4-1 C a r b o h y d r a t e S y n t h e s i s Up to this point in the course, the main focus has been the breakdown of metabolites, including carbohydrates, lipids and amino acids. The primary purpose of these pathways is to extract energy in useable form with the common end product being ATP, the "energy currency" of the cell. In the case of glucose, the break down can be expressed by: (CH 2 O) 6 + 6 O 2 6 CO 2 + 6 H 2 O ∆G' o = -2868 kJ/mol (energy released) Obviously, the material to be degraded must have originated somewhere and the starting point for all organic carbon is the fixation of CO 2 into carbohydrate via photosynthesis. 1 . P h o t o s y n t h e s i s - I n t r o d u c t i o n To reverse the above reaction such that CO 2 is reduced or converted into glucose, energy must be supplied and in photosynthetic cells light energy is used. 6 CO 2 + 6 H 2 O (CH 2 O) 6 + 6 O 2 ∆G' o = +2868 kJ/mol (energy used) In this one process, not only is reduced carbon (ie. carbohydrate) produced but molecular oxygen, required for respiration, is produced. When the two processes of photosynthesis and respiration are combined, the carbon cycle is generated. carbohydrates lipids amino acids organic carbon O 2 respiration photosynthesis CO 2 energy released (ATP) energy used (hν or light energy) Lec # 11 nucleotides
25
Embed
1. Photosynthesis - Introduction · 1. Photosynthesis - Introduction To reverse the above reaction such that CO 2 is reduced or converted into glucose, energy must be supplied and
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
4-1Carbohydrate Synthesis
Up to this point in the course, the main focus has been the breakdown of metabolites, including carbohydrates, lipids and amino acids. The primary purpose of these pathways is to extract energy in useable form with the common end product being ATP, the "energy currency" of the cell.In the case of glucose, the break down can be expressed by:
(CH2O)6 + 6 O2 6 CO2 + 6 H2O ∆G'o = -2868 kJ/mol (energy released) Obviously, the material to be degraded must have originated somewhere and the starting point for all organic carbon is the fixation of CO2 into carbohydrate via photosynthesis.
1. Photosynthesis - Introduction
To reverse the above reaction such that CO2 is reduced or converted into glucose, energy must besupplied and in photosynthetic cells light energy is used.
6 CO2 + 6 H2O (CH2O)6 + 6 O2 ∆G'o = +2868 kJ/mol (energy used) In this one process, not only is reduced carbon (ie. carbohydrate) produced but molecular oxygen, required for respiration, is produced. When the two processes of photosynthesis and respiration are combined, the carbon cycle is generated.
Both procaryotes and eucaryotes are capable of carrying out photosynthesis. While the overall reactions are similar, there are differences apparent and the following section compares three casesto highlight the commonalities.
1. Green plants and algae
6 CO2 + 6 H2O (CH2O)6 + 6 O2 is the usual representation but if 6 H2O are added to both sides we get: 6 CO2 + 12 H2O (CH2O)6 + 6 O2 + 6 H2O 2. Green sulfur bacteria
6 CO2 + 12 H2S (CH2O)6 + 12 S + 6 H2O 3. Purple non-sulfur bacteria
6 CO2 + 12 CH3CH(OH)CH3 (CH2O)6 + 12 CH3COCH3 + 6 H2O (isopropanol) (acetone) Comparison of the three overall reactions produces the generalized overall reaction:
6 CO2 + 12 H2X (CH2O)6 + 12 X + 6 H2O electron electron acceptor donor Reflection on this generalized reaction in relation to the most common reaction from plants and algae leads to the realization that the oxygen atoms in the product waters must have originated in the input CO2 while the molecular oxygen (O2) must have originated from the input electron donor (H2O). In short, there may be two stages to the process as follows:
Stage 1 12 H2X 24 H+ + 24 e- + 12 X
Stage 2 24 H+ + 24 e- + 6 CO2 (CH2O)6 + 6 H2O
Two key experiments addressed and confirmed this idea.
1. The first experiment identified the electron acceptors that became reduced electron carriers (Hill reagents after the scientist) generated when electrons are removed from H2X (clearly electrons don't float around loose in solution), and demonstrated that they were generated independent of CO2.
4-3
Hill found that isolated chlorplasts were capable of generating molecular oxygen (O2) in the absence of CO2 if they were provided with an electron acceptor. A variety of electron acceptors were found to work in vitro and these became known as "Hill reagents". hν H2O + 2 Fe3+ 1/2 O2 + 2 Fe2+ + 2 H+
donor acceptor Eventually, it was determined that the actual in vivo electron acceptor in plants was NADP+.
H2O + NADP+ 1/2 O2 + NADPH + H+
2. In order to prove that molecular oxygen was derived from the H2O and not from CO2, heavy water (H2
18O) was used with the result that only 18O2 was found - no 16O2 (from the C16O2) or (CH2
18O)6. Then if the overall reaction is written normally, it is not balanced correctly.
6 C16O2 + 6 H218O (CH2
16O)6 + 618O2 However, it can be balanced simply by adding 6 H2
18O to the left and 6 H216O to the right side.
6 C16O2 + 12 H218O (CH2
16O)6 + 618O2 + 6 H216O
This makes it very clear that the input and output waters are treated separately and this is easily explained by there being two stages to the process as deduced above. hνStage 1 12 H2
Because stage 1 requires light energy, it is referred to as the light stage or light reactions. On the other hand, stage 2 does not require light energy and is referred to as the dark stage or dark reactions.
Within plants, the whole process occurs within the intracellular organelle, the chloroplast which is believed to have bacterial origins.
stroma or cytoplasm
thylakoid vesicles
4-4All of the reactions to do with the light stage (absorption of light energy and oxidation of water) occur in the membrane of the thylakoid vesicles and all of the reactions to do with the dark stage (CO2 reduction to carbohydrate) occur in the stroma.
2. Light Reactions
The principal light absorbing molecule is the chlorphyll of which there are several different types varying in substituents on the porphyrin ring. There are light harvesting complexes composed of many proteins, chlorophylls and other pigments that absorb light energy and transfer it to one of two complex apparatuses called photosystems I and II (PS I and PS II). The complexity of the photosystems is evident in photosystem I which contains 16 proteins, 168 chlorphylls, as well as carotenoids, Fe-S clusters and phylloquinones.
N
N N
N
Mg
H3C CH2CH3
CH3H3C
C20H39OOCH2CH2COH3COOCphytol
This is the structure of a chlorophyll but you are NOT responsible for it. The point here is to illustrate the conjugated double bond system.
To understand how light energy is absorbed, we must first briefly review what light energy is. Light energy has the properties of both a particle or photon and a wave. The speed of light is expressed by:
c = λν (where λ is the wavelength and ν is the frequency.) The energy of light is expressed by:
E = hν or E = hc/λ (where h is Planck's constant) It is more common to use wavelength than frequency as a measure and the energy of an Einstein (6 x 1023) of photons increases with decreasing wavelength.
The important point here is that the energy of a photonof light increases as you move to shorter wavelength. This is an important consideration for exposure to sunlight.
All electrons in a molecule have the capability to exist in a number of different energy levels or energy states. The most stable is the ground state, but absorption of energy can result in an electron being excited to a higher energy level. Light energy can be used for this photoexcitation.
E3E2
E1
E0
hν hν(red) (purple)
hν must equal the energy required for excitation from one level to another.
The energies of the transitions vary with the type of electronand π electrons in an extended aromatic system are not as tightly bound and can be excited at lower energy levels whereas those more tightly bound (n electrons) require higher energy.
The absorbance spectrum of a chlorophyll exhibits two main peaks of absorbance, one in the red region (650 to 700 nm) and the second in the blue region (400 nm) of the spectrum. The trough between the two peaks where light is not absorbed falls in the green region of the spectrum where light is reflected giving chlorophyll and plants their green color.
The light reactions actually involve two key photoexcitations involving photosystems I and II once. In fact, some of the light energy is transferred in from the light harvesting complexes but the photosystems themselves have a very complex array of chlorophylls and pigments that can absorblight. The light energy is eventually transferred to a key chlorophyll or reaction center in each of the photosystems, P700 in PS I and P680 in PS II (denoted by the wavelength of maximum absorbance). hν P680 P680* (in PSII) E'o = +1.0 v E'o = -0.6 v hν P700 P700* (in PSI) E'o = +0.4 v E'o = -1.0 v
300 400 500 600 700Wavelength (nm)
Abs
orba
nce
4-6
The significance of an electron photoexcited to a higher energy level is that it is less tightly bound tothe molecule and can therefore be donated or given up to another molecule more easily. In other words, the molecule becomes a better reducing agent or is more easily oxidized.
This is evident in the change in reduction potentials from +1.0 v to -0.6 v and +0.4 v to -1.0v. Whencompared to the standard reduction potential of NADP+ of -0.32 v, it means that the photoexcitationof the reaction center can lead to reduction of NADP+.
Eo'
-1.0v
+1.0v
NADH
O2 H2O
NADP+
energy out as ATP
light energy in
Effectively, electrons are pumped uphill or against an energy barrier using light energy.
Two key photoexcitations in PS II and PS I are used to bring about theelectron transfer from H2O to NADP+.
H2OO2
P680
P680*
-1.0v
Eo'
+1.0v
0.0v
P700
P700*
hν
hν
OEC
PSII
PSIATP
NADP+
This diagram is often referred to as the "Z" diagram of the light reactions of photosynthesis and its basic tenet is that it delineates (1) how electrons are photoexcited from water to NADP+, (2) that O2 is evolved and (3) that ATP is generated as a result of electron flow (the mechanism will come shortly).
O2
4-7
H2O1/2O2
P680
P680*
-1.0v
Eo'
+1.0v
0.0v
P700
P700*
hν
hν
OEC
PS II
PS I
NADP+
2H+
pheophytin
plastoquinone
cytochrome bf- quinone complex
plastocyanin
Fe-S protein
ferridoxin
The OEC or oxygen evolving complex is a manganese containing complex that is responsible for pulling electrons out of water and generating molecular oxygen.
The series of electron transfer reactions linking P680* and P700 are similar in concept to the series of oxidation-reduction reactions that make up the Electron Transport Chain in the membranes of mitochondria and bacteria. The similarity extends further to there being protons pumped across the thylakoid membrane to generate a pH and electrical gradient. This gradient or energized state is then used by ATPase to generate ATP.
Getting a little bit ahead of ourselves, 2 NADPH and 3 ATP are required to fix 1 CO2 into organic form and the quantum yield refers to the number of photons required to fix 1 CO2 or the number of photons required to generate 2 NADPH and 3 ATP.
From what we have seen so far, determining the number of photons required to generate 2 NADPH is clear. 8 hν 2 H2O O2 + 2 H+ 4 electrons are involved and each electron must be photoexcited 2 times: 2 x 4 = 8 or 2 NADP+ 2 NADPH it takes 8 photons to generate 2 NADPH.
cytochrome bf
Lec #12
Fe-S protein
ferridoxin
see p 4-9
OEC PS II Cyt bf
4hν
4hν
8 H+
4-8
ATPase
12 H+
3 ATP + 3 H2O3 ADP + 3 Pi
2 NADP+ + 2 H+
2 NADPH
Stroma (outside)
Thylakoid vesicle (inside)
++++++++++++++
- - - - - - - - - - -
(low pH)
(high pH)
The next question is how is ATP generated and how many photons are required to generate 3 ATP? The answer lies in the chemiosmotic theory, first introduced for oxidative phosphorylation in the mitochondria. As the electrons flow through the system, protons are pumped across the membrane INTO the thylakoid vesicle. This creates a region of positively charged electrical character and low pH inside the vesicle and a region of negatively charged electrical character and high pH in the stroma. In other words, an energized state is created which can be dissipated by theflow of protons across the membrane. 4 protons are generated at the OEC and another 8 protons are pumped at the cytochrome bf / quinone complex for a total of 12 H+ per 4 electrons.
As in the mitochondria, there is an ATPase in the thylakoid membrane through which protons can flow and the released energy is coupled to the phosphorylation of ADP to form ATP. The yield is also similar with 4 H+ yielding 1 ATP.
In response to 8 photons: 1. 2 H2O + 2 NADP+ O2 + 2 NADPH + 2 H+ and,
2. 12 H+ are pumped across the membrane and, 3. 3 ADP + 3 Pi 3 ATP + 3 H2O as protons flow back through the ATPase. This gives rise to a quantum yield of 8 photons per 2 NADPH and 3 ATP or 8 photons per 1 CO2fixed into organic form.
PS I
2 H2O O2 + 4 H+
(1 H+ per e-) (2 H+ per e-)
4-9
P700
P700*
hν
PS I
plastoquinone
cytochrome bf quinone complex
plastocyanin
Fe-S protein
ferridoxin
cytochrome bf
The Z-diagram as just presented depicts a process of non-cyclic electron flow during which electrons are pumped from H2O to form NADPH. There also exists a modification to this system that allows a process of cyclic electron flow that seems to have evolved as a means to generate additional ATP.
The system is a hybrid of PS I and the plastoquinone/cytochrome bf complex that bypasses NADPH formation.
Electrons are photoexcited in PS I and as they flow through the plastoquinoneand cytochrome bf complexes, protonsare pumped across the thylakoidmembrane. This generates the pHgradient to be used by the ATPase to generate ATP.
The locations or distribution of ATPase, PS I, PS II and the cytochrome bf - quinone complex differin the thylokoid vesicles. The ATPase and PS I are found mainly in the unstacked regions giving them access to NADP+ and ADP in the stroma. PS II is found mainly in the regions of stacked lamellae while the cytochrom bf - quinone complex is spread evenly throughout the membrane.
ATPase ATPasePS I
PS I
PS II
PS II Cyt bf
Cyt bf
4-103. Dark Reactions
The dark stage or dark ractions are responsible for the fixation of CO2 into organic form and are collectively known as the Calvin Cycle.
3 C5 + 3CO2 [ 3 C6] 6 C3 5 C3 1 C3 (profit) As presented at this lowest common denominator of intermediates, the process requires 6 NADPH and 9 ATP (3 CO2 are fixed each requiring 2 NADPH and 3ATP).
To generate one hexose (glucose), this process would occur twice generating 2 C3 which would be combined into a C6, and the energy requirement would be 12 NADPH and 18 ATP.
The dark reactons can be sub-divided into two stages; 1. the fixation stage and 2. the rearrangement stage. In neither stage is light energy used, and only in the fixation stage are the NADPH and ATP used.
Ribulose bisphosphate carboxylase is a multimer of 8 large and 8 small subunits, L8S8 and is probably the most abundant protein in nature.
C
HC OH
CH2OPO32
O O
2X3- phosphoglycerate
C
HC OH
CH2OPO32
O OPO3
2X1,3-bisphosphoglycerate
2
2 ATP 2 ADP
3-Phosphoglycerate kinase
C
HC OH
CH2OPO32
O OPO3
2X1,3-bisphosphoglycerate
2
3
C
HC OH
CH2OPO32
O H
2Xglyceraldehyde- 3-phosphate
CH2OH
C O
CH2OPO32
1Xdihydroxy acetone phosphate
4
2 NADPH + 2 H+ 2 NADP+
2 PiGlyceraldehyde-3-phosphate dehydrogenase
Triose phosphate isomerase
4-12In summary, for the fixation stage reaction 1 CO2 + 1 C5 2 C3, 3 ATP and 2 NADPH are required. OR 3 CO2 + 3 C5 6 C3 5 C3 1 C3 (profit) B. Rearrangement stage The overall reaction is 5 C3 3 C5 and much of the process should be considered along side the pentose phosphate pathway. Like the pentose phosphate pathway, the easiest way to keep things straight is to have an overall scheme that illustrates duplicated processes and then fit the details into the overall template.
C3 1 C6C3 2 C5 + C4C3 3 C7C3 4 C5 + C5C3 OR
5 C3 3 C5
Steps 1 and 3 utilize almost the same enzymes, and steps 2 and 4 also utilize a second enzyme.
C
HC OH
CH2OPO32
O H
glyceraldehyde- 3-phosphate
CH2OH
C O
CH2OPO32
dihydroxy acetone phosphate
Aldolase
C O
CHHO
HC OH
HC OH
CH2OPO32
fructose-1,6-bisphosphate
CH2OPO3
+
C O
CHHO
HC OH
HC OH
CH2OPO32
fructose-6-phosphate
CH2OH2
H2O Pi
Fructose, 1,6-bisphosphatase
1 C3 + C3 C6
Lec #13
4-132 C6 + C3 C4 + C5
C O
CHHO
HC OH
HC OH
CH2OPO32
fructose-6-phosphate
CH2OH
C
HC OH
CH2OPO32
O H
glyceraldehyde- 3-phosphate
+HC OH
HC OH
CH2OPO3
2
erythrose-4-phosphate
HC O
CH2OH
C O
CHHO
HC OH
CH2OPO32
xylulose-5-phosphate
HC O
HC OH
HC OH
HC OH
CH2OPO32
ribose-5-phosphate
+Transketolase
HC OH
HC OH
CH2OPO3
2
erythrose-4-phosphate
HC O
+
CH2OH
C O
CH2OPO32
dihydroxy acetone phosphate
CHHO
HC OH
HC OH
HC OH
CH2OPO32
sedoheptulose-1,7-bisphosphate
C
CH2OPO3
O
CHHO
HC OH
HC OH
HC OH
CH2OPO32
sedoheptulose-7-phosphate
C
CH2OH
O
2
Aldolase
H2O Pi
Sedoheptulose-1,7-bisphosphatase
3 C4 + C3 C7
C7 + C3 C5 + C54
CHHO
HC OH
HC OH
HC OH
CH2OPO32
sedoheptulose-7-phosphate
C
CH2OH
O
C
HC OH
CH2OPO32
O H
glyceraldehyde- 3-phosphate
+ Transketolase +
CH2OH
C O
CHHO
HC OH
CH2OPO32
xylulose-5-phosphate
TPP
TPP
4-14
CH2OH
C O
HC OH
HC OH
CH2OPO32
ribulose-5-phosphate
HC O
HC OH
HC OH
HC OH
CH2OPO32
1Xribose-5-phosphate
CH2OH
C O
CHHO
HC OH
CH2OPO32
2Xxylulose-5-phosphate
To finish up reactions 2 and 4, it is necessary to convert the 2 xylulose-5-P and 1 ribose-5-P to ribulose-5-P and this is accomplished as follows:
Ribulose phosphate 3-epimerase
Ribose phosphate isomerase
In summary the overall process of fixation and rearrangement is: 6 NADPH + 9ATP fixation 3 C5 + 3 CO2 6 C3 rearrangement 5 C3 1 C3 (profit)
The process just described occurs in all plants and produces a C3 carbohydrate (3-phosphoglycerate) as the immediate product of CO2 fixation.
Some plants have evolved an accessory system that results in a C4 carbohydrate being the immediate product of CO2 fixation. Such plants are referred to as C4-plants and the primary reason for the auxiliary pathway is to allow the plants to grow more efficiently at lower CO2 concentrations. That is, C3 plants express only the Calvin Cycle process while C4 plants expressboth the Calvin Cycle enzymes and the enzymes of the C4 process.
The reason that the C4 process may have evolved is that there is an inherent inefficiency in ribulose bisphosphate carboxylase in the form of a side reaction which leads to the oxidation of ribulose bisphosphate, rather than its carboxylation, and subsequent cleavage of the product to 2-phosphoglycolate and 3-phosphoglycerate. In other words, the enzyme uses up both oxygen and carbohydrate.
5. C4 plants
4-15
H2C OPO3
C O
HC OH
HC OH
CH2OPO32
Ribulose-1,5-bisphosphate
2
C
HC OH
CH2OPO32
O O
3 - phosphoglycerate
+ O2
CH2OPO3
OO
2 - phosphoglycolate(salavageable but expensive)
2
Ribulose bisphosphatecarboxylase/oxygenase
The problem for the enzyme lies in its relative affinities for O2 and CO2 in comparison to the aqueous concentrations of the two compounds.
for O2 KM = 350 M compared to the aqueous [O2] of 250 M This means the enzyme will work as an oxidase at less than 1/2 Vmax. for CO2 KM = 9 M compared to the aqueous [CO2] of 10 M This means the enzyme will work as a carboxylase at about 1/2 Vmax. Or in other words, under normal conditions, the enzyme is a reasonably efficient oxidase as wellas a carboxylase. "Product pull" caused by the presence of NADPH and ATP generated in the light reactions during day time creates a favorable environment for the other reactions of the CalvinCycle and pushes the enzyme to be a carboxylase. However, at night, in the absence of NADPH and ATP, the enzyme works effectively as an oxidase and some estimates have has much as 50%of fixed carbon actually being metabolized as a result.
C4 plants have evolved a system that circumvents this problem by creating an effective "CO2 pump" that increases the intracellular [CO2] available for ribulose bisphosphate carboxylase. The system involves four additional enzymes and an extra cell type. We will look at the enzymes individually and then consider the overall picture.
1
PEP
CO2
C OPO3
CH2
2
PEP carboxylase
CO2
CO2
C O
H2C
CO2
Pi
OAA
G'o=-28.6 kJ/mol
(HCO3-)
4-16
Pyruvate
CO2
C O
CH3Malic enzyme
CO2
CO2
HC OH
H2C
CO2
Malate
NADPH + H+NADP+3
2
CH2
CO2
C
CO2
H OH
Malate
CH2
CO2
C
CO2
O
OAA
Malate dehydrogenase
∆G'o=+1.7 kJ/mol
NADP+NADPH + H+
Pyruvate
CO2
C O
CH3
4
PEP
CO2
C OPO3
CH2
2
Pyruvate orthophosphatedikinase
Pi+
ATP AMP + PPi 2 Pi
IPPase
to ribulose bisphosphate carboxylase
H2O
In this case, it is the inorganic pyrophosphatase that pulls the reaction towards PEP generating an overall ∆G'o for both reactions of ~0.
CO2 (atmosphere)
OAA PEP
malate pyruvate
malate pyruvate
CO2 Calvin Cycle
Mesophyll cell
Bundle sheath cell
The process occurs in two different cell types and results in the CO2 beingpumped into the bundlesheath cell creating anelevated intracellularconcentration of CO2 for use by ribulose bisphosphatecarboxylase.
∆G'o=-29.7 kJ/mol
2 ATP equivalents
4-17This system provides better reaction conditions for ribulose bisphosphate carboxylase in the form ofa higher [CO2] making possible a more efficient fixation of CO2 and a more rapid accumulation of organic carbon with less oxidation reaction.
Generally, C3 plants are found in temperate regions and C4 plants are found in the tropics, but thereare obvious exceptions. Rapidly growing plants such as crab grass and corn are C4 plants and the growth advantage provided by the C4 process is obvious.
The C4 process requires more energy but with the benefit of faster growth.
C3 plants 12 NADPH + 12 H+ 12 NADP+
6 CO2 (CH2O)6 + 6 H20 18 ATP + 18 H2O 18 ADP + 18 Pi C4 plants 12NADPH + 12 H+ 12 NADP+
6 CO2 (CH2O)6 + 6 H2O 30 ATP + 30 H2O 30 ADP + 30 Pi In other words, an additional 2 ATP per CO2 are required for the C4 process to proceed.
6. Carbohydrate from acetate (AcCoA)
From section 1, 2 and 3 involving the degradation of various substrates to form acetyl CoA, one take home lesson should have been that the only thing that can happen to acetate (a 2 carbon molecule) is that it can be fed into the TCA cycle to generate 2 CO2. In other words, there can be no net gain in organic carbon atoms using acetate as a carbon source for growth
acetyl CoA (citrate)
(C2) (OAA)
CO2 CO2
4-18However, there are many obvious examples in nature where acetate can be used as a carbon source from which larger organic molecules are generated. The most obvious lies in plants during seed germination when lipids in the seeds are broken down to acetate (β-oxidation) and used to generate rootlets and stems. Also, bacteria can grow using acetate as the sole carbon source.
In order to do this, it is necessary to bypass the two decarboxylation steps in the TCA cycle and thisis achieved by the glyoxalate shunt. This involves two additional enzymes superimposed on the TCA cycle.
CO2CH
HC
CO2
CH2
CO2
Isocitrate
OH
CO2H2C
HC
CO2
CH2
CO2
glyoxalate
O
succinate
Isocitrate lyase CH2
CO2
C
CO2
HHO
Malate
Malate synthase
acetyl-CoA CoASH
TCA cycle
Ac-CoAcitrate
isocitrate
glyoxalate
sucinate
fumarate
malate
OAA
α-KG
succinyl-CoA
CO2
CO2
1
2
The difference between the TCA cycle and the glyoxalate shunt is as follows: OAA + 2 AcCoA 2 OAA (glyoxalate shunt)
OAA + 2 AcCoA OAA + 4 CO2 (TCA cycle)
bypassed reactions
4-19
As noted earlier, plants utilize the glyoxalate shunt during seed germination which is a very specific growth stage. As a result the glyoxalate enzymes are synthesized for only a short length of time as they are needed and then synthesis is turned off. This is illustrated in the graph.
Another way of looking at the process to reinforce its role is as follows:
AcCoA 2 OAA (net gain of 4 C via glyoxalate) (C2) (2C4)AcCoA + OAA Isocitrate (C2) (C4) (C6) OAA (no net gain of C in TCA cycle) 2 CO2 (1C4)
7. Synthesis and storage of glucose
Once photosynthesis produces the glyceraldehyde-3-P and the glyoxalate shunt produces OAA, the products can be converted into glucose via the gluconeogenesis pathway which is basically the reversal of glycolysis with a couple extra enzymes added. This was covered in detail in section 1 ofthe notes.
Pyr OAA PEP 2-PGA 3-PGA 1,3-bisPGA Ga-3P Frc-1,6-bisP Frc-6-P Glc-6-P
Excess glucose produced in this way can then be stored as glycogen:
glc-6-P glc-1-P glycogen The following section considers the reactions involved in glycogen synthesis and breakdown and then looks at the more interesting (and complicated) regulatory system that controls the process.
Time
Gly
oxal
ate
enzy
mes
seed germination plant
Lec #14
4-20
OO3POH2C
HOHO
OH OH
glucose-6-phosphate
2
OHOH2C
HOHO
OH
glucose-1-phosphate
2OPO3
Phosphoglucose mutase
OHOH2C
HOHO
OH
glucose-1-phosphate
2OPO3
OHOH2C
HOHO
OH
uridine diphosphoglucose (UDPG)
OPO2OPO2O
NH
O
ON
O
OHOH
HH
HHUTP PPi
2 Pi
UDPG pyrophosphorylase
IPPase
OHOH2C
HOHO
OHO
HOH2C
OHO
OH
OHOH2C
OHO
OHO
HOH2C
OHO
OH
OHOH2C
HOHO
OH
n glc
n glc
UDP
Glycogen synthase (GS)
added to end of chain lastremoved from chain first
last in - first out
glycogenn+1
glycogenn+2
H2O
4-21
OHOH2C
OHO
OHO
HOH2C
OHO
OH
OHOH2C
HOHO
OH
n glc
OHOH2C
HOHO
OHO
HOH2C
OHO
OH
n glc
OPO3
OHOH2C
HOHO
OH
Pi
Glycogen phosphorylase (GP)
glycogenn+1
glycogenn+2
glucose-1-phosphate
2
gluconeogenesis
glc-6-P glc-1-P glycolysis
UDPG
glycogenn+2glycogenn+1
Pi
UDPUTP
PPi
GP
GS
In the absence of a system to control these reactions, there is the potential for a "futile cycle" which would use up UTP (equivalent to ATP in energy terms). Therefore, glycogen metabolism is highly regulated with hormones affecting the activity and synthesis of the enzymes.
Overall
2 Pi
4-228. Regulation of glycogen metabolism
While there are several levels of control of glycogen metabolism, the best studied and understood is the one activated by adrenalin (epinephrin or norepinephrin) secreted in the adrenal cortex in response to some external stimulus. The hormone binds to a β-adrenergic cell surface receptor present on certain tissue types. Closely associated with the receptor is the enzyme adenylate cyclase and whatever change in the receptor is induced by adrenalin bindingresults in activation of the cyclase and an increase in cAMP levels.
N
NN
N
NH2
O
OHOH
HHHH
OPO
O-
O
PO
O-
O
P-O
O-
O
N
NN
N
NH2
O
OHO
HHHH
O
P
O-
OATP
cAMP
PPi
2 PiH2O
Adenylate cyclase
IPPase
N
NN
N
NH2
O
OHOH
HHHH
OP-O
O-
O
5'-AMP
H2O
Phosphodiesterase
The phosphodiesterase is active at a low level at all times (except when inhibited by caffeine) and the levels of cAMP are determined by the activity of the cyclase and whether it is turned on or not.
Therefore, levels of cAMP rise when it is turned on and drop when it is turned off as a result of the slow action of the phosphodiesterase.
cAMP is an intracellular messenger with roles in many different processes. In the case of glycogen metabolism regulation, it interacts with the regulatory subunit (R) of protein kinase (PK) causing its dissociation from the catalytic subunit (C) which is activated as a result.
C R
R
C
cAMP
cAMP
inactive active
protein
protein-P
ATP
ADP
Pi
H2O
Proteinphosphatase
Any protein that is phosphorylated has to be dephosphorylated and this is accomplished by protein phosphatase (PP).
4-23Once activated the protein kinase phosphorylates three proteins that affect glycogen metabolism.
1. Glycogen synthase (GS) is inactivated. Protein kinase GS GS-P (active) (inactive) ATP ADP This results in the turning off of glycogen synthesis. 2. Glycogen phosphorylase kinase (GPK) is activated to phosphorylate and activate glycogen phosphorylase (GP). Protein kinase GPK GPK-P (inactive) (active) ATP ADP GPK-P GP GP-P (inactive) (active) ATP ADP This results in the turning on of glycogen degradation. 3. Protein phosphatase inhibitor (PPI) is activated so that it can bind to and inactivate protein phosphatase (PP). Protein kinase PPI PPI-P (inactive) (active) ATP ADP PP PP-PPI-P (active) (inactive) This results in the phosphorylation reactions not being reversed. The net effect of these three reactions is that glycogen synthesis is stopped, glycogen breakdown is turned on and the enzyme that reverses the effect of protein kinase, protein phosphatase, is turned off.
This is summarized in the two diagrams on the following page. The top diagram shows how the presence of adrenalin activates energy release and the bottom diagram shows how in the absenceof adrenalin, the system is shifted to carry out energy storage.
4-24Adrenalin
-adrenergicreceptor
adenylatecyclase
cAMP
PKinactive
PKactive
GSactive
GS-Pinactive
GPKinactive
GPK-P active
GP-Pactive
GPinactive
PPIinactive
PPI-Pactive
glycogenn+1
glycogenn+2
energy storage
energy release
PPactive
PP - PPI-P inactive
X
X
Adrenalin
-adrenergicreceptor
adenylatecyclase
cAMP
5'AMP
PKinactive
PKactive
GSactive
GS-Pinactive
GPKinactive
GPK-P active
GP-Pactive
GPinactive
PPIinactive
PPI-Pactive
glycogenn+1
glycogenn+2
energy storage
energy release
PPactive
PP - PPI-P inactive
X
X
X
X
X
X
X
X X
X
X
X
4-25
9. Summary of carbohydrate synthesis
1. Photosynthesis
a) overall reactions b) light reactions - Z-diagram, NADPH and ATP synthesis c) dark reactions - CO2 fixation d) C4 reactions 2. Growth on acetate - glyoxalate shunt