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