GLUCONEOGENESIS

GLUCONEOGENESIS

• Where does the majority of gluconeogenesis occur?

• Mainly in the liver, and to a lesser extent in the kidneys and intestinal epithelium

• Name four substrates that can be used for gluconeogenesis:

• 1. Lactate
• 2. Pyruvate
• 3. Glycerol

• 4. Substances that can be converted to oxaloacetate (amino acid carbon skeletons)

• Name four enzymes that circumvent the irreversible steps in glycolysis:

• 1. Pyruvate carboxylase
• 2. PEP carboxykinase
• 3. Fructose-1,6-bisphosphatase
• 4. Glucose-6-phosphatase

• Where in the cell can pyruvate carboxylase be found?

• Mitochondria

• What reaction does pyruvate carboxylase catalyze?

• Pyruvate → oxaloacetate (which leaves the mitochondria)

• Pyruvate carboxylase requires what coenzyme in order to function?
• Biotin

• What molecule in excess activates pyruvate carboxylase?

• Acetyl CoA

• Where in the cell can PEP carboxykinase be found?

• Cytosol and mitochondria

• What reaction does PEP carboxykinase catalyze?

• Oxaloacetate → PEP

• What triphosphate must be present in order for PEP carboxykinase to function?

• GTP

• What is the major manifestation of PEP carboxykinase deficiency?

• Hypoglycemia after fasting

• Where in the cell can fructose-1,6-bisphosphatase be found?

• Cytosol

• What reaction does fructose-1,6-bisphosphatase catalyze?

• Fructose-1,6-bisphosphate → fructose-6-phosphate

• Where in the cell can glucose-6-phosphatase be found?
• Cytosol

• What reaction does glucose-6-phosphatase catalyze?

• Glucose-6-phosphate → glucose

• Glucose-6-phosphatase deficiency is also known as what disorder?

• von Gierke disease

• In the aforementioned disorder, the liver becomes metabolically similar to what organ?

• Muscle

• What hormone is the main regulator of gluconeogenesis?

• Glucagon

• How does glucagon exert allosteric regulation on gluconeogenesis?

• Glucagon increases levels of cyclic adenosine monophosphate (cAMP), which increases the activity of cAMP-dependent protein kinase. This leads to decreased levels of fructose-2,6 bisphosphate, thereby activating fructose-1,6 bisphosphatase and inhibiting PFK-I.

• How does glucagon regulate pyruvate kinase?

• Glucagon increases levels of cyclic adenosine monophosphate (cAMP), which increases the activity of cAMP-dependent protein kinase. This causes pyruvate kinase to become inactive via covalent modification by the dependent protein kinase.

• How does this mechanism alter the level of gluconeogenesis that occurs?

• Decreasing the amount of active pyruvate kinase decreases the conversion of PEP to pyruvate, and diverts PEP toward glucose.

• How can a decrease in insulin alter gluconeogenesis?

• Decreased insulin favors the mobilization of amino acids from muscle to the liver, which are used as carbon skeletons for gluconeogenesis.

• During starvation, which molecule acts as an activator of gluconeogenesis?

• β-Oxidation of fatty acids during starvation increases the amount of acetyl CoA, exceeding the capacity of the liver to oxidize it to CO2.
• How does this molecule stimulate gluconeogenesis?

• Excess acetyl CoA activates pyruvate carboxylase, increasing gluconeogenesis.

• Which by-product of exercising or ischemic muscle is used for gluconeogenesis?

• Lactate

TRICARBOXYLIC ACID CYCLE

TRICARBOXYLIC ACID CYCLE

• For each acetyl CoA pushed through the TCA cycle, how much of the following molecules are produced:

• NADH? Three
• FADH2? One
• CO2? Two
• Guanosine triphosphate (GTP)? One

• How much ATP is produced per acetyl CoA pushed in the TCA cycle?(NADH=3 ATP & FADH=2ATP)

• 12 ATP (2 × everything per glucose)

• What reaction does citrate synthase catalyze?
  Oxaloacetate + acetyl CoA → citrate
• The presence of what molecule inhibits citrate synthase?

• ATP (demonstrates there is a sufficient amount of energy in the cell)

• What reaction does isocitrate dehydrogenase catalyze?

• Isocitrate + NAD+ → α-ketoglutarate + CO2 + NADH

• What molecules inhibit isocitrate dehydrogenase?

• ATP, NADH

• The presence of what molecule activates isocitrate dehydrogenase?

• ADP

• What enzyme catalyzes the formation of succinyl CoA from α-ketoglutarate?

• α-Ketoglutarate dehydrogenase

• This enzyme requires what cofactors in order to function?

• Vitamins B1, B2, B3, and B5 and lipoic acid

• The formation of succinyl CoA also releases what molecules?

•  NADH, CO2

• What molecules inhibit the aforementioned enzyme?

• Succinyl CoA, NADH, ATP

• The formation of what molecule in the TCA cycle results in GTP liberation?

• Succinate (via succinyl-CoA thiokinase)

• The formation of what molecule in the TCA cycle results in FADH2 liberation?

• Fumarate (via succinate dehydrogenase)

• What reaction does malate dehydrogenase catalyze?

• Malate + NAD+ → oxaloacetate + NADH

• Is the reaction that malate dehydrogenase catalyzes reversible?

• Yes (important in gluconeogenesis)

 

OXIDATION OF PYRUVATE

OXIDATION OF PYRUVATE

• What are the three enzymes of the pyruvate dehydrogenase (PDH) complex?

• Pyruvate decarboxylase
• Dihydrolipoyl transacetylase
• dihydrolipoyl dehydrogenase

• What reaction does the PDH complex catalyze?

• Pyruvate + NAD+ + CoA → acetyl CoA CO2 + NADH

• What coenzymes are required by this enzyme?

• 1.Thiamine pyrophosphate (vitamin B1)
• 2.coenzyme A (vitamin B5),
• 3.NAD+ (vitamin B3),
• 4.flavin adenine dinucleotide (FAD) (vitamin B2),
• 5. lipoic acid

• The PDH complex is similar to what other enzyme?

• α-Ketoglutarate dehydrogenase complex

• Is the PDH complex active in the phosphorylated or nonphosphorylated state?

• Nonphosphorylated state

• What enzyme phosphorylates the PDH complex?

• PDH kinase

• What molecules activate PDH kinase (thus inhibiting the PDH complex)?

• Acetyl CoA, NADH

• What molecules inhibit PDH kinase (thus activating the pyruvate dehydrogenase complex)?

• Pyruvate, decreased levels of adenosine diphosphate (ADP)

• What enzyme dephosphorylates the PDH complex?

• PDH phosphorylase (PDH phosphatase)

• What molecule activates PDH phosphatase (thus activating the PDH complex)?

• Ca2+

• What are the major manifestations of PDH deficiency?

• Lactic acidosis, neurological manifestations

• What is the treatment for PDH deficiency?
• Increased intake of ketogenic nutrients

GLYCOLYSIS

GLYCOLYSIS

• What are the three major classes of carbohydrates?

• 1.Monosaccharides:
• Fructose, glucose, galactose (The diet contains more disaccharides)
• 2. Disaccharides:

• Sucrose = fructose and glucose disaccharide in cane sugar
• Lactose = galactose and glucose disaccharide in milk
• 3. Polysaccharides:

• Starches = large glucose-dominant polysaccharides present in almost all non-animal foods, particularly in potatoes and grains.
• Other carbs: amylose, glycogen, pectins, dextrins, ....
• The diet also contains cellulose, but humans do not have enzymes capable of hydrolyzing cellulose, only normal flora do express cellulase in the large intestine.

• What enzymes hydrolyze starch and glycogen?

• ptyalin (Salivary amylase) and pancreatic amylase digest all the starch and glycogen to small polysaccharides before chyme enters the distal duodenum.

• What enzymes in the gut hydrolyze residual sugars obtained by hydrolysis of pancreatic and salivary amylase?
• Enterocytes contain four major enzymes (lactase, sucrase, maltase, and α-dextrinase) to hydrolyze the disaccharides lactose, sucrose, maltose and small glucose polymers into monosaccharides.

         α-dextrinase= Isomaltase

         malthose = glucose and glucose =disaccharide

• What carbohydrates are absorbed in the intestine?

• Monosaccharides

• What is the simplest of these carbohydrates?

• Monosaccharides

• What is the major fuel source of the brain?

• Glucose

• What are the major metabolic pathway(s) of the brain?

• Glycolysis and amino acid metabolism

• The brain can not use fatty acids because they do not penetrate the blood-brain barrier

• What cells do not contain mitochondria and thus rely only on glycolysis for energy production?

• Erythrocytes

• What type of tissue stores, synthesizes, and mobilizes triglycerides?

• Adipose tissue

• Name the family of glucose carrier proteins that transport glucose into the cell:

• The GLUT proteins → via facilitated diffusion

• What glucose transporter is used by the liver?

• GLUT2

• What glucose transporter is used by adipose tissue and skeletal muscle?

• GLUT4 (dependent insulin)

• Which one of the above transporters is sensitive to insulin?

• GLUT4

• What is the mechanism of action of insulin on this transporter?

• Facilitates movement of the transporter to cell membrane

• What glucose transporter is located on the brush-border membrane of both intestinal and kidney cells?

• SGLT1, SGLT2 →via Secondary active transport (apical side)

• Epithelial cells are polarized with an apical surface facing the lumen or external environment and a basal surface facing the basement membrane.

• What glucose transporter is located in the intestinal and kidney cells that transpoted glucose into blood?

• GLUT2 (basal side)

• The above enzyme is coupled to the transport of what ion to provide energy for glucose transport?

• Na+

• In most tissues, glucose is trapped in the cell by phosphorylation by what enzyme?

• Hexokinase

• What inhibits the above enzyme?

• Feedback inhibition by its product glucose-6-phosphate

• In the liver, glucose is phosphorylated by what enzyme?
• Glucokinase
• What is the major distinction between hexokinase and glucokinase?

• Glucokinase differs from hexokinase in that it requires a much larger glucose concentration (Km) to achieve half saturation.

• Does glucokinase or hexokinase prevent hyperglycemia following a carbohydrate-rich meal?

• -Glucokinase functions to prevent hyperglycemia following a carbohydrate-rich meal.

• Which two organs express glucokinase?

• 1. Liver
• 2. Pancreas

• Describe the kinetics of glucokinase:

• It has a high Km and high Vmax and is not subject to feedback inhibition by glucose-6-phosphate.
• -Glucokinase is indirectly inhibited by fructose 6-phosphate, and is indirectly stimulated by glucose.

• Describe the kinetics of hexokinase:
  -It has a low Km and low Vmax and is subject to feedback inhibition by glucose-6-phosphate.
• What is the effect of insulin on this glucokinase?

• Insulin induces synthesis of the glucokinase.

• Name two functions of glycolysis:

• 1. Degrading glucose to generate adenosine triphosphate (ATP)
• 2. Providing building blocks for synthetic reactions (such as the formation of long-chain fatty acids)

• How much ATP is consumed per mole of glucose that undergoes glycolysis?

• -2 moles are consumed.

• How much ATP is generated per mole of glucose that undergoes glycolysis?

• 4 moles

• What is the net generation of ATP per mole of glucose that undergoes glycolysis?

• 2 moles

• What is the major regulatory enzyme in glycolysis?

• -Phosphofructokinase-I (PFK-I)

• Name the three enzymes of glycolysis that catalyze virtually irreversible reactions:

• -1. Hexokinase
• -2. Phosphofructokinase-I (PFK-I)
• -3. Pyruvate kinase

• What reaction does PFK-I catalyze?

• Fructose-6-phosphate → fructose-1,6-bisphosphate (coupled to the hydrolysis of ATP)

• Name a positive allosteric regulator of this enzyme:

• -Adenosine monophosphate (AMP), fructose-2,6-bisphosphate

• Name an allosteric inhibitor of this enzyme:

• -ATP, citrate

• What reaction does PFK-II catalyze?

• Fructose-6-phosphate → fructose-2,6-bisphosphate

• Is activity of PFK-II a sign of the fed or fasting state?

• Fed state

• Which two glycolytic intermediates liberate enough energy for driving ATP synthesis?

• 1. 1,3-Bisphosphoglycerate
• 2. Phosphoenolpyruvate (PEP)

• What are the two ATP-producing enzymes of glycolysis?

• -1. 3-Phosphoglycerate kinase
• -2. Pyruvate kinase
  (Tip: remember “kinase”)

• Pyruvate kinase catalyzes what reaction?

• Phosphoenolpyruvate (PEP) → pyruvate

• What covalent modification inhibits pyruvate kinase?

• Phosphorylation → via Protein kinase A

• Name the allosteric inhibitors of pyruvate kinase:

• ATP, acetyl coenzyme (CoA); alanine

• Name the allosteric activator of pyruvate kinase:

• Fructose-1,6-bisphosphate

• What are the signs of pyruvate kinase deficiency?

• Anemia, reticulocytosis with macrovalocytosis, increased 2,3-bisphosphoglycerate (BPG) (Tip: remember red blood cells [RBCs] metabolize glucose anaerobically and thus depend solely on glycolysis)

• This disorder is inherited in what pattern?

• Autosomal recessive

• Which enzyme produces nicotinamide adenine dinucleotide (NADH) in glycolysis?

• Glyceraldehyde-3-phosphate dehydrogenase

• How much NADH is produced per mole of glucose oxidized to pyruvate?

• 2 moles

• Since erythrocytes do not contain mitochondria, what is the NADH produced in glycolysis used for?

• To reduce pyruvate to lactate

• Where is 2,3-bisphosphoglycerate (BPG) created?

• In the glycolysis cycle from 1,3-BPG to 3-phosphoglycerate (PG)→Red blood cells

• How is the reducing power of NADH transferred to the mitochondria?

• Via the glycerol-3-phosphate shuttle or malate aspartate shuttle

• What are the possible fates of pyruvate produced in the cell?

• -1) oxidatively decarboxylated by pyruvate dehydrogenase, producing acetyl CoA
• -2) carboxylated to oxaloacetate (a TCA cycle intermediate) by pyruvate carboxylase (biotin=vit B7)

• 3) reduced by microorganisms to ethanol by pyruvate decarboxylase (coenzyme: thiamine pyrophosphate=TPP=vit B1)
• -4) reduced by lactate dehydrogenase to lactate.
• -5) It can be converted to alanine by ALT (alanine amoinotransferase)(PLP=vit B6)

• How many moles of ATP are required to generate glucose from pyruvate?(NADH=3 ATP)

• 8 moles

• Under anaerobic conditions, pyruvate is converted to what molecule?

• Lactate (anaerobic conditions result in less ATP production than aerobic conditions) = 2 mole

• What enzyme catalyzes the aforementioned reaction?
• Lactate dehydrogenase

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