Lecture 14: Carbon Fixation: The C3 and C4 Pathway Flashcards
C3 Pathway
converts CO2 and RuBP to 3phosphoglycerate
first step of Calvin cycle
cataluzed by Rubisco
How many turns of Calvin cycle to get GAP?
three turns
need 3 CO2s
What regulates Calvin Cycle enzymes
Light
ways light regulates Calvin cycle enzymes
1) Inc Rubisco activity in response to elevated pH and Mg2 (during the day) in stroma
2) Thioredoxin-mediated reduction of disulfide bonds
Photorespiration
wasteful side reaction of Rubisco
uses O2, makes 2-phosphoglycolate instead of CO2 to make GAP.
C4 and CAM pathways help to minimize its effects by inc concs of CO2
Whats the product of carbon fixation?
GAP
its a 3 carbon sugar
we later make it a hexose nrg so chem nrg can be used at night
What 2 pathways use GAP as a metabolic intermediate
glycolysis and gluconeogenesis
What does the Calvin Cycle generate?
triose phosphates (GAP!)
types of hexose sugars we can make using GAP
1) sucrose (transport to other plant tissues)
2) starch (nrg stores in cells)
3) cellulose (cell wall synth)
4) pentose phosphates (5 carbon sugars for metabolic intermediates)
What we get out of the light dependent reactions of photosynth
ADP and NADP+
Calvin Cycle Stage 1
Rubisco combines 3 RuBPs and 3 CO2s to make 6 3-PGA
Calvin Cycle Stage 2: Reduction Stage
reduce 6 3-PGA to make 6 GAP
one GAP used by gluconeogensis to make sugar
How many carbon to make 1 GAP?
3
Whats the source of carbon in the Calvin Cycle
CO2
Calvin cycle Stage 3
5 GAPS used to replinish 3 RuBP
“Dark Reactions”
another name for Calvin Cycle
BUT CALVIN CYCLE IS MOST ACTIVE DURING THE DAY (b/c there is lots of NADPH and ATP in the day b/c photosynth makes them)
So what property of Calvin Cycle reactions gave them the name “Dark Reactions”
these reactions don’t “need” light directly to run
Stage 1: CO2 fixation to RuBP to make TWO 3-PGA
catalyzed by rubisco
4 steps
very favorable
ADOL CLEAVAGE STEP: major contributor to the favorable free nrg change
once CO2 added at a time
Rubisco
very SLOW
(only 3 molecs of CO2 fixed per second
most abundant enzyme (in plants)
WHY is there so much rubisco?
because it is really freaking slow
Stage 2: Reduction of 3phosphoglyceradte to form glyceraldehyde
3-PGA converted to GAP
6ATP and 6NADPH needed in stage 2 for EVERY THREE CO2 converted to ONE GAP (2 molecs of 3PGA, 3 turns of Calvin Cyclel; 2x3=6)
lots of nrg used
(for every 3 CO2 fixed by carboylation of 3 RuBP molecs, 3 3phosphoglyceradte are made in stage 1)
How many ATP and NADPH for each GAP that leaves the cycle?
6ATP
6NADPH
(Stage 2!!! 3 turns of cycle)
(3 more ATP used in stage 3 to regner 3 RuBP molecs)
What happens to glyceraldehyde 3-phosphate? DIfferent fates of GAP
1) converted to DHAP
2) Combined with DHAP to make starch
3) converted to DHAP for export from chloroplasts
4) cystolic DHAP used for glycolysis or sucrose
can leae or stay in chloroplast, depends on concentrations
see slide 10
Stage 3
enzyme rxns convert FIVE C3 molecs (GAP or DHAP) into THREE C5 molecs (RuBP) to replinish CO2 supplies
REQUIRES THREE MORE ATP
enzymes: transketose, transaldose
by carbon shuffle rxns
WHY carbon shuffle reactions?
so we don’t lose and waste any carbon
SEE SLIDE 11!!!
Transketolase enzymes
transfer 2 carbon fragments
Transaldolase enzymes
transfer 3 carbon fragments
Carbon Shuffle Reactions
we don’t want to lose any carbons
we need to create 5 carbon molecules (RuBP)
see slide 12!
Common irreversible reactions in carbon shuffle rxns
when inorganic phosphate is released
when ATP is used
Light Regulation of the Calvin Cycle reactions
At NIGHt: plants rely on glycolysis and mitochondrial aerobic respiration to make ATP
CALVIN CYCLE ONLY ACTIVE IN LIGHT
when we have NADPH and ATP
What would happen if glycolysis, pentose phosphate pathway, and the Calvin Cycle were active at the same time?
starch degredation and carbohydrate biosynthesis happening at the same time would quickly drain ATP and NADPH pools in stroma, wasting energy
Two light regulations of Calvin Cycle: pH and MG2+
at night: Calcin cycle has reduced activity b/c flux is very decreased
no pumping, pH in lumen and stroma are both 7
during the day: differnt pH, pH 5 in stroma, 8 in lumen b/c protons pumped into lumen b/c of light activation.
lower pH inside, higher outside
balanced in charge by transport of Mg from lumen into stroma. Thus Rubisco and FBPase acrivities maximal at pH8 and high Mg conc
phopshorylation of lysine reside when the pKa is right due to this process!
slide 14
Two light regulations of Calvin Cycle: Thioredoxin
Thioredoxin is a redox protein that interconverts disulfide bridges in cystine residues
active in calvin cycle in REDUCED STATE (reduced by photosynthetic ETS)
when no light, spontaneous oxidation occurs, then they’re inactive
slide 15
Wasteful Photorespiration: Rubsico and Oxygenase
if we use oxygen instead of CO2 (b/c rubisco), we get out 1 molec of 3-phosphoglycerate and 1 of 2-phosphoglycerate, instead of two 3-phosphoglycerate
we’re short one carbon b/c we didn’t use CO2
this is wasteful because we have to use nrg to make another 3-phosphoglycerate
More on Photorespiration
it is rubisco oxygenastion reaction and glycolate pathway together
O2 consumed and CO2 released
requires NRG input in the form of ATP
see slide 17
So why is rubisco so dang slow?
there were lower O2 concs in the atmosphere long ago
slow reaction may favor CO2 over oxygen
Evolutions fix for Photorespiration
High temps: higher O2:CO2 ratio
more O2 dissolved
plants at higher temps have to deal with more photorespiraton. but they have very low levels of photorespiration b/c of some fixes that they have
fix for Photorespiration
Hatch-Slack patwhay:
put CO2 on PEP (a 3 carbon molec) to make store on OAA (a 4 carbon molec)
OAA is a
transiet CO2 carrier molec
4 carbon molec
Two ways of dealing with Photorespiration
C4 Pathway in tropical plants (SEPARATE CELL TYPES)
CAM Pathway in succulents (SEPARATE TIMES)
C4 Pathway
Mesophil Cells and Bundle Sheath cells
Stomata gaurd cells let O2 and CO2 in
CO2 used to from OAA
Mesophyll Cells
OAA enters, converts to malate
malate goes to bundle sheath cell
Bundle Sheath Cells
Calvin cycle enzymes are only in these cells
malate comes in and is decarboxylated and the CO2 can be used by Calvin Cycle.
C4 Pathway and separation in Space
eliminates oxygenase reaction in rubisco, thus blocks photorespiration
REVIEW SLIDE 20
Explain why C4 plants exist if they cost more nrg to run than the C3 pathway?
they don’t cost more nrg. they end up costing less nrg b/c they dont have to deal with bad rxns of photorespiration
C4 high temp advantage
can reduce photorespiration process
don’t have wasteful oxygenase rxn happening
DOES take NRG input to temp store CO2
AT high temps, O2:CO2 increases, we have more dissolved oxygen at high temps
CAM pathway
separation in time
NIGHT: stomata open when temps cooler, CO2 captured by mesophyll cells and incorporated into OAA by PEP carboxylase. Malate stored in vacule. Calvin cycle inactive
DAY: stomata close, CO2 released, stored malate goes into Calvin cycle, CO2 fixed into carbohydrates/starch
