Sjogren - Exam 3 Flashcards
What is the purpose of metabolism?
The purpose of metabolism it to convert exogenous sources of energy (such as food) to usable energy.
What are the principles of redox reactions?
- many redox reactions have both an electron and a proton transferred
- conversion of pyruvate and NADH to lactate and NAD+ is under anaerobic conditions
What is true about the substrate that accepts electrons in redox reactions?
It also gains protons.
What is the Gibbs free energy for ATP hydrolysis?
Hydrolysis of ATP has a largely negative delta G.
Why are the steps that require ATP irreversible?
- The negative delta G of the hydrolysis of ATP indicates that reversing the ATP hydrolysis would have a highly positive delta G. This indicates that it would require an input of energy to complete.
Where does metabolic regulation frequently occur at?
the rate-limiting or commitment steps
Rate-limiting step
the slowest step in the pathway
commitment step
the first irreversible step unique to the pathway
Which steps usually involve high-energy substrates?
irreversible
Allosteric regulation
- fast
- activators and inhibitors
Transcriptional/translational regulation
- slow
- induction of genes for enzymes involved in metabolism
Protein degradation
- slowest
- ubiquitin-proteasome pathway
- lysosomal proteolysis
Post-translational modification
- fast
- phosphorylation
Compartmentation
- fast
- shuttling substrates to a compartment for biochemical reactions
- Ex: fatty acid biosynthesis in the cytosol and oxidation in the mitochondria
How is hexokinase (glucose –> G6P) regulated?
- feedback (product inhibition)
- concentrations of glucose-6-phosphate inhibits hexokinase
What regulates pyruvate kinase (PEP –> pyruvate) via feedforward?
- fructose-1,6-bisphosphate
- positive allosteric regulation
What is a futile cycle?
- futile cycles are formed by the activation of two irreversible reactions that occur in opposite directions
- wastes cellular energy
What is one physiological example of a futile cycle?
The liver uses a futile cycle between glucose and glucose-1-phosphate as a buffer to maintain blood glucose levels.
What is the Gibbs free energy of fructose-6-phosphate to fructose-1,6-bisphosphate?
- largely negative (indicating that its irreversible)
What is the commitment step for glycolysis?
Fructose-6-phosphate –> fructose-1,6-bisphosphate mediated by PFK-1
Where does the glucose come from?
digestion and absorption of carbohydrates
What must happen to dietary carbohydrates in order for absorption to occur?
Di, oligo, and polysaccharides must be hydrolyzed to monosaccharides to be absorbed.
What is GLUT-1 responsible for?
basal non-insulin-stimulated glucose uptake into many cells
What is GLUT-2 responsible for?
- glucose sensing for pancreatic B-cells
- Together with glucokinase, it forms the B-cell’s glucose sensor and allows glucose to enter the B-cell at a rate proportional to the extracellular glucose level
What is GLUT-3 responsible for?
non-insulin mediated glucose uptake into brain neurons
What is GLUT-4 responsible for?
insulin-stimulated glucose uptake in muscle and adipose tissue, and thus the classic hypoglycemic action of insulin
Which tissues can increase glucose uptake in response to insulin?
muscle and adipose tissue
Which organ can complete gluconeogenesis?
liver
How does insulin resistance occur?
- defects in intracellular signlaing
- downregulation of insulin receptors
What is a consequence of insulin resistance?
Type II Diabetes
Where is glucose-6-phosphate used outside of glycolysis?
- glycogen synthesis
- pentose phosphate pathway
What do liver and pacreatic B-cells use in place of hexokinase?
glucokinase
When does glucokinase utilize glucose?
Glucokinase only utilizes glucose when the concentration is sufficiently high
- after meals
Where does the switchover from Hexokinase I-III to glucokinase occur?
glucose = 5mM (90 mg/dL)
What inhibits PFK-1 (F6P –> Fructose 1,6 -bisphosphate)
- negative regulation by ATP, citrate, and H+
What promotes PFK-1 (F6P –> Fructose 1,6-bisphosphate) (commitment step)
- AMP
- fructose-2,6-bisphosphate
- hormonal regulation in liver
How can cells use AMP to sense energy status?
A very small % decrease in ATP will be indicated by a very large % increase of AMP. This indicates glycolysis is needed.
What does high ATP regulate?
- inhibits hexokinase, PFK-1, and pyruvate kinase
What does high AMP regulate?
- activates PFK-1 and pyruvate kinase
What has a critical role in cellular energy homeostasis?
AMP-activated protein kinase (AMPK)
Which stage (and steps) of glycolysis consumes ATP?
Priming stage
- Hexokinase converting Glucose–> G6P
- PFK converting F6P –> F1,6bisphosphate
Which stage (and steps) of glycolysis generates ATP?
Energy extraction
- Phosphoglycerate kinase converting glycerate-1,3-bisphosphate to Glycerate-3-phosphate
- Pyruvate kinase converting PEP to pyruvate
Are the reversible steps of glycolysis regulated?
- Not really
- The enzymes keep the reactions near equilibrium
What is the role of pyruvate kinase?
Pyruvate kinase converts phosphoenolpyruvate (PEP) into pyruvate
How is pyruvate kinase regulated?
- activated by Fructose-1,6bisphosphate
- inhibited by ATP and alanine
What are the fates of pyruvate in anaerobic conditions?
- transamination to form alanine
- reduction to form lactate
What happens to pyruvate when oxygen is available?
- it undergoes oxidative phosphorylation and forms Acetyl CoA
How is ATP generated in red blood cells?
- RBCs don’t have mitochondria and use anaerobic fermentation to generate ATP
Where does one carbon source for gluconeogenesis come from?
- Lactate can be transported to the liver via the Cori cycle to serve as a substrate for gluconeogenesis.
Regulation of PFK-1 by citrate is an example of:
feedback inhibition
Describe the metabolic problem of a fructose-only diet?
Fructose metabolism bypasses the highly regulated PFK-1 step in glycolysis. This means that glycolysis continues to happen – leading to the overproduction of pyruvate and glycerol-3-phosphate which are both precursors to triglycerides.
What are all of the carbon sources for gluconeogenesis?
lactate, amino acids, glycerol, fructose
What is the rate-limiting step of gluconeogensis?
PEP carboxykinase converting oxaloacetate to phosphoenolpyruvate (PEP) is the rate limiting step.
Identify the regulation mechanism to suppress glycolysis and to promote gluconeogenesis:
- G6P accumulates only during gluconeogenesis – this inhibits hexokinase
- allosteric
How does alcohol interfere with gluconeogenesis?
- oxidation of ethanal depletes NAD+
- NAD+ is used to convert lactate into pyruvate
- Because there’s an excess of NADH, pyruvate is depleted at is it converted to lactate
- no pyruvate = no gluconeogenesis
What are the fates of glucose-6-phosphate in cells?
- gluconeogenesis (liver)
- glycogen synthesis
- glycolysis
- pentose phosphate pathway
What does the liver use glycogen for?
The liver uses glycogen to maintain blood glucose levels. (The liver generates glucose from glycogen)
What do muscles use glycogen for?
The muscles use glycogen as a storage molecule as more G6P is regenerated.
How is glycogenesis regulated?
- allosterically
- glycogen synthase
- G6P promotes glycogen synthase
How is glycogenolysis regulated?
- phosphorylase
- phosphorylase is allosterically promoted by AMP
- phosphorylase is allosterically inhibited by G6P and ATP
True statements for gluconeogenesis:
- It should be off when glycolysis is on
- It can use several different carbon sources
- Most of the enzymes used are the same as glycolysis
- It is activated in the liver during fasting
- It is negatively regulated by ADP
What is the function of the pyruvate dehydrogenase complex?
It catalyzes the conversion of pyruvate to acetyl CoA
What are the 3 reactants of the pyruvate dehydrogenase complex?
- pyruvate
- CoA
- NAD+
What are the 3 products of the PDH enzyme complex?
- CO2
- Acetyl-CoA
- NADH + H+
What is the function of Coenzyme A (CoA) in the PDH enzyme complex?
- acts as an acetyl group carrier
What are the different sources of Acetyl CoA?
- produced from glucose (glucose –> pyruvate –>)
- produced from fatty acids
- produced from ketogenic amino acids
What are the different fates of Acetyl CoA?
- Used in the TCA cycle –> CO2
- –> ketone bodies
- –> fatty acids
- –> cholesterol
Under what conditions are ketone bodies produced?
- during prolonged fasting
- during ketogenic (low carb - high fat) diet
How are ketone bodies utilized to produce energy?
- Ketone bodies are converted to acetyl-CoA in nonhepatic tissues
- Acetyl-CoA is used as a fuel in the TCA cycle
What is the rate-limiting step of ketone synthesis from acetyl-CoA?
HMG-CoA synthase
- promoted by glucagon
- inhibited by insulin
Describe the uses of TCA cycle intermediates in biosynthesis:
- critical building blocks for gluconeogenesis and amino acid synthesis (neurotransmitters)
What is the purpose of anaplerotic reactions?
- supply TCA cycle intermediates
- When amino acids are not sufficient, pyruvate is the main source of TCA cycle intermediates.
Explain why triheptanoin is used during pyruvate carboxylase deficiency.
- Pyruvate carboxylase is essential in gluconeogenesis. Triheptinoin is used to during deficiency because it can be converted to acetyl-CoA and succinyl-CoA to continue supplying the TCA cycle intermediates.
Why isn’t the initial reaction in glycolysis the commitment step?
Glucose-6-phosphate can be utilized in other pathways.
Why does high intake of sucrose increase the risk of obesity?
Fructose metabolism bypasses the highly regulated PFK-1 step in glycolysis.
True statements about ketone bodies:
- High levels of ketone bodies in the blood result in exhaling acetone
- Ketone body production involves HMG-CoA as an intermediate
- Ketone bodies are mainly produced in the liver
- Ketone bodies are water-soluble
What are the 5 coenzymes involved in the PDH complex?
- Thiamine pyrophosphate (TPP)
- Lipoamide
- FAD
- CoA
- NAD+
True Hexokinase reaction statements:
- The reaction requires energy in the form of ATP
- It results in trapping glucose inside the cell
- The reaction has a negative delta G
- The reaction is negatively regulated by G6P
In order for oxaloacetate to cross the mitochondrial membrane, it must first be converted to either ___________ or _________.
- aspartate
- malate
Why can’t the liver utilize ketone bodies as a source for energy?
The liver lacks the enzyme thiophorase that converts acetoacetic acid to acetoacetyl-CoA.
What is the overall “reaction” of oxidative phosphorylation?
carbon fuel + O2 –> CO2 + H2O + Energy
How does the flow of electrons in the ETC result in ATP synthesis?
- Electrons from NADH and FADH2 are transferred to O2
- This creates a proton gradient
- ATP synthase produces ATP by using the proton gradient
True statements regarding coenzymes:
- NADH is the reduced form
- FAD is the oxidized form
- Flavin coenzymes function as prosthetic groups
- Both NADH and FADH2 are produced in the TCA cycle
How would cyanide affect the ETC and oxidative phosphorylation?
- cyanide inhibits cytochrome c oxidase (complex IV)
- this would increase the pH of the intermembrane space
- It would inhibit ATP synthesis
Describe the mechanism of energy loss caused by uncouplers:
- uncouplers (2,4-dinitrophenol) increase the rate of O2 consumption
- they dissipate the proton gradient which prevents ATP synthesis
- the energy is lost as heat
What is the role of uncouplers in humans?
- Expressed in brown adipose tissue, which play a major role in non-shivering thermogenesis, especially in newborns
Identify key enzymes that remove reactive oxygen species:
- superoxide dismutase (SOD) and catalase are the primary defense mechanism
- glutathione peroxidase uses glutathione as a reductant
Identify the organs and tissues especially sensitive to reactive oxygen species in diabetic patients:
- cells that cannot restrict the uptake of glucose are vulnerable
- retina
- kidney
- peripheral nerves
Why is brown adipose tissue common in hibernating animals?
They need to generate heat while energy need is low
True statements regarding nicotinamide coenzymes:
- They are polar and water soluble
- They act as high energy electron carriers in the TCA cycle (mitochondria)
- They act as high energy electron carriers in glycolysis (cytosol)
- They cannot be used to produce ATP in the absence of mitochondria
Why does the oxidation of FADH2 generate less ATP than oxidation of NADH in oxidative phosphorylation?
Oxidation of FADH2 occurs in complex II, bypassing on proton-pumping step across the inner mitochondrial membrane
True statements regarding Flavin coenzymes:
- They can act as high energy electron carriers in the TCA cycle
- They act as prosthetic groups on enzymes
- They cannot be used to produce ATP in the absence of mitochondria
- A vitamin is part of the structure of Flavin coenzymes
Which component of ETC is also an enzyme in the TCA cycle?
succinate dehydrogenase/ Complex II
