24 Calvin cycle & PPP

What are the products of light reactions that will be used in carbon reactions?

Energy of light/photons generate NADPH via e- transfer & ATP via H+ gradient (in the stroma), used in the Calvin cycle to convert CO2 into carbohydrates

What are the 3 phases of Calvin cycle?

1. carboxylation (CO2 to 3PG)

2. Reduction (3PG to GAP)

3. Regeneration of starting material (GAP to Ru5P)

What occurs in phase 1 of Calvin cycle?

Co2 is fixed to 6C ribulose bisphosphate (RuBP) by the enzyme RuBisCO, resulting in the formation of two 3-phosphoglycerate (3PG)

What is Rubisco strucutre?

D4 sym dimer, 8 large subunits, 8 small subunits, RuBP core center

How does Rubisco split RuBP 6C to 2 3C 3PG?

Rubisco active site utilizes carbamylated Lys & Mg+ to attach CO2 to RuBP

What occurs in Phase 2 of Calvin cycle?

Reduce 3PG to CAP in reverse of reactions 6&7 of glycolysis. ATP & NADPH(from light rxn) are used to convert 3PG into glyceraldehyde-3-phosphate (G3P), enabling the formation of glucose.

-acyl-phosphate cleave DRIVES REACTION! (NADP+ created)

What are net gains for each 3 turns of the cycle?

Start w 15 C from 3 RuBP’s, add 3C from 3 CO2 (1C), 3C siphoned off for 1 GAP to form glucose

RESULT: 1 G3P per 3 turns.

How many GAP’s are created? Where do they go?

6 GAP’s created: 1 goes to carbohydrate synthesis, 5 used for regeneration of RuBP

What occurs in Phase 3 of Calvin cycle?

Regeneration of RuBP; 5 G3P (3C) converted back to 3 RuBP (5C) using ATP

-GAP/DHAP readily isomerized by TIM

-catalyzed by aldolase, transketolase, bisphosphates, isomerases, and epimerases

What is the role of Aldolase in phase 3 of Calvin cycle?

Combines one 3C aldose & one 3C ketose into a 6C carbohydrate

What is the role of Transketolase in phase 3 of Calvin cycle?

Transfers 2C from ketose to aldose acceptor (5C)

How is SBP formed?

Aldolase adds 3C DHAP to 4C E4P to make 7C SBP

How are the final 2 5C molecules made?

Transketolase transfers 2C from S7P to GAP to make 2 5C molecules; R5P and Xu5P

Where do R5P and Xu5P go?

R5P & 2 Xu5P converted to 3 Ru5P’s & phosphorylated to regenerate 3 RuBP’s

Which are the regulated/irreversible reactions?

Rubisco & kinase/phosphatase steps

• all other reactions are reversible.

Where do the GAP’s go?

Amylose(stroma) by ATP & sucrose by UTP (cytoplasm)

Key concepts of light reaction

• Light reactions use excitation of chlorophyll to oxidize water to O2, pump H+, and reduce NADP+ to NADPH

• Roles of chloroplast, thylakoid lumen, stroma

• Significance of reduction potentials

• Antenna pigments

Key concepts of dark reaction

• “Dark reactions” start with CO2 fixation by RuBisCO

• “Dark reactions” use ATP and NADPH from light reactions

• 3 CO2s and 3 Ribulose 5 Ps produce 6 GAPs

• 5 GAPs are used to regenerate 3 RuBPs

• 1 GAP used to make starch or sucrose

• Transketolase reactions, epimerase, isomerase

• Rubisco, bisphosphatases, and phosphoribulokinase are regulated

What is the Pentose Phosphate Pathway?

Oxidation of glucose in the cytosol

Why does Pentose Phosphate Pathway occur?

• To store reducing power in cytosolic NADPH for biosynthetic reactions

• To generate sugar phosphates for nucleotide and amino acid biosynthesis

• To metabolize dietary pentoses (from nucleic acids)

Where is Pentose Phosphate Pathway occur?

Everywhere, particularly in anabolic tissues (liver/adipose tissue)

What is first phase of PPP? Oxidative phase

Glucose-6-phosphate oxidized to ribulose-5-phosphate (Ru5P) + CO2 to generate 2 NADPH

What is first phase of non-oxidative phase in PPP?

Ru5P converted to R5P (& other 5C sugars) using transketolase/transaldolase

• bc cell doesn’t need all that Ru5P

What is second phase of non-oxidative phase in PPP?

3 5C sugars (Ru5P) converted to 2 6C (F6P) sugars + 1 3C sugar (GAP), both of which can go through glycolysis

What is third phase of non-oxidative phase in PPP?

Production of G6P from 3C & 6C sugars & then ready to repeat cycle

When is oxidative stage most needed vs not?

needed: when cells need NADPH/nucleotides, oxidative phase provides R5P

not: high levels of NADPH inhibit G6PDH so oxidative phase turned off

How can NADPH be useful in the cell?

NADPH protects cells from oxidative damage by reducing glutathione since G6P-DH levels are insufficient to maintain levels during oxidative stress.

KEY concepts of PPP

• Main source of cytoplasmic NADPH in many non-photosynthetic organisms, particularly eukaryotes

• Generates sugar phosphates of varying carbon number, which are used in multiple biosynthetic pathways

• Including ribulose-5-phosphate, a key building block for DNA/RNA

• Oxidizes glucose in the cytoplasm and stores the e- in NADPH

• It is in many ways the reverse of the Calvin cycle (key difference –Rubisco!)

• It ties in to glycolysis at two points, making it very versatile and able to respond to the needs of the cell

• Another use of NADPH, beyond biosynthetic pathways, is keeping glutathione reduced to prevent oxidative damage