Lecture 24 Flashcards
electron entrance
ox of h2o with production of O2
cyt b6f –>
proton translocation across thylakoid membrane–> atp production
electron exit
2Nadp +2h+ –> 2Nadph
reduction
The Light Independent “Dark” Reactions
- also called the “Calvin Cycle”
- actually happen during daylight … more later
- located in stroma
- Reduction of CO2 to carbohydrates (glyceraldehyde- phosphate, GAP) by using ATP and NADPH produced in the light reactions. “Ru5P” = Ribulose 5 Phosphate
Part I:
Fixation of CO2 and reduction
Ribulose Bisphosphate Carboxylase/Oxygenase: “RuBisCO”
- slow (3 per second)
- can be up to 50% of leaf protein
- most abundant protein on earth (40 million tons on earth!)
- important for regulation: needs Mg2+ for activity and works best at basic pH (optimum at pH 8)
“Reverse of glycolysis” (Note the use of
NADP+/NADPH instead of NAD+/NADH)
TRANSKETOLASE REACTION
- Transfers 2 carbon units
- TPP is cofactor
- (also used by Pyruvate Decarboxylase, Pyruvate Dehydrogenase)
Part II
Regeneration of RuBP (the CO2-acceptor)
Summary: 5 GAP (C3)–> 3 RuBP (C5)
so one C3 is left over!!
isomerase
are a general class of enzymes which convert a molecule from one isomer to another. Isomerases can either facilitate intramolecular rearrangements in which bonds are broken and formed.
Triose phosphate isomerase: converts all G3P molecules into DHAP
aldolase
reverse of typically characterized as a glycolytic enzyme with the ability to split fructose 1,6 bisphosphate into DHAP and G3P.
regulate a reverse reaction in the Calvin cycle
epimerase
converts the Xu5P into Ru5P
kinase
an enzyme that catalyzes the transfer of a phosphate group from ATP to a specified molecule
phosphorylates Ru5P into ribulose-1,5-bisphosphate
transketolase
removes two carbon molecules in fructose 6-phosphate to produce erythrose 4-phosphate (E4P); the two removed carbons are added to G3P to produce xylulose-5-phosphate (Xu5P)
what rxns are not reversible
phosphoribulokinase reaction and the bisphosphatase reactions
step 6
c3 + c3 ——(aldolase)—–> c6
step 8
c3 + c6 ——(transloketolase)—–> c5 + c4
step 9
c3 + c4 ——(aldolase)—–> c7
step 11
c3 + C7 ——(transketolase)—–> c5 + c5
which steps are regulated
phosphoribulokinase, RuBisCO, fructose biphosphatase, sedoheptulose bisphosphatase
Regulation means that the Calvin Cycle is coupled to the light reactions.
All enzymes that are regulated catalyze irreversible reactions
Regulation of the Calvin cycle by light
- RuBisCO
- - pH optimum at pH 8
- - Mg2+ is essential
Note that in chloroplasts, there is no electrochemical gradient (in contrast to mitochondria).
Regulation of the Calvin cycle by light
3 CO2 + 3 C5 –> 6 C3–> 1 C3 —> starch
5 C3
Lots of Energy Consumed to convert CO2 into GAP
3 CO2 + 9 ATP + 6 NADPH –> GAP + 9 ADP + 8 Pi + 6 NADP+
GAP —> Starch
how many GAP’s are used to make F6P
2 GAP’s are used to make F6P
2 GAPs from 2 rounds of Calvin cycle
Mutase
catalyzes shift of a functional group from one position to another.
Starch Synthesis (in the STROMA of the chloroplast)
Fructose-6-P –isomerase–> Glucose-6-P –Mutase–> Glucose-1-P –ATP–> ADP-Glucose –Starch Synthase–> Amylose
Sucrose Synthesis in the cytoplasm
study this on paper
Some Key Concepts for Photosynthesis
- Light Reactions use oxidation of chlorophyll to oxidize water to O2, pump protons and reduce NADP+ to NADPH.
- Compartments of chloroplast, thylakoid lumen, stroma
- Significance of reduction potentials
- Antenna pigments
- “Dark reactions” start with CO2 fixation by RUBISCO
- “Dark Reactions” use ATP and NADPH from light reactions.
- 3 CO2’s and 3 Ribulose 5 P’s produce 6 GAP’s
- 5 GAP’s are used to regenerate Ru5P’s.
- 1 GAP used to make starch or sucrose
- Transketolase reactions, epimerase, isomerase
- RUBISCO and bisphosphatases are regulated.
The Pentose-Phosphate-Pathway (PPP) What does it do ?
– oxidation of glucose in the cytosol
ppp- Why?
- to generate NADPH for biosynthesis, e.g. of fatty acids. NADPH is also needed to reduce glutathione, which is a cellular defense mechanism against oxidative stress.
- to generate pentoses for nucleotide biosynthesis
- to metabolize pentoses
ppp Where?
- mainly in liver and adipose tissue
- - in dividing tissue, e.g. tumors
Overview: 2 stages
- oxidative (irreversible)
Glc-6-P + 2NADP+ –> + H2O Ribulose-5-P + CO2 + 2NADPH + 2H+ - non-oxidative (reversible)
3Ribulose-5-P 2Fructose-6-P + GAP
3 x C5 2 x C6 + C3
together:
3Glc-6-P + 6NADP+ + 3H2O –> 3CO2 + 6NADPH + 6H+ + 2Fru-6-P + GAP
What is energetically more efficient, to use PPP or glycolysis –
TCA cycle to obtain 3 CO2, 2 Fru-6-P and 1 GAP from 3 Glc-6-P?
glycolysis/TCA cycle 16 ATP, PPP 15 ATP. Know why!
Lactone
cyclic ester
Stage 2
The non-redox stage
Transketolase (TK)
transfers C2-units
Transaldolase (TA):
transfers C3 units
TRANSALDOLASE REACTION
- Transfers 3 carbon units
* Catalyzed by lysine side chain and Schiff base formation
Pentose Phosphate Pathway (PPP): Non-redox reactions (stage 2) Summary:
- All reactions shown here are reversible. (ΔG ~ 0)
* Some products (substrates) of stage 2 reactions are glycolytic intermediates (F6P and GAP)
Regulation of PPP:
- Flux of pathway depends on the need of the cells.
- Examples:
- When NADPH is being used NADP+ drives the oxidative phase
- When ribose is not needed carbons are diverted to glycolysis.
Note that pentoses can be made even without running the oxidative phase of PPP.
(Ribose-5-phosphate is a precursor for Nucleotides needed to make DNA and RNA.)
When Rapidly dividing cells already have lots of NADPH but need to make lots of DNA:
High levels of NADPH inhibit G6PDH (oxidative phase is not used)
(Intermediates of glycolysis are used by PPP to make R5P)
g6p –> f6p –> f 1,6 bisP glyc 3-p –> ribose 5 phosphate
When cells need both NADPH and nucleotides:
NADP+ supports Oxidative phase
Importance of PPP for detoxification
The cellular weapon against damaging reactive oxygen species (e.g. peroxides) is glutathione:
NADPH generated in PPP is used to regenerate (reduce) glutathione
People with mutations that decrease Glc-6-P-DH
activity are hypersensitive to oxidative stress.
Glucose-6-Phosphate Dehydrogenase Deficiency
- 400 million people worldwide have G6PDH deficiency
- They are more resistant than normal people to malaria.
- But they also are more sensitive to oxidizing agents because they can’t make enough NADPH to keep Glutathione reduced.
- In erythrocytes reduced glutathione helps keep Fe in the Fe2+ oxidation state.
- Fe3+ hemoglobin does not bind O2 and it causes erythrocytes to change shape.
- Erythrocytes get broken down - causing anemia.
