Lecture 12 - Carbohydrate Anabolism 3 Flashcards
What effect does increased light have on dark reactions?
- Causes higher pH and Mg in stroma which increases rubisco activity
- Thioredoxin reduces disulfide bonds of some enzymes, activating them
What is important about C4 plants?
They have an accessory system that results in a C4 carbohydrate being the intermediate product of CO2 fixation, so this allows the plant to grow more efficiently at low CO2
What is different about C3 and C4 plants?
- C3 plants have the Calvin cycle
- C4 plants have the Calvin cycle and enzymes of the C4 process
Why do C4 plants have the extra pathway?
A side reaction leads to the oxidation of ribulose bisphosphate and cleavage, rather than carboxylation
When will rubisco act as an oxidase and why?
- Less than 1/2 Vmax
- Km is greater than the aqueous O2 concentration (350 uM compared to 250 uM)
When will rubisco act as a carboxylase and why?
- About 1/2 Vmax
- Km is less than the aqueous CO2 concentration (9 uM compared to 10 uM)
At normal aqueous conditions, what will rubisco work as?
It will work efficiently as both an oxidase or a carboxylase
Describe the day-night shift of rubisco
- In the presence of ATP and NADPH (from light reactions), “product pull” creates favourable conditions for Calvin cycle and carboxylase activity is predominant
- At night, with less ATP and NADPH, oxidase activity predominates
Where do C4 plants have their extra enzymes?
In the mesophyll cell and bundle sheath cell
What is the function of the extra enzymes of C4 plants?
Increase the concentration of CO2 for rubisco, so carboxylase activity increases
______ is more efficient that rubisco because ____
- PEP carboxylase
- Has a higher affinity for CO2 and incorporates it more readily, and doesn’t use O2
Where are C3 plants found?
In temperate regions
Where are C4 plants found?
- In the tropical regions where plants are faster growing
- Exceptions are crab grass and corn
What does the C4 process require?
More energy (30 ATP compared to the C3 process that only requires 18 ATP; an extra 2 ATP per CO2)
What does the glyoxylate cycle provide to plants and bacteria?
A gain in organic carbon from acetate
What does the glyoxylate shunt bypass?
The 2 decarboxylation steps of TCA cycle
What are the 2 glyoxylate shunt enzymes?
Isocitrate lyase and malate synthase
Where does the glyoxylate cycle occur in seeds?
In special compartments called glyoxysomes
What determines how the glyoxylate cycle and TCA cycle work together and how is this done?
- Regulation of isocitrate dehydrogenase activity
- Protein kinase and phosphatase work together to regulate isocitrate dehydrogenase
What does low energy levels mean for the glyoxylate cycle?
Isocitrate will divert to the TCA cycle
When does the glyoxylate cycle take place in plants?
During seed germination
Do animals have the enzymes for the glyoxylate cycle?
No!
What is the main difference between the glyoxylate cycle and the TCA cycle?
- Glyoxylate cycle converts isocitrate to 2 oxaloacetate with the addition of acetyl-CoA (net gain of 4 C)
- TCA cycle converts isocitrate to 1 oxaloacetate with the release of 2 CO2 (no net gain of C)
Which polymer of glucose is stored in plants?
Starch (amylose and amylopectin)
Which polymer of glucose is stored in animals?
Glycogen
Where is glycogen stored?
In the liver and some in muscle
How is glycogen produced?
Glucose 6-P -> glucose 1-P -> glycogen
How is starch produced?
Glucose 1-P -> ADP-glucose + starch (n) -> starch (n+1)
What enzyme converts glucose 6-P to glucose 1-P?
Phosphoglucose mutase
What enzyme converts glucose 1-P to glycogen/starch and what is the cofactor?
- Phosphorylase
- Pi out
What enzyme converts glycogen/starch to glucose and what is the cofactor?
- Alpha-amylase
- H2O in
How is glycogen catabolized?
The last glucose added to the chain is the first one removed (from non-reducing end)
Why does the metabolism of glycogen have to be highly regulated and how is it regulated?
- There will be UTP waste
- Hormones that affect the activity and synthesis of the enzymes
What is the function of insulin?
Promotes synthesis (storage) of glycogen
What is the function of glucagon?
Stimulates breakdown of glycogen (“hungry hormone” from pancreas)
What is the function of adrenaline/epinephrine?
Promotes breakdown of glycogen
Where is adrenalin secreted?
In the adrenal medulla
What is adrenalin secreted in response to?
External stimulus
What does adrenalin bind to?
Beta-adrenergic cell surface receptors present on certain tissue types
Which enzyme is closely associated with beta-adrenergic cell surface receptors?
Adenylate cyclase
What occurs after adrenalin binds to its receptor?
Change in beta-adrenergic receptor -> activation of adenylate cyclase -> increased cAMP
What does cAMP do with respect to glycogen and how?
Regulates glycogen synthesis and breakdown via protein kinase
Activation of ______ increases cAMP
Adenylate cyclase
_____ converts cAMP to 5’-AMP
Phosphodiesterase
When is phosphodiesterase active?
All the time, at a low level
What does cAMP activate and what does this lead to?
- Protein kinase
- Controlled enzyme activity via phosphorylation
How does cAMP activate protein kinase?
cAMP interacts with the regulatory subunit of protein kinase, causing its dissociation from the catalytic subunit, allowing activation
How does protein kinase regulate proteins?
By phosphorylating them
What do protein phosphatases do?
Remove phosphates
What does protein phosphatase inhibitor do?
Blocks protein phosphatase activity
What are 3 ways that protein kinase can regulate glycogen metabolism?
1) Inactivate glycogen synthase, so glycogen synthesis is turned off
2) Activate glycogen phosphorylase, so glycogen degradation is turned on
3) Inactivate protein phosphatase, so phosphorylation of proteins is not reversed
How is glycogen phosphorylase activated?
Via glycogen phosphorylase kinase
How is protein phosphatase inactivated?
Via protein phosphatase inhibitor
What is the net effect of protein kinases?
1) Glycogen synthesis is stopped
2) Glycogen breakdown is turned on
3) Protein phosphatase is turned off, allowing protein kinase to remain active, continuing 1 and 2
What happens in the absence of adrenalin?
Energy storage occurs
