Carbohydrates Flashcards
What is the definition of metabolism?
metabolism is the process through which living systems acquire and utilize the free energy they need to carry out their various functions
i.e. How food is transformed to provide energy
It is an open system as the energy is lost in the form of work, heat and wastes and new food/energy is constantly needed as input
Metabolism = Catabolism + Anabolism
What are the inputs and outputs of the metabolism
Inputs:
- Chemical energy (food) → Carbs, Fats, etc.
Outputs:
- Chemical wastes → CO2, H2O
- Heat
- Work
- Organic wastes (Urine, feces)
How much energy is stored in glucose when completely digested?
∆G˚ = -2850 kJ/mol
Glucose + 6 O2 → 6 CO2 + 6 H2O
How much energy is stored in fats when completely digested?
∆G˚ = -9781 kJ/mol
Fat + 23 O2 → 16 CO2 + 16 H2O
What is the ∆G˚ of the phosphorylation of ATP into ADP?
ATP + H2O → ADP + Pi
∆G˚ = -30.5 kJ/mol
What is catabolism?
Catabolism = degradation
Breakdown of nutrients and cell constituents to use their components or generate free energy
Complex → Simple
What is anabolism?
Anabolism = biosynthesis
Biomolecules are synthesized from simpler compounds
Simple → Complex
What is the Standard Metabolic Rate?
It is the metabolic rate of an organism not digesting food, at themoneutrality, under resting and stress free conditions
→ Basic energy consumption, at rest
→ Comprises transcriptional activity, translation of protein, cellular respiration, other usage specific to cells
In the energy balance, what are the 3 sources of energy expenditure?
- Standard metabolic rate
- Activities
- Exercise
What are examples of negative and positive energy balance?
Negative energy balance → anorexia, cachexia, death
Positive energy balance → weight gain, obesity, type 2 diabetes, death
What is the definition of metabolic pathways?
They are series of consecutive enzymatic reactions that produce specific products
What are examples of metabolites?
reactants, intermediates and products of metabolic pathways
What are the 5 principals of metabolic pathways?
- Are irreversible
- Have a first committed step (irreversible step that commits the intermediates)
- Are regulated
- Catabolic and Anabolic pathways must differ
- Occur in specific locations in eukaryotic cells (cell subcompartments)
What confers directionality to a metabolic pathway?
A highly exergonic step which is irreversible
*Regulates the directionality
What are mechanisms of regulation of metabolic pathways?
- Feedback inhibition
- Hormones
- Enzymes activate and inactivate
- Allosteric regulation
- Covalent modifications
- Substrate cycles
- Genetic control (translation, transcription)
Why is cellular location important for metabolic pathways?
- Allows for the specific enzymes to be in the specific locations
- Allows for better proximity of metabolites
- Prevents futile cycle
Are metabolic pathways at equilibrium?
What big law regulates metabolism?
NO
Overall, living organisms maintain an non-equilibrium state
- A process at equilibrium cannot be directed
- Living organisms are open system that require a cst energy input
→ Metabolism is regulate by law of supply and demand
What is ∆G˚?
It indicates the nature of the reaction, if all substrates are at 1mol concentration
Exergonic (∆G˚ < 0, produce energy)
Endergonic (∆G˚ > 0, requires input of free energy)
What is ∆G?
∆G is a true indication of the direction of a reaction → irreversibility vs reversibility of the reaction in vivo
∆G = 0 → equilibrium → reversible in vivo
∆G < 0 → releases energy → irreversible in vivo
What is the energy currency if the cell?
ATP
What is the Gibbs Free Energy formula?
∆G = ∆G˚ + RT ln ([C][D]/[A][B])
∆G = ∆G˚ + RT ln Keq
What molecules is sucrose formed with?
Sucrose = Glucose + Fructose (joined by alpha bond)
*Digested by sucrase
What molecules is lactose formed with?
Lactose = Galactose + Glucose (joined by beta bond)
*digested by lactase
What is the structure of cellulose?
How is it different than Startch and Glycogen?
polymer of glucose with alternating alpha and beta bonds
Difference bc its 1 line/linear structure and has alternating alpha and beta bonds, whereas others only have alpha bonds
What chanels are responsible transport of the digested glucose from the lumen of the small intestine to the blood stream?
Done through epithelial cells forming the brush border lining the microvilli
- Glucose entry into epithelial cells done by Na+/Glu symport → 2ndary active transport because relies on active transport of Na+ out of the cell by Na+/K+ ATPase
- Glucose uniport follows concentration gradient to passively transport glucose from the cell to the blood stream
- Na+/K+ ATPase primary active transport pumps Na+ out of the cell to create low intracellular Na concentration on capillaries side
*Active transport takes glucose against its concentration gradient and passive transport brings in down its gradient
How do pancreatic b-cells operate are rest? (when blood glucose <= 5.5 mM)
Basal glucose level enter the b-cells by Glut2 passive transporter → Basal glycolysis → low levels of pyruvate → low oxphos activity in mitchondrias → low ATP production → no effect
K+ channel is open at rest and allows K+ to exit the cell keeping the membrane hyperpolarized
What is the minimal concentration of glucose that triggers release of insulin into the blood by b-cells of the pancreas?
[glucose] > 5.5mM
*It is also considered to be the basal blood glucose level?
How do pancreatic b-cells operate at high blood glucose levels (> 5.5 mM)? (i.e. after a meal)
Higher glucose level entre the b-cells by Glut2 passive transporter → More glycolysis → higher levels of pyruvate → more oxphos activity in mitchondrias → higher ATP production → inhibition of K+ channel → depolarization of the membrane (because K+ stays in the cell) → openning of Ca++ channel → entry of Ca2+ into the cell → insulin release in the blood stream from b-cells
Which 2 transporters allow glucose uptake in the tissues?
*Both passive transporters
Glut2:
- Ubiquitously expressed
- Not insulin responsive
Glut4:
- Increase glucose uptake in presence of insulin by being recruited to the surface of the cells
- only in Adipose tissue (lipogenesis) and Muscles (glycogen synthesis) to store glucose
What is the main role of the liver in metabolism?
Allows Glucose management → keeps [glucose] in the blood = 5.5mM
Fed state → stores glucose as glycogen
Fasting state → breaks down glycogen + gluconeogenesis
- Liver also distributes glucose to other tissues
- Does glycogen synthesis in the presence of insulin (NO Glut4) → Reduces glucose concentration → more glucose uptake by Glut2
Why are there so many different glucose transporters?
- Transport is specific to the needs of the tissue (depending on storage capacity and consumption)
- Sugar specificity
- Tissues often express more than 1 glucose transporter and relative functions controlled by levels of expression
What is SGLUT1? Where is it expressed?
Expressed in intestinal mucosa and kidney tubules
Symport of 1 Glucose or Galactose with 2 Na+ ions
*Does NOT transport fructose
What is GLUT2? Where is it expressed?
Expressed in Liver, pancreatic beta cells, small intestine, kidney
- Transports glucose, galactose, fructose
- Low affinity, high capacity glucose transporter
- “Glucose sensor” in pancreatic beta cells
What is GLUT3? Where is it expressed?
Expressed in the brain, placenta and testes
Transport glucose (high affinity) and galactose (not fructose)
- Primary glucose transporter in neurons
What is GLUT4? Where is it expressed?
Expressed in skeletal and cardiac muscles, and in adipocytes
Insulin-responsive glucose transporter
- High affinity for glucose
What are, in order, the different sources of ATP production when exercising?
- ATP
- Creatine phosphatase
- Anaerobic glycolysis → dominant over 8 sec
- Aerobic glycolysis (CHO) → dominant over 60 seconds
*Steps 3 and 4 = breakdown of glycogen in the muscles to glucose - Aerobic lipolysis → dominant over ~3 hours
*They all overlap, but have different times they are dominant at
How does creatine phosphatase provide ATP in the early stages of exercise?
Creatine phosphatase: P-creatine + ADP → creatine + ATP
Creatine kinase: creatine + ATP → P-creatine + ADP
What general stimuli increases the formation of ATP and the increase in all the processes involved?
The lack of products
*Goes backwards
Lack of ATP → increase in OXPHOS → lack in NADH and FADH → increase in Citric acid cycle → lack of pyruvate → increase in Glycolysis
What are the end products of Oxidative phosphorylation, of the Citric Acid Cycle and of Glycolysis that are relevant to the other processes?
Glycolysis → 2 pyruvate + 2 ATPs + 2 NADH (out of 1 glucose)
*Glycolysis does NOT require O2, but requires a fresh supply of NAD+
Citric acid cycle → NADH, FADH (out of Acetyl-Coa)
Oxidative phosphorylation → ATP (out of NADH FADH2, O2)
Which products regulate Glycolysis and how?
Low [ATP] → Stimulates
Low [NAD], High [ATP] → Inhibits
Which products regulate the Citric Acid Cycle and how?
High [NADH] → inhibits
*Not ATP bc this step produces 0 ATP (glycolysis produces 2)
How does the lack of O2 affect Glycolysis?
Glycolysis does NOT require O2 so it is not affected by the lack of O2, but is requires a fresh supply of NAD+
Without O2:
- Pyruvate → Lactate in the muscles converts NADH to NAD+ homolactic fermentation
- Pyruvate → Ehtanol in Yeast and bacteria converts NADH to NAD by alcoholic fermentation
Which products regulate Oxidative Phosphorylation and how?
Low [ATP] → stimulates
High [ATP], Low [O2] or [NADH/FADH2] → inhibits
For 1 molecules of glucose what is the net output of glycolysis?
- 2 pyruvate
- 2 ATP
- 2 NADH
What is the main source of ATP of most cancer cells?
Glycolysis
→ No need for O2, but 17X less efficient than OxPhos to have to use much more glucose to compensate (PET Scan uses that)
Which are the main regulatory steps of Glycolysis?
Step 1: Glucose → G6P (invest 1 ATP)
Step 3: F6P → FBP (invest 1 ATP)
Step 10: PEP → Pyruvate (produces 1 ATP/Glucose)
What is the first step of Glycolysis?
Hexokinase traps the glucose inside the cell by adding 1 phosphate (uses 1 ATP/glucose) as phosphorylated compounds don’t transport readily (don’t fit in Glut transport)
*Hexokinase needs Mg2+ to be in active conformation
∆G = -27 kJ/mol → physiologically irreversible
→ Keeps the concentration of glucose in the cytoplasm low to allow more glucose to be taken up (no need for energy expenditure, simple osmosis)
What is the difference between Hexokinase and Glucokinase?
Glucokinase has much lower affinity for glucose than hexokinase, needs higher concentration to do the reaction
What is GKRP?
Glucose Kinase Regulatory Protein is a Binding partner that sequesters Glucokinase in the nucleus when glucose levels are low → only Hexokinase acts
When high [Glucose], Glucokinase is released in the cytoplasm to produce G6P
What explains the fact that multiple systems are activated for different duration of exercise?
The shortage of ATP (or of substrate to make ATP) from 1 mechanism will lead activation of the next one
What are the ∆G or Step 1 and 3 of glycolysis?
Step 1: ∆G = -27 kJ/mol
Step 3: ∆G = -26 kJ/mol
*Physiologically irreversible
What are the differences between Hexokinase and Glucokinase?
(substrate, tissue, affinity)
Same reaction for both: Glucose + ATP → Glucose-6-phosphate + ADP
Substrate specificity: Hexokinase → Hexoses (6C sugars) // Glucokinase → only glucose
Tissue prevalance: Hexokinase → all cell types // Glucokinase → LIVER and a bit in b-cell of pancreas
Affinity constant: Hexokinase → high affinity Km = 0.1 mM // Glucokinase → Low affinity Km = 5mM
*Glucose uptake in the liver can be increased/more efficient when glucose levels are higher to be converted to glycogen more efficiently
What are the different modulators of Hexokinase vs Glucokinase
Hexokinase: negative allosteric inhibition by G6P
Glucokinase: no feedback inhibition, but Nuclear Glucokinase regulatory protein → Binds GK to sequester it in the nucleus when glucose levels are low
What factor decides which way the reversible step 2 of glycolysis goes?
Enzyme Phosphoglucose Isomerase makes G6P or F6P depending on which one is more concentrated, can go both ways
*At equilibrium
Why is the 3rd step of glycolysis THE rate determining step and the one that commits cell to metabolize glucose?
G6P can be used in many pathways → pentose phosphate pathway, conversion to glycogen, etc.
But F1,6BP can only be used in glycolysis
How is PFK-1 regulated?
Activated by AMP, F2,6BP
Inhibited by ATP, citrate
When bound to ATP → T state (inactive dimers)
When not bound (when more F2,6BP or AMP) →R state (active tetramer)
Is ATP or AMP more potent at modulating PFK? (at equal concentrations which dominantes)
AMP dominates by far → low [AMP] still overcomes ATP inhibition
What is the role of Adenylate Kinase (ADK)?
It is en enzyme that re-generates ATP from 2 ADP molecules
*Also good because AMP activates PFK
2 ADP ⭤ ATP + AMP
Ex: Have 100 ATP; 20 ADP ; 2 AMP
Use 10% of ATP → 10 ATPs → 10 ADP
Have 90 ATP; 30 ADP; 2 AMP
With ADK, 95 ATP; 20 ADP; 7 AMP which is a 350% increase in AMP and only 5% loss in ATP compared to a 10% use
What enzyme regulates the inverse reaction to step 3? (FBP → F6P)
Fructose-1,6-bisphosphatase
It promotes Gluconeogenesis → ∆G = -9kJ/mol (not as favourable as the inverse in vivo, but if they were running at the same rate, would could no net ATP hydrolysis)
*Allows regulation by substrate cycles
How is PFK1 allosterically modulated?
What is its active/inactive state?
- AMP and F2,6P are allosteric activators
- ATP is an allosteric inhibitor
- Transition from the T state to the R state activates PFK1
*AMP is more potent at activating PFK1 than ATP is at inhibiting it
What are the ratios of PFK and FBPase at rest and during exercise ? (Substrate cycle)
*FBPase is involved in gluconeogenesis
*Step 3
At rest, equilibrium between both (looks like futile cycle, but price to pay to have both necessary pathways)
G6P ⭤ F6P (→ = PFK1) / (← FBPase) FBP ⭤ GAP + DHAP
During exercise, more AMP and F2,6P activate PFK1 which favours the forward
What is the importance of Fructose-2,6-bisphosphate?
F2,6P is a potent activator of PFK and inhibitor of FBPase (AMP as well)
Made from F6P ⭤ (PFK-2/FBPase-2) ⭤ F2,6P
*Not an intermediate of Glycolysis
What are the major differences between PFK-1 and PFK-2?
PFK-1 //PFK-2
1) PFK-1 = tetramer // PFK-2 = dimer (when active)
2) F6P → F1,6P // F6P → F2,6P
3) Effector and rate limiting step // Modulator (not involved in glycolysis pathway) → covalent modification regulation
4) Kinase // Kinase and phosphatase
What are the modulators of PFK-1 vs PFK-2?
PFK-1:
Inhibited by ATP
Activated by AMP and F2,6P
PFK-2:
Glucagon (in the liver)
Epinephrine → modulate cAMP (2nd messenger) in the muscles
How does regulation of PFK2 occur in the liver vs in the heart muscle cells?
Liver isoenzyme = inverse of heart muscle isoenzyme
Liver: Fasting or prolonged exercise → low [blood glucose] → increase in glucagon → increase [cAMP] → PKA active → phosphorylation of PFK-2 → More FBPase activity → decrease [F2,6P]
*Favours gluconeogenesis
Heart muscle isoenzyme: increase in epinephrine → increase [cAMP] → PKA active → phosphorylation of PFK-2 → less FBPase activity → favours glycolysis
*To generate ATP (if need to run away)
What occurs in Step 4 and 5 of glycolysis?
Step 4: Aldolase splits 6C (FBP) → 2 x 3C (GAP + DHAP)
*Only GAP can be used for glycolysis
Step 5: Triose phosphate isomerase (TIM) converts DHAP ⭤ GAP, but GAP is quickly consumed by Glycolysis so DHAP → GAP is favoured by TIM to maintain equilibrium ([DHAP]»_space; [GAP])
Generation of 2 GAP/glucose
*GAP enters the payoff phase
Describe steps 6 and 7 of glycolysis (coupled).
Step 6: GAP ⭤ 1,3-BPG (high energy intermediate), generates NADH
∆G˚ = 6.7 kJ/mol
Step 7: 1,3-BPG ⭤ 3PG, generate 1 ATP
PGK requires Mg2+
∆G˚ = -18.8 kJ/mol (drives step 6)
*Generation of 2 NADH + 2ATP/glucose
What is substrate channeling? Give an example in glycolysis.
Step 6 and 7 are coupled as step 6 forms 1,3-BPG high energy intermediate which has very short life → rapidly passed along to make 3PG
→ Both enzymes are very close physically in the cell
Describe steps 8 and 9 of glycolysis.
Step 8: PGM (mutase) transfers a phosphate from position 3 to position 2
3PG → 2PG
Step 9: enolase (with Mg2+) dehydrates 2PG → PEP + H2O
*PEP = high-energy intermediate
Describe step 10 of glycolysis.
PEP + ADP → {PK (pyruvate kinase) w/ Mg2+, K+} → Pyruvate + ATP
∆G = -23kJ/mol
How is step 10 of glycolysis regulated?
F1,6P = allosteric activator
ATP (end product) = allosteric inhibitor of PK
In the Liver only,
Glucagon Glucagon activated cAMP → activated PKA → PKA phosphorylated PK → hormonal inhibition
Which are the 3 enzymes responsible for the 3 irreversible steps of glycolysis?
Step 1: Hexokinase
Step 3: PFK-1
Step 10: PK
What is 1 adv of Anaerobic glycolysis and 1 adv of aerobic glycolysis?
Anaerobic glycolysis → 2ATP, bit much more rapid than mitochondrial ox. phos.
Aerobic glycolysis → 32 ATP, 16x more efficient
The aerobic catabolism of which of the following generates more ATP?
Glucose, Galactose, Fructose, Sucrose, Lactose, Maltose
Sucrose = glucose + fructose
Lactose = glucose + galactose
Maltose = glucose + glucose
*Glucose, Galactose and fructose all make 2 net ATP from glycolysis
How is fructose digested to enter glycolysis in the liver (major pathway) ?
Fructose → {Fructokinase consumes ATP} → F1P ⭤ F1P open chain → {F1P aldolase B} → GAP + DHAP (→ {triose phosphate isomerase} → GAP) → Glycolysis
*95% of fructose of the body is metabolized in the liver by fructokinase
How is fructose digested to enter glycolysis in the muscles (minor pathway) ?
Fructose → {Hexokinase consumes 1 ATP} → F6P → Glycolysis: PFK-1 → F1,6P → {Aldolase A} → GAP + Dihydroxyacetone phosphate
*5% of the fructose bypasses the liver and is metabolized in other tissues (muscles)
What are the differences between Fructose digestion for glycolysis in the liver vs in the muscles?
Aldolase A (muscles) uses F1,6P (normal glycolysis aldolase)
Aldolase B (liver) uses F1P
Unlike hexokinase, fructokinase is NOT allsoterically inhibited →
In muscles, F6P enters before PFK point of control
In liver, GAP enters after PFK point of control → could contribute to dyslipidemia
*Glucose → G6P → F6P → GAP → Pyruvate
What are the 2 options for Glyceraldehyde made from F1P aldolase in the fructose metabolism in the liver?
Option 1: Glyceraldehyde → {Glyceraldehyde kinase (Consumes ATP and puts it in P3)} → GAP
Option 2: Glyceraldehyde → … → Glycerol-3-phosphate → Glycerophospholipids Triacylglycerol for Fatty acid synthesis OR DHAP → triose phosphate isomerase → GAP
*Both consume net 1 ATP
GAP = Glyceraldehyde-3-phosphate
What is the difference between Glucose and Fructose metabolism in Liver in a Fed State?
In fed state → Liver is ANABOLIC organ → generates glycogen
Glucose → G6P → G1P → Glycogen
Fructose → F1P → DHAP/GAP after PFK control point → Glycolysis → CAC (Citrate) → ATP and NADH inhibit the CAC → Citrate accumulates → Activator of fatty acid pathway → Fatty acid synthesis
What is the difference between fructose metabolism in the liver and in non-liver?
In Liver:
Fructose → F1P → DHAP/GAP after PFK control point → Glycolysis → CAC (Citrate) → ATP and NADH inhibit the CAC → Citrate accumulates → Activator of fatty acid pathway → Fatty acid synthesis
Non-liver:
Fructose → F6P → Glycolysis {with PFK control} → Citric Acid Cycle → Generation of ATP
Or glycogen in the muscle
What is Leloir Pathway?
Where does it mostly occur?
It is the way Galactose is metabolized to enter glycolysis
*Galactose is NOT a substrate for Hexokinase
*MOSTLY in the liver
Galactose → {galactokinase (consumes 1 ATP)} → Gal1P → Glucose1P → {glucose mutase} → G6P → glycolysis
Galactose1P + UDP-Glucose → {galactose-1-phosphate uridylyl transferase} → G1P + UDP-galactose
In Leloir Pathway, where does UDP-glucose come from?
UDP-glucose comes from UDP-galactose of the previous galactose metabolism run
UDP-galactose → { UDP-galactose-4-epimerase + NAD+} → UDP-glucose
What is galactosemia and its different types?
It is a lack of capacity to metabolize galactose
Type I → Uridyltransferase
Type II → Galactokinase (1st step)
Type III → Epimerase (regeneration of UDP-glucose)
Its a rare autosomal recessive genetic disorder
Symptoms: liver disease, vomiting, lethargy, mental retardation, cataracts
*Milk is toxic to newborns with galactosemia
Can a galactosemic mother produce milk? How?
In the mammary gland:
The ENERGY released from PPi → 2Pi when making the UDP-glucose (G1P + UTP → PPi + UDP-glucose, highly exergonic reaction) used to invert epimerase reaction to produce UDP-galactose from the UDP-glucose
UDP-galactose + Glucose → {Lactose synthase} → Lactose + UDP
Lactose = b-galactose(1→4)glucose
Where is mannose found in the body?
How is it metabolized for glycolysis?
It is a minor sugar as it is not found circulation
Mostly found in cells, when eat meat → in glycoproteins
Mannose → {Hexokinase} → mannose-6-phosphate → {phosphomannose isomerase} → F6P → {PFK} → F1,6P → Glycolysis
What are the entry points into glycolysis of Galactose, Fructose and Mannose?
Galactose → G6P
Fructose (muscles) → F6P
Mannose → F6P
PFK control
Fructose (liver) → GAP (only problematic)
In a petri dish, when cancer cells undergoing the “Warburg effect” are fed with radiolabelled 13C-glucose tracer, which radioactive by-product is expected to be massovely detected?
A) 13C-acetyl-CoA in mitochondria
B) 13CO2, in the vapor phase above the cells
C) 13C-Lactate, in the cells culture medium
D) 13C-citrate, in the cytosol
E) 13C-NADPH, in the cytosol
C) 13C-Lactate, in the cells culture medium
D) 13C-citrate, in the cytosol
