storage mechanisms and control in carbohydrate metabolism Flashcards
(45 cards)
what happens when we consume a meal high in carbohydrates
body converts the excess glucose into glycogen for storage
primarily in the liver and muscles
glycogen serves as a highly efficient energy reserve, capable of being mobilized rapidly when the body needs glucose
it is a branched polymer of glucose
glycogen
what is the average chain length of the branches in glucose
13 glucose residues
it is important for its ability to
- store [ long-lasting source of glucose for activities ranging from short bursts of exercise to extended fasting]
- deliver energy quickly [enables rapid glycogenolysis (breakdown of glycogen into glucose)]
= maintaining homeostasis
how does starch and glycogen store glucose for energy in plants and animals
Starch (Used by plants):
Amylose: A straight chain of glucose molecules (simple and unbranched).
Amylopectin: A chain with fewer branches compared to glycogen.
Plants store glucose as starch in seeds and roots to use as energy later.
Glycogen (Used by animals):
Highly branched structure: This allows glycogen to pack tightly and be broken down quickly when energy is needed.
Animals store glycogen in their liver (to regulate blood sugar) and muscles (for quick energy during movement).
The Difference:
Branching: Glycogen is much more branched than starch (amylopectin). This is what makes glycogen faster to access when the body needs energy.
Who uses it?
Starch = plants’ energy storage.
Glycogen = animals’ energy storage.
In simple terms: glycogen is like a “fast-access energy bank” for animals, while starch is a slower but steady energy source for plants.
briefly explain the glycogen breakdown
- in the liver
- in the muscle
LIVER
* The release of glycogen stored in the liver is triggered by low levels of glucose in blood.
* Liver glycogen breaks down to glucose-6 phosphate, which is hydrolyzed to give glucose which then released into the bloodstream
MUSCLE
* When it needs energy, it converts from glycogen to glucose-6-phosphate
* It uses for muscle movement instead of converting to glucose
what are the 2 steps in Glycogenolysis
[1] Phosphorolysis: Formation of Glucose-1-P
- from glycogen to G1P
- enzyme: glycogen phosphorylase (this breaks the a(1-4) linkages) + debranching enzyme —> degrade (1-6) linkages
- phosphorolysis (glucose + P = G1P)
- NO ATP IS HYDROLYZED
- hence, glycogen produce net of 3 ATP instead of 2 bcs the conversion of glucose to G6P is skipped as that takes 1 ATP
= save, more efficient energy production
[2] Isomerization: Formation of Glucose-6-P
- from G1P to G6P
- enzyme: PGM, phosphoglucomutase
- isomerization (transfer P from 1 to 6. why? so that it can enter the glycolysis)
function of debranching enzyme
- branch transfer
- take a small branch of 3 glucose molecules and moves to the end of another main branch - bond breaking
- breaks a(1-6) bond at the branch point
what are the 2 types of glycosidic bond
a(1-4) bonds:
form long linear chains of glucose units
a(1-6) bonds:
create branch points where side chains connect to the main chains
what is the net gain of ATP molecules when when glycogen rather than glucose is the starting
material for glycolysis
3 rather than 2 because
because the hexokinase step, which consumes ATP is removed
briefly explain the glycogen production
Energy is Needed: Making glycogen requires energy, which comes from a molecule called UTP (similar to ATP).
Stage 1 – Preparing the Building Block:
G1P reacts with UTP to form another molecule called UDP-glucose with a by-product of
pyrophosphate (PPi)
enzyme: UDP-glucose pyrophosphorylase.
step 2:
UDP-glucose (UDPG) added to the growing glycogen chain
enzyme: glycogen synthase
it connects the glucose from UDPG to the chain.
creates a new bond called an α(1 → 4) glycosidic bond, which links glucose molecules in a straight chain.
*Glycogen synthase can only add glucose to an existing chain. It can’t start a new chain from scratch.
another enzyme: branching enzyme
takes a small segment (7 glucose units) from the end of the growing glycogen chain.
moves this segment to a different part of the chain and creates a branch point by forming a new bond called an α(1 → 6) glycosidic linkage.
In Short:
Glycogen synthase adds glucose units one by one in a straight line using α(1 → 4) bonds.
The branching enzyme creates branches by forming α(1 → 6) bonds, making the glycogen structure more efficient for energy storage and release.
what is the energy release in glycogen production
G1P reacts with UTP = UDPG + PPi
- energy change (ΔG°’) is close to zero because this reaction can easily go forward or backward.
PPi breaks down into 2 phosphate molecules (2Pi) with the help of water
- releases energy (ΔG°’ = -30.5 kJ/mol or -7.3 kcal/mol). it ensures the reaction moves forward by removing PPi
Overall Charge:
Glucose-1-phosphate + UTP → UDPG + 2Pi
ΔG°’ = -30.5 kJ/mol (-7.3 kcal/mol).
The energy released makes the reaction irreversible, which helps the body efficiently prepare UDP-glucose for glycogen synthesis without reversing the process.
what are the forms of glycogen phosphorylase
Phosphorylase a (active): This form is phosphorylated (a phosphate group is added), making it active and able to break down glycogen.
Phosphorylase b (inactive): This form is dephosphorylated (no phosphate group), so it’s less active.
Phosphorylation (Activation):
Dephosphorylation (Deactivation):
Phosphorylation (Activation):
An enzyme called phosphorylase kinase adds a phosphate group to glycogen phosphorylase (converts it to the active “a” form).
This process uses energy from ATP.
Dephosphorylation (Deactivation):
Another enzyme, phosphoprotein phosphatase, removes the phosphate group (converts it back to the inactive “b” form).
R and T States
R form (Relaxed and Active):
When glycogen phosphorylase is active, it’s in the R form, ready to break down glycogen.
Molecules like AMP (a signal of low energy) promote this state.
T form (Tense and Inactive):
When inactive, it’s in the T form. Molecules like ATP or glucose-6-phosphate (G6P) (indicators of enough energy) stabilize this state.
what controls the Glycogen phosphorylase
Allosteric control: Molecules like AMP (activates) or ATP/G6P (inhibits) regulate its activity.
Covalent modification: Phosphorylation (activation) and dephosphorylation (deactivation) control its form.
This regulation ensures glycogen is only broken down when the body needs energy.
_______________________________________________________
Covalent Modification:
Enzymes involved in glycogen breakdown or synthesis can be activated or deactivated by adding or removing phosphate groups. This is influenced by hormones like glucagon or epinephrine.
Allosteric Control:
Molecules like AMP, ATP, glucose, and glucose-6-phosphate act as signals to tell the enzymes when to start or stop glycogen metabolism. For example:
AMP signals low energy and promotes glycogen breakdown.
ATP or glucose-6-phosphate signals enough energy and slows glycogen breakdown.
what is the allosteric control of glycogen phosphorylase in the liver and muscles
- In the Liver
Key Player: Glucose
Glucose acts as an inhibitor for the active form of glycogen phosphorylase (phosphorylase a).
When glucose binds to the enzyme, it makes the enzyme shift to the inactive T state.
This helps stop glycogen breakdown because the liver senses there’s already enough glucose in the blood.
This explanation focuses on allosteric control (how molecules regulate enzyme activity) of glycogen phosphorylase in the liver and muscles.
- In the Liver
Key Player: Glucose
Glucose acts as an inhibitor for the active form of glycogen phosphorylase (phosphorylase a).
When glucose binds to the enzyme, it makes the enzyme shift to the inactive T state.
This helps stop glycogen breakdown because the liver senses there’s already enough glucose in the blood. - In the Muscles
Key Players: ATP, AMP, and G6P
These molecules regulate glycogen phosphorylase activity based on the muscle’s energy needs:
Low ATP, High AMP (low energy):
High AMP signals low energy in the muscle.
AMP activates glycogen phosphorylase b by shifting it to the active R form to break down glycogen and provide energy.
High ATP, Low AMP (high energy):
High ATP means the muscle has enough energy.
ATP stabilizes the inactive T form, reducing glycogen breakdown.
High Glucose-6-phosphate (G6P):
High G6P indicates plenty of stored energy.
G6P also stabilizes the inactive T form, further preventing glycogen breakdown.
Opposite Behavior to Glycogen Phosphorylase
Glycogen phosphorylase (breaks down glycogen): Active when phosphorylated.
Glycogen synthase (builds glycogen): Active when unphosphorylated.
- When phosphorylated, glycogen synthase becomes inactive, stopping glycogen production.
How Phosphorylation Happens
Hormonal Signals (Glucagon or Epinephrine):
- These hormones signal the body that more energy is needed (e.g., during fasting or stress).
- The signal triggers an enzyme called cAMP-dependent protein kinase, which adds phosphate groups to glycogen synthase.
- This phosphorylation inactivates glycogen synthase, halting glycogen synthesis.
At the same time:
- The hormonal signal activates glycogen phosphorylase (to break down glycogen).
- It inactivates glycogen synthase (to stop making glycogen).
This ensures the body doesn’t simultaneously break down and build glycogen, optimizing energy use.
In Simple Terms
Glycogen synthase builds glycogen but is turned off by phosphorylation when hormones like glucagon or epinephrine signal the need for energy.
This allows the body to prioritize breaking down glycogen for energy instead of storing it.
What is Glycogen?
store glucose for later use
Why is Glycogen Important?
When your body needs energy (e.g., during exercise or fasting), glycogen is broken down to release glucose.
When you don’t need energy (e.g., after eating), glucose is stored as glycogen for future use.
Process by which pyruvate is converted to glucose
Gluconeogenesis
what are the 3 irreversible steps of gluconeogenesis
- Production of pyruvate and ATP from phosphoenolpyruvate
- Production of fructose-1,6-bisphosphate from fructose-6-phosphate
- Production of glucose-6 phosphate from glucose
Conversion of Pyruvate to Phosphoenolpyruvate
[Step 1] Pyruvate is carboxylated to oxaloacetate
* Requires energy, which is available from the hydrolysis of ATP
* Catalyzed by pyruvate carboxylase, which is activated by acetyl-CoA
* Reaction requires biotin, which is a CO2 carrier, and Mg2+
- biotin is a carrier f carbon dioxide, it has a specific site for covalent attachment of CO2
[Step 2] Conversion of oxaloacetate to phosphoenolpyruvate
* Catalyzed by phosphoenolpyruvate carboxykinase (PEPCK)
* Involves the hydrolysis of GTP
what are the Fates of the Oxaloacetate Formed in the Mitochondria
- Pathway 1: Stay and Turn into PEP
Oxaloacetate can be converted directly into phosphoenolpyruvate (PEP) by an enzyme.
PEP is then transported out of the mitochondria to continue building glucose in the cytosol (outside the mitochondria). - Pathway 2: Turn into Malate and Travel
If oxaloacetate can’t leave the mitochondria directly, it gets converted into malate using an enzyme called mitochondrial malate dehydrogenase. This step also uses NADH (a helper molecule that carries energy).
Malate acts as a “sneaky traveler” and exits the mitochondria.
Once malate is outside in the cytosol, it gets converted back into oxaloacetate by cytosolic malate dehydrogenase. This step regenerates NADH, which is needed for other steps in gluconeogenesis.
Why These Options?
The direct PEP route is faster but might not always be possible.
The malate route helps balance NADH levels outside the mitochondria, which is necessary for gluconeogenesis to continue smoothly.
