Lecture 24

Question 1

electron entrance

Answer 1

ox of h2o with production of O2

Question 2

cyt b6f –>

Answer 2

proton translocation across thylakoid membrane–> atp production

Question 3

electron exit

Answer 3

2Nadp +2h+ –> 2Nadph

reduction

Question 4

The Light Independent “Dark” Reactions

Answer 4

• • 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

Question 5

Part I:

Answer 5

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)

Question 6

TRANSKETOLASE REACTION

Answer 6

• Transfers 2 carbon units
• TPP is cofactor
• (also used by Pyruvate Decarboxylase, Pyruvate Dehydrogenase)

Question 7

Part II

Answer 7

Regeneration of RuBP (the CO2-acceptor)

Summary: 5 GAP (C3)–> 3 RuBP (C5)
so one C3 is left over!!

Question 8

isomerase

Answer 8

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

Question 9

aldolase

Answer 9

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

Question 10

epimerase

Answer 10

converts the Xu5P into Ru5P

Question 11

kinase

Answer 11

an enzyme that catalyzes the transfer of a phosphate group from ATP to a specified molecule

phosphorylates Ru5P into ribulose-1,5-bisphosphate

Question 12

transketolase

Answer 12

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)

Question 13

what rxns are not reversible

Answer 13

phosphoribulokinase reaction and the bisphosphatase reactions

Question 14

step 6

Answer 14

c3 + c3 ——(aldolase)—–> c6

Question 15

step 8

Answer 15

c3 + c6 ——(transloketolase)—–> c5 + c4

Question 16

step 9

Answer 16

c3 + c4 ——(aldolase)—–> c7

Question 17

step 11

Answer 17

c3 + C7 ——(transketolase)—–> c5 + c5

Question 18

which steps are regulated

Answer 18

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

Question 19

Regulation of the Calvin cycle by light

Answer 19

1. RuBisCO
   - - pH optimum at pH 8
   - - Mg2+ is essential

Note that in chloroplasts, there is no electrochemical gradient (in contrast to mitochondria).

Question 20

Regulation of the Calvin cycle by light

Answer 20

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

Question 21

how many GAP’s are used to make F6P

Answer 21

2 GAP’s are used to make F6P

2 GAPs from 2 rounds of Calvin cycle

Question 22

Mutase

Answer 22

catalyzes shift of a functional group from one position to another.

Question 23

Starch Synthesis (in the STROMA of the chloroplast)

Answer 23

Fructose-6-P –isomerase–> Glucose-6-P –Mutase–> Glucose-1-P –ATP–> ADP-Glucose –Starch Synthase–> Amylose

Question 24

Sucrose Synthesis in the cytoplasm

Answer 24

study this on paper

Question 25

Some Key Concepts for Photosynthesis

Answer 25

• 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.

Question 26

The Pentose-Phosphate-Pathway (PPP) What does it do ?

Answer 26

– oxidation of glucose in the cytosol

Question 27

ppp- Why?

Answer 27

• • 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

Question 28

ppp Where?

Answer 28

• • mainly in liver and adipose tissue

- - in dividing tissue, e.g. tumors

Question 29

Overview: 2 stages

Answer 29

1. oxidative (irreversible)
   Glc-6-P + 2NADP+ –> + H2O Ribulose-5-P + CO2 + 2NADPH + 2H+
2. 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

Question 30

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?

Answer 30

glycolysis/TCA cycle 16 ATP, PPP 15 ATP. Know why!

Question 31

Lactone

Answer 31

cyclic ester

Question 32

Stage 2

Answer 32

The non-redox stage

Question 33

Transketolase (TK)

Answer 33

transfers C2-units

Question 34

Transaldolase (TA):

Answer 34

transfers C3 units

Question 35

TRANSALDOLASE REACTION

Answer 35

• Transfers 3 carbon units

* Catalyzed by lysine side chain and Schiff base formation

Question 36

Pentose Phosphate Pathway (PPP): Non-redox reactions (stage 2) Summary:

Answer 36

• All reactions shown here are reversible. (ΔG ~ 0)

* Some products (substrates) of stage 2 reactions are glycolytic intermediates (F6P and GAP)

Question 37

Regulation of PPP:

Answer 37

• 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.)

Question 38

When Rapidly dividing cells already have lots of NADPH but need to make lots of DNA:

Answer 38

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

Question 39

When cells need both NADPH and nucleotides:

Answer 39

NADP+ supports Oxidative phase

Question 40

Importance of PPP for detoxification

Answer 40

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.

Question 41

Glucose-6-Phosphate Dehydrogenase Deficiency

Answer 41

• 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.