Chapter 20 The Calvin cycle and Pentose Phosphate Pathway
an evolutionary kinship
§ Light reaction (Ch. 19) § Dark reaction (Calvin cycle)
Calvin cycle – take place in the stroma of chloroplast
6 CO2 + 18 ATP + 12 NADPH + 12 H2O C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+
3 stages:
or 10
Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the rate-limiting step
G°= 12.4 kcal/mol
Rubisco located on the stromal surface of the thylakoid membrane;
probably the most abundant protein in the biosphere;
8 large subunits: each contains catalytic site and regulatory site
8 small subunits: enhance the catalytic activity of L subunits
a slow enzyme (kcat), 3 s-1
requires a magnesium ion and a CO2 other than the substrate
CO2
the activity increase markedly on illumination (CAM?)
also catalyzes oxygenase reaction – photorespiration
Rubisco activase faciliate
How to mimic?
active form
substrate
Rubisco activase:
exhibit an ATPase activity
two polypeptide (42 and 47 kDa)
Taiz and Zeiger, 2006
Oxygenase activity of Rubisco
2C
still requires that Lys210 be in the carbamate form
no CO2, no oxygenation
The rate of carboxylation is four times that of oxygenation under normal
conditions.
Stromal [CO2]: 10 M, [O2]: 250 M
A salvage pathway for phosphoglycolate
6 CO2 + 18 ATP + 12 NADPH + 12 H2O
C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+
53C
6(3CO2)
35C
+ 3 H2O
10
Rubisco activity analysis?
Synthesis of sucrose and startch in cytoplasm and chloroplasts, separately.
Phosphate translocator
Storage forms in plants: sucrose, starch, fructan
ADP-G starch
(diel)
§ 20.2 The activity of the Calvin cycle depends on
environmental conditions
alkaline pH and [Mg2+]
rhythm phenomenon
Thioredoxin: plays a key role in the regulating the Calvin cycle a 12 kDa protein that have a catalytically active disulfide group containing two redox-active cysteine residues (-Trp-Cys-Gly-Pro-Cys).
C4 pathway of tropical plants sugar cane
PEP carboxylase
Malate dehydrogenaseMalic enzyme
concentrating CO2 , little phosphorespiration
2 ATP
Pyruvate-Pi dikinase
6 CO2 + 30 ATP + 12 NADPH + 24 H2O C6H12O6 + 30 ADP + 30 Pi + 12 NADP+ + 18 H+
Kranz (wreath) cells:
Crassulacean acid metabolism (CAM)Cacti, pineapple, vanilla, agave and mature
Phalaenopsis. Spatial and temporal
Water use efficiency
Nocturanl acidification
§ 20.3 Pentose Phosphate Pathway in plants and
animal generate NADPH, CO2, and 5C sugar.
5C sugar is components of RNA, DNA, ATP, NADH, FAD, CoA.
take place in cytoplasm
+ CO2
G6P dehydrogenase
[gluconeogenesis]
35C26C + 13C (p. 579)
transaldolase
isomerase epimerase
Oxidative phase of PPP
Phase 1
Glucose 6-P + 2 NADP+ + H2O
ribose 5-P + 2 NADPH + 2 H+ + CO2
Intramolecular ester
C-1 carboxyl group
C-5 hydroxyl group
C-1
Nonoxidative phase of PPP
transketolase and transaldolase (Ex.1)
linked between PPP and glycolysis
(2C)
-Ketoglutarate dehydrogenase pyruvate dehydrogenase
TPP
§ 20.4 The fate of glucose 6-phosphate – the need for NADPH, ribose 5-P, and ATP
5C NADPH
e.g., rapidly dividing cells
Net: 5 G6P + ATP 6 R5P + ADP + H+
5C NADPH, G6P CO2
Net: G6P + 12 NADP+ + 7 H2O 6CO2+ 12 NADPH + 12H+ + Pi
The cytoplasm of a live cell from a well-fed rat:
NADP+/NADPH 0.014≒
NAD+/NADH= 700
Both NADPH and ATP are required
Net: 3 G6P + 6 NADP+ + 5 NAD+ + 5 Pi + 8 ADP
5 pyruvate + 3 CO2+ 6 NADPH + 8H+ + 5 NADH + 2 H2O + 8 ATP
ATP
Calvin cycle:
CO2 fixation
to use NADPH
C6 + C3 C5 in regeneration phase
PPP:
CO2 production
to produce NADPH
C5 C6 + C3
Reactive oxygen species removed
superoxide dismutase (p. 518) catalase peroxidase (glutathione, ascorbate) G6P dehydrogenase reduced glutathione (GSH)GSSG reduced ascorbate G6P dehydrogenase level [NADPH] sensitive to oxidative stress
especially important in red blood cells
Phytochelatin
(-Glu-Cys)n-Gly
n= 2~7
The functions of reduced glutathione (GSH)
Serves as a sulfhydryl buffer
maintains the cysteine residues of hemoglobin and
other red-blood-cell proteins in the reduced state.
in normal red blood cells: [GSH]/[GSSG] 500
To maintain the normal structural of red blood cells
To keep hemoglobin in the ferrous state
To detoxify hydrogen peroxide and organic peroxides
glutathione peroxidase: 2 GSH + ROOH GSSG + H2O +
ROH
§ How to regenerate GSH
mediate glutathione reductase – a flavoprotein
NADPH FAD-Enz (FADH2-Enz)
Enz-cys-cys-Enz (Enz-cys-SH)
GSSG (GSH)
Glucose 6-phosphate dehydrogenase– plays a key role in protection against ROS
Glucose 6-phosphate dehydrogenase deficiency– is inherited on the X chromosome
– pamaquine, a purine glycoside of fava beans, leads to the generation of peroxides, then induced hemolytic anemia
– urine turned black, jaundice developed, and the hemoglobin content of the blood dropped sharply
– cross-linked hemoglobins to form Heinz bodies on cell membrane caused membrane damage and cell lysis
Glucose 6-phosphate dehydrogenase deficiency– not all bad
protect against falciparum malaria
the parasites required GSH and the products of PPP for optimal growth
11% among Americans of African heritage
the interplay of heredity and environment in the production of disease
atypical reactions to drugs may have a genetic basis
瘧疾分為四種,其中,最嚴重的是惡性瘧 ( falciparum malaria ),這種瘧疾會對生命造成威脅。 其他三種瘧疾 -- 隔日瘧、三日瘧卵圓形瘧疾、(vivax, malariae 及 ovale) 則較不嚴重,並且沒有立即致命的危險。
Fenton reactionfrom plant physiol biochem (2000) 38: 125-140
The exceedingly active oxygen species can be produced in a Fenton reaction involving Cu+ and H2O2
O2 + AH2 → H2O2 + A
2 Cu2+ + AH2→ 2 Cu+ + A + 2 H+
Cu+ + H2O2 → Cu2+ + OH + OH-