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Metabolic Regulation > Mansfield > Flashcards

Mansfield Flashcards

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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

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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

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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

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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

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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?

Study These Flashcards
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

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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
Synthesis of SSU subunit
Encoded in the nucleus Synthesised on cytosolic ribosomes Post translational import to stroma (TIC and TOC)
25
Synthesis of LSU subunit
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
activation of Rubisco 1
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
Activation of Rubisco 2 mechanism
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
How Mg, pH and 3-PGA influence Rubisco
High Mg and pH favour carbamylation | Inhibited by 3-PGA as this is a product
29
Activation of Rubisco by carbamylation
LSU contains Lysine at position 201 (total 420 AAs) Lys + CO2 -> carbamate (seperate from the substrate CO2) Carbamate then binds Mg2+ Rubisco activated
30
Reactions catalysed by Rubisco- carboxylase reaction
Carboxylase reaction- cannon reductive cycle RuBP + CO2 +H2O --> 2x 3-PGA + 2H+ Virtually irreversible Inhibited by O2
31
Reactions catalysed by Rubisco- oxygenase reaction
RuBP + O2 --> 3-PGA + 2-phosphoglycolate (no immediate use) | Inhibited by CO2
32
Carboxylase vs oxygenase activity
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
Oxygenase activity of Rubisco
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
Problems with Rubisco
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
Photorespiratory C2 cycle 1
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
Photorespiratory C2 cycle 2 | Glycine decarboxylase
Glycine decarboxylase and serine hydroxymethyltransferase | 2 glycine --> serine + CO2 + NH3 + NADH
37
Photorespiratory C2 cycle 3 | Peroxisome
Serine -> hydroxypyruvate by amino transferase Hydroxypyruvate -> glycerate Glycerate back to chloroplast -> 3-PGA Ammonia used to synthesise glutamine
38
Cost/role of Photorespiration
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
What is a C4 plant?
Eliminate Photorespiration by concentrating CO2 Allows Rubisco to work at max rate No loss of CO2 or 2-PG produced. Specialised leaf anantomy
40
Leaf anatomy of C4 plants
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
The C4 pathway
- 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
Activation of C4 enzymes by light
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
Further adaptation of C4 leaf anatomy
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
CAM (crassulacean acid metabolism) Low water and CO2 IN THE DARK
- stomata open to allow CO2 entry - CO2 entering needs light to be fixed by Rubsico - CO2 fixation catalysed by PEPcase - oxaloacetate -> malate
45
CAM (crassulacean acid metabolism) Low water and CO2 IN THE LIGHT
-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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