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Gluconeogenesis
Synthesis of glucose from non-carbohydrate precursors e.g. lactate and alanine via pyruvate conversion
Reaction doesn’t occur in all cells → liver and kidney
Maintains glucose levels in the blood → provides energy to brain and muscle
How do glycolysis and gluconeogenesis handle reversible and irreversible steps?
Most reactions are reversible.
use the same enzymes for these steps.
3 Irreversible Steps:
Gluconeogenesis uses different enzymes to bypass these steps e.g. Phosphatase in the last step counteracts the kinase used in the first step of glycolysis.
How does gluconeogenesis convert pyruvate back into phosphoenolpyruvate (PEP)?
Two-Step Process: Requires two enzymes for a more complex conversion than the reverse reaction in glycolysis.
Enzymes:
Pyruvate Carboxylase: Converts pyruvate to oxaloacetate.
PEP Carboxykinase: Converts oxaloacetate to PEP.
Non-Carbohydrate Pre-Cursors Used In Gluconeogenesis
Lactate ⇄ pyruvate
Some amino acids (e.g. alanine) ⇄ pyruvate
Removal of NH3 group
Some amino acids (e.g. aspartate) ⇄ oxaloacetate
Glycerol (fats) ⇄ dihydroxyacetone phosphate
Overall Equation for Gluconeogenesis
2 Pyruvate + 2 NADH + 4 ATP + 2 GTP + 6 H2O + 2 H+ → Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi
Net energy investment/gain of gluconeogenesis
Formation of glucose from pyruvate
Net Cost: 6 ATP/GTP per glucose molecule.
Pyruvate Decarboxylase (PC)
An important enzyme that catalyses the metabolically irreversible reaction, that occurs in the mitochondrial matrix
addition of CO2 to pyruvate to form oxaloacetate
Pyruvate + CO2 + ATP → oxaloacetate + ADP + P
Requires prosthetic group biotin
Regulation of PC
Allosterically activated by acetyl CoA
An accumulate of acetyl-CoA signals an abundance of energy and so directs pyruvate to oxaloacetate for gluconeogenesis
Liver cells receive a signal
Structure of Pyruvate Decarboxylase
Tetramer of 4 subunits
2 x biotin carboxylase domains
Site where pyruvate is carboxylate using CO2 to then form oxaloacetate
Binding of a molecule to its domains induces a conformational change, activating the enzyme
Biotin
Co-factor
Described as a prosthetic group when covalently attached to PC via Lys side chain
Carrier of Activated CO2 - captures CO2 from the atmosphere to form oxaloacetate
Carboxylbiotin
How Is Glucose Stored in Plants and Animals
Starch
Glycogen - accumulates in the liver
Glycogen
Polymers of glucose α(1,4), with α(1,6) branching linkage
Stored as cytosolic granules in the liver and muscles
Provides a low, but quick source of glucose for a sudden demand of energy
Can provide energy in the absence of oxygen - anaerboic activity
1-4 Linkage
Ensure a linear structre/ polymer
1-6 Linkage
Introduces branching into the glucose polymer
Glycogenolysis
Degradation of glycogen to glucose-1,6-phosphate
Polymer is depolymerised to release glucose in the blood
Polymer then converted to Glucose-6-Phosphate by phosphoglucomutase
Uses of Glucose-6-Phosphate
Glycolysis → ATP energy
Phosphate removed and then released into the liver/ blood stream
How Does The Liver Raise Blood Glucose Levles
Glycogen → G1P → G6P → Glucose (glucose 6-phosphatase)
glycogen slowly degraded to maintain levels between sleeping and meals
Structure of Glycogen
Branched polysaccharide chains
increase the amount of available ends – allow rapid hydrolysis from these ends to allow the faster release of glucose units for energy
Has 1-4 and 1-6 glycosidic linkages
Glycogenin
A protein at the centre of glycogen
Acts as a primer for glycogen synthesis
Glycogen Phosphorylase
Enzyme that catalyses the sequential removal of glucose residues at the then of of glycogen branches chains
Glycogen (n residues) + Pi ⇄ glucose-1-phosphate + Glucogen (n – 1 residues) from glycogen chain
removes 1 molecule of glucose from the end of the chain
Phosphoglucosemutase
Enzyme that easily converts G1P to G6P
Mutase enzyme that changes the position of phosphate
Phosphorylated serine present in the enzyme to which glucose binds and the phosphate group is then transferred
This glucose is then phosphorylated to form a bisphosphate and can then change to form G-6-P
Significance of glycogen phosphorylase stopping at 4 residues from a branch point
Enzyme stops at 4 residues from a branch point, leading to the truncation of glycogen chains, resulting in what is known as limit dextrin.
What 2 Enzymes Remodel The Resulting ‘Limit Dextrin’ In Glycogen Degredation
Transferase
a-1,6-glucosidase
Transferase
Used in limit dextrin remodelling
Shifts 3 glucose from one outer branch to anohter
Further linearises the glycogen chain
α-1,6-Glucosidase
Used in limit dextrin remodelling
Glycogen debranching enzyme that removes the branched glucose and leaves an elongated unbranched chain
Can be further degraded by glycogen phosphorylase
Steps Involved In Glycogen Sythesis
Glucose Conversion
requires energy
Isomerisation
Glucose Conversion
Glucose + ATP → Glucose-6-phosphate (G6P) + ADP
Enzyme: Glucokinase (in the liver) or Hexokinase (in other tissues)
1st step in glycogen synthesis
Isomerisation
G6P → Glucose-1-phosphate (G1P)
Enzyme: Phosphoglucomutase (reversible)
Step in glycogen synthesis
How is Glucose-1-phosphate activated and added to the glycogen chain?
Activation:
Glucose-1-phosphate + UTP → UDP-glucose + PPi
UDP-glucose is the activated form of glucose
Incorporation: using glycogen synthase which adds UDP-glucose to the growing glycogen chain, releasing UDP.
UDP-Glucose Formation
The activated form of glucose for glycogen synthesis
Catalysed by UDP-glucose pyro phosphorylase
Liberates the outer two phosphate groups from UTP as PPi (pyrophosphate)
UDP Glucose
An actived sugar formed by combining glucose-1-phosphate with UTP.
makes glucose suitable for addition to the growing glycogen chain
UTP
Energetically similar to ATP (UTP + ADP ⇌ UDP + ATP)
How much energy (ATP equivalents) are required to incorporate one glucose molecule into glycogen?
2 ATP Equivalents:
1 ATP: Glucokinase/Hexokinase reaction (glucose phosphorylation).
1 UTP (equivalent to ATP): Formation of UDP-glucose.
Addition of Glucose To Glycogen Chain
Free energy required to continue this continuation required UDP free energy
1-4 glycosidic bonds form to form the linearised version of glycogen
Glycogen Synthesis: Nulceation
The need for a starting point or "core" to initiate a process.
Glycogenin acts as a primer, attaching the first few (10-20) glucose molecules to itself.
This creates a core that glycogen synthase can then extend using 1-4 glucosidic bonds
Linear Growth
Allows the continuation of Glycogen Synthesis
Chain formed from 1-4 glycosidic bond
Glycogen Synthesis: Branch Formation
A 1,4 glycosidic bond is broken and a block of ~7 glucose is transferred to a more interior site forming a 1,6 linkage
glycogen synthase can then enhance this branching
α1,6-glycosidic linkage assists reaction
Increases glycogen solubility, and rates of glycogen synthesis and degradation
Glucokinase
a liver enzyme that catalyses the phosphorylation of glucose to glucose-6- phosphate
Diverts glucose when present at high levels to the synthesis of glycogen in the liver
Inhibition of Hexokinase
Occurs in response to high levels of G6P
Key characteristics of glucokinase.
Location: Primarily found in the liver.
Affinity: Lower affinity for glucose (higher Km) compared to hexokinase.
Function:
Active mainly when blood glucose levels are high.
Promotes glycogen synthesis (glucose storage).
Key characteristics of hexokinase
Location: Found in most tissues, including brain and muscle.
Affinity: High affinity for glucose (lower Km).
Function:
Captures glucose for immediate energy needs.
Inhibited by its product, glucose-6-phosphate (G6P), providing feedback regulation.
Glucose Transport
Glucose enters mammalian cells down a concentration gradient (thermodynamically downhill)
Passive transport, but can’t cross the membrane as glucose is polar – requires transporters
no ATP required – moves down a concentration gradient
GLUT
a family of passive hexose transporters (up to 12 coding genes present in humans) that facilitate glucose transport – no ATP used
GLUT1 and GLU3
Present in nearly all mammalian cells, constantly transport glucose into cell under normal conditions, at essentially constant rate
GLUT2
Present in liver and pancreatic β cells, transports glucose when concentration in blood is high → pancreas detects level → insulin production
Pancreatic B-cells have mechanisms to detect levels of glucose in the blood
When high – insulin production is triggered
GLUT4
Insulin dependent transporter present in muscle and fat cells,
the numbers increase rapidly in the presence of insulin
Excess glucose present after meals is stored as glycogen using insulin – accelerates the interconversion
Structure of GLUT1
Single polypeptide chain, ~500 amino acids
12 membrane-spanning α-helices.
Six of these form a polar channel in the membrane through which glucose travels.
Both N- and C-termini are on the cytoplasmic side of membrane.
How does insulin trigger increased glucose uptake into cells?
Insulin (hormone) is secreted by the pancreas when blood glucose levels are high.
Insulin binds to the insulin receptor (a tyrosine kinase) on target cells.
This activates a signalling pathway inside the cell, increasing the number of GLUT4 receptors in the membrane
How Does Insluin Promote GLUT4 Translocation
Insulin binds to its receptor, activating a signalling cascade within the cell.
The signal triggers vesicles containing GLUT4 to move towards the cell membrane and fuse with it.
This increases the number of GLUT4 transporters on the cell surface.
More GLUT4 enables greater glucose uptake from the blood.
Inside the cell, glucose is phosphorylated and can be used for energy or stored as glycogen in liver and muscles
Insulin
Peptide hormone - Produced by β-cells of the pancreas
Two polypeptide chains (a and B) linked by disulphide bonds.
The α chain has an intramolecular disulphide bond
Synthesized as a single-chain precursor, proinsulin, processed by proteases
Increases the rate of glucose transport into muscle, adipose tissue
Stimulates glycogen synthesis in the liver
Regulation of glycolysis and gluconeogenesis.
Coordinated: When one pathway is active, the other is generally less active.
Control by Substrate Availability:
Glycolysis: Driven by (high) glucose levels.
Gluconeogenesis: Driven by the availability of precursors like lactate (and low glucose levels).
Fructose 2,6- Bisphosphate (F2,6BP)
Synthesis by the phosphorylation of F-6-P by liver PFK2
Degraded by Fructose bisphosphatase back to F6P
Effect of F2,6BP On Glycolysis
A STRONG activator of phosphofructokinase – when F26BP is present in high levels, reaction is activated due to the upregulation of PFK
increased rate of reaction
Effect of F2,6BP On Gluconeogenesis
Inhibits F1-6-BP a key enzyme the the metabolic pathway, decreasing rate of reaction
Structure of PFK2/FBPase-2
1 polypeptide with 2 domains
Kinase domain and phosphate domains→ allows the removal/ addition of phospahte
Reglation of PFK-2/FBPase-2 fUNCTION
Controlled by the covalent modification of a single Ser residue
When phosphorylated - phosphatase activity is prominent; kinase activity is low
When unphosphorylated - kinase activity is dominant
Glucagon
Produced by α-cells of the pancreas in response to low blood glucose
Stimulates glycogen degradation
Maintains blood glucose levels to ensure the brain and muscle function
How does glucagon signalling affect the activity of PFK-2/FBPase-2?
Glucagon binds to its receptor on liver cells.
This activates Protein Kinase A (PKA).
PKA phosphorylates the Ser residue at position 29 in PFK-2/FBPase-2.
This favors FBPase-2 activity (phosphatase), decreasing F2,6BP levels.
Effect of Low Blood Glucose on Glucagon
Glucagon levels rise, and insulin levels fall leading to phosphorylation of PFK-2/FBPase-2.
F2,6BP Decrease This favours the phosphatase activity of the enzyme, decreasing F2,6BP levels.
Glycolysis is inhibited due to the lack of its activator, F2,6BP.
Gluconeogenesis is stimulated, causing the liver to produce glucose
Effect of High Blood Glucose on Glucagon
Glucagon levels fall and insulin levels rise leading to a dephosphorylation of PFK-2/FBPase-2.
F2,6BP Increase: This favours the kinase activity of the enzyme, increasing F2,6BP levels.
Glycolysis is stimulated by high F2,6BP, allowing the body to use excess glucose.
Gluconeogenesis is suppressed.
Epinephrine (Adrenaline)
Produced. by the adrenal glands in response to sudden energy requirements
Stimulates the breakdown of glycogen to glucose and its subsequent release in the blood