triazine
Block transfer of electrons from P680 to PQ
Engineering of crops resistant to this

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16
Q

Calvin cycle- carboxylation stage

A

RuBP + CO2 + H2O –> 2 molecules of 3-phosphoglycerate

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17
Q

Calvin cycle- reduction stage

A

The NADPH and ATP from light reactions is used
3phoshoglycerate kinase and G3P dehydrogenase
Reduces 3-phosphoglycerate –> glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (isomers).
1/6 -> sucrose/starch
5/6 -> regenerated to RuBP

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18
Q

Calvin cycle- regeneration stage

A

3CO2 –> 3 RuBP + 1 leftover triose phosphate
Every 5 for RuBP, 1 is used for sucrose
Autocatalytic- intermediate removal does not affect rate.
1 triose phosphate = 3CO2, 9ATP, 6NADPH

19
Q

Regulation of Calvin cycle enzymes

A

Key enzymes are regulated by reduction of S-S bonds by PSI e

Changes in pH and Mg from illumination

20
Q

4 Calvin cycle enzymes that are active SH inactive SS

The role of reduced thioredoxin, how it is formed

A 
Ribulose 5 phosphate kinase
Fructose 1,6 bisphosphatase
Sedoheptulose 1,7 bisphosphatase
Glyceraldehyde 3 phosphate dehydrogenase
Thioredoxin reduced by PSI electrons and reduced ferredoxin
 Ferredoxin-thioredoxin reductase--> REDUCED THIOREDOXIN
ACTIVATES ENZYMES
No light -> enzymes deactivated
21
Q

Changes in pH and Mg

A

Generation of the proton gradient removes H from stroma
Increased pH -> flow of Mg from lumen to stroma
pH7 night, pH8 light
This increases Rubisco activity
F16Bphosphatase activity x100 when in light

22
Q

Which reaction does Rubisco catalyse?

A

Fixation of CO2 with RuBP -> 3phosphoglycerate
Reaction is virtually irreversible
Can act as both a carboxylase and an oxygenase

23
Q

Structure of Rubisco

A

8 LSU and 8 SSU subunits
Central core of 4LSU is capped by 4SSU
SSU holds complete together, but also increases specificity
50% of total protein in chloroplasts

24
Q

Synthesis of SSU subunit

A

Encoded in the nucleus
Synthesised on cytosolic ribosomes
Post translational import to stroma (TIC and TOC)

25
Q

Synthesis of LSU subunit

A

Encoded in the chloroplast genome
Each contains catalytic site
Synthesised in the chloroplast
Assembly of both subunits to form holo protein. (8 of each)

26
Q

activation of Rubisco 1

A

Action by Rubisco activase
Rubisco activase activated by reductive state of PSI (ferredoxin-thioredoxin).
Rubisco activase promotes binding of CO2 to Lys on LSU

27
Q

Activation of Rubisco 2 mechanism

A

Inactive with RuBP bound
Activase binds, releasing RuBP (ATP used)
Free enzyme binds CO2 (carbamylation) to Lys and Mg
Removes CA1P inhibitor which accumulates in dark

28
Q

How Mg, pH and 3-PGA influence Rubisco

A

High Mg and pH favour carbamylation

Inhibited by 3-PGA as this is a product

29
Q

Activation of Rubisco by carbamylation

A

LSU contains Lysine at position 201 (total 420 AAs)
Lys + CO2 -> carbamate (seperate from the substrate CO2)
Carbamate then binds Mg2+
Rubisco activated

30
Q

Reactions catalysed by Rubisco- carboxylase reaction

A

Carboxylase reaction- cannon reductive cycle
RuBP + CO2 +H2O –> 2x 3-PGA + 2H+
Virtually irreversible
Inhibited by O2

31
Q

Reactions catalysed by Rubisco- oxygenase reaction

A

RuBP + O2 –> 3-PGA + 2-phosphoglycolate (no immediate use)

Inhibited by CO2

32
Q

Carboxylase vs oxygenase activity

A

Much lower affinity for O2
CO2 and O2 compete for active site
Lots more O2 in chloroplast
Oxygenase reaction proceeds at 25% rate of carboxylation

33
Q

Oxygenase activity of Rubisco

A

Only one molecule of 3-PGA produced (not 2) with O2
2-phosphoglycolate enters photo respiratory cycle and returns 3-PGA
Substantial loss of carbon through CO2
Wasteful process

34
Q

Problems with Rubisco

A

Filature to discriminate between CO2 and O2
Slow activity, so large quantities needed
More catalytic sites than substrate
Wastes carbon during oxygenase reaction
Improvement of CO2:O2 ration would stop water wastage

35
Q

Photorespiratory C2 cycle 1

A

Rubisco -> 3-PGA and 2-phosphoglycolate
2-PG -> glycolate -> glyoxylate + H2O2 By glycolate oxidase
Glyoxylate -> glycine by amino transferase in peroxisome
H2O2 hydrolysed by catalase

36
Q

Photorespiratory C2 cycle 2

Glycine decarboxylase

A

Glycine decarboxylase and serine hydroxymethyltransferase

2 glycine –> serine + CO2 + NH3 + NADH

37
Q

Photorespiratory C2 cycle 3

Peroxisome

A

Serine -> hydroxypyruvate by amino transferase
Hydroxypyruvate -> glycerate
Glycerate back to chloroplast -> 3-PGA
Ammonia used to synthesise glutamine

38
Q

Cost/role of Photorespiration

A

Reduces productivity, but may protect against photo inhibition.
Stomata closed - no CO2 entry, lack of water, NADP cannot accept electrons, no gradient to generate ATP.

39
Q

What is a C4 plant?

A

Eliminate Photorespiration by concentrating CO2
Allows Rubisco to work at max rate
No loss of CO2 or 2-PG produced.
Specialised leaf anantomy

40
Q

Leaf anatomy of C4 plants

A

Chloroplast containing cells arranged into 2 ring layers around the vascular bundles (Kranz)
Bundle sheaths thick walled, contain Rubisco.
Both bundle and mesophyll carry out reductive stage

41
Q

The C4 pathway

A
  • CO2 first fixed in mesophyll cells by PEP carboxylase.
  • oxaloacetate -> malate -> Pyruvate + CO2
  • CO2 then used by Rubisco
  • Pyruvate + ATP -> PEP and cycle restarts
42
Q

Activation of C4 enzymes by light

A

Malate dehydrogenase- via thioredoxin system

PEPcase- phosphorylation. High affinity for HCO3-, more efficient than Rubsico (no O2) fixes CO2 in malate/Aspartate.

Pyruvate phosphate kinase- by dephosphorylation.

1CO2 = PEP (2ATP needed)

43
Q

Further adaptation of C4 leaf anatomy

A

Low flux through reduction stage of Calvin cycle
NADPH supplied by NADP-magic enzyme
NADPH is not need form light reactions
High ATP needed. Accommodated by cyclic electron transport.
Half of 3-PGA formed by Rubisco -> triose P -> sucrose
Rest reduced using malic enzyme, no NADPH needed.
NADP-malic enzyme, has no PSII

44
Q

CAM (crassulacean acid metabolism)
Low water and CO2
IN THE DARK

A
  • stomata open to allow CO2 entry
  • CO2 entering needs light to be fixed by Rubsico
  • CO2 fixation catalysed by PEPcase
  • oxaloacetate -> malate
45
Q

CAM (crassulacean acid metabolism)
Low water and CO2
IN THE LIGHT

A

-Stomata are closed (stops water loss)
-malate provides CO2 for Rubisco
-prevents Photorespiration by temporally separated reactions
(In C4 they are spacially separated by anatomy)

Metabolic Regulation (3 decks)

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