Glucose Transport and Metabolism Quiz

Introduction to Metabolism & Glycolysis

• Metabolism is the collection of enzymatic reactions occurring in pathways.
• Enzmatic reactions are not isolated; they occur in pathways with each product becoming a substrate for the next reaction, acting like an assembly line.
• Different pathways intersect; collectively, these reactions are called metabolism.
• Catabolic pathways break down complex molecules into simpler ones.
• Anabolic pathways synthesize complex molecules from simpler ones.
• Nutrients (carbohydrates, fats, proteins) are the energy sources for metabolism.
• Complex molecules (proteins, polysaccharides, lipids, nucleic acids) are broken down to precursor molecules (amino acids, sugars, fatty acids, nitrogenous bases).

Metabolic Map

• Each pathway in metabolism is composed of multienzyme sequences.
• Each enzyme may have catalytic or regulatory features.
• Blockages in metabolic pathways can cause disorders.

Catabolic Pathways

• Functions of catabolic pathways:
  • To capture ATP from energy-rich molecules.
  • To convert molecules into building blocks for other components.

Stages of Catabolism

• Stage 1: Hydrolysis of complex molecules into component building blocks (proteins, carbohydrates, & fats).
• Stage 2: Conversion of building blocks into acetyl-CoA (or simple intermediates).
• Stage 3: Oxidation of acetyl-CoA and oxidative phosphorylation.

Anabolic Pathways

• Combine simple components into complex ones. For example, amino acids combine to form proteins.
• Require energy (endergonic) – often ATP, where the ATP is converted into ADP and Pi
• Often requires reducing power NADPH.
• Catabolism is a convergent process, anabolism is a divergent process.

Regulation of Metabolism

• Intracellular communication
• Intercellular communication
• Second messenger systems
• Adenylyl cyclase
  • GTP-dependent regulatory proteins
  • Protein kinases
  • Dephosphorylation of proteins
  • Hydrolysis of cAMP
• Direct contact
  • Signaling cell
  • Target cells

Overview of Glycolysis

• All tissues use glycolysis.
• Functions:
  • Oxidize glucose to provide energy in the form of ATP.
  • Provides intermediates for other metabolic pathways.
• Glycolysis is at the hub of CHO metabolism.
  • Virtually all sugars are converted into glucose prior to entering glycolysis.

Aerobic and Anaerobic Glycolysis

• Aerobic glycolysis occurs in cells with mitochondria and a sufficient oxygen supply and produces pyruvate as its end product.
• Anaerobic glycolysis occurs in the absence of oxygen and converts pyruvate to lactate by oxidizing NADH to NAD+. It occurs in tissues lacking mitochondria (e.g., red blood cells) and during anoxia.

Transport of Glucose into Cells

• Facilitated diffusion (sodium-independent) is mediated by a family of glucose transporters (GLUTs) in the cell membrane (GLUT1-GLUT14).
• GLUTs have tissue specificity determined by gene expression.
• Examples of GLUTs include GLUT-1(erythrocytes, blood barrier), GLUT-2 (liver, kidney, β-cells), GLUT-3 (neurons), GLUT-4 (muscle & adipose, insulin dependent), and GLUT-5 (fructose transporter in small intestine & testes).

Sodium-Monosaccharide Cotransport

• Energy-requiring process (against concentration gradient):
• Transporter-mediated process where glucose movement is coupled to the sodium (Na+) concentration gradient.
• Called sodium-dependent glucose transporter (SGLT).
• Occurs in epithelial cells of the intestine, renal tubules, & choroid plexus.

Glucose Transport in Intestinal Epithelial Cells

• Glucose movement occurs through the different compartments of the cell.
• Active transport of Na+ coupled to glucose
• Facilitated diffusion of glucose out of the cell

Reactions of Glycolysis

• Phase 1: Energy investment phase (first 5 reactions)
• Phosphorylated intermediates formed using ATP molecules
• Phase 2: Energy generation phase (subsequent reactions)
• Net of 2 ATP formed during the substrate-level phosphorylation step.

Phosphorylation of Glucose

• Phosphorylated sugars cannot easily penetrate cellular membranes.
• Phosphorylation of glucose traps it inside the cell.
• Glucose-6-phosphate is the first step in glycolysis which is irreversible.
• Hexokinase and glucokinase are two enzymes that catalyze the phosphorylation of glucose.
• Hexokinase has broad substrate specificity, low Km, and is inhibited by its product.
• Glucokinase is found in the liver & β-cells of the pancreas. It is a glucose sensor that regulates insulin secretion.

Regulation of Glucokinase Activity by Glucokinase Regulatory Protein

• GKRP regulates the activity of glucokinase.
• In the presence of fructose-6-phosphate, GKRP binds tightly to glucokinase and inactivates it.
• When glucose concentrations increase, glucokinase is released from GKRP, permitting phosphorylation of glucose.
• Glucokinase functions as a glucose sensor in glucose homeostasis.

Isomerization of Glucose 6-Phosphate

• Catalyzed by phosphoglucose isomerase.
• Reaction is reversible
• Not a rate-limiting or regulated step

Phosphorylation of Fructose 6-phosphate

• Catalyzed by phosphofructokinase-1 (PFK-1).
• Important control point in glycolysis.
• Regulation by energy levels within the cell (inhibited by high ATP & citrate, activated by high AMP).
• Regulation by fructose 2,6-bisphosphate (potent activator of PFK-1, can override ATP inhibition).

Cleavage of Fructose 1,6-bisphosphate

• Aldolase cleaves fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate.
• Reaction is reversible and not regulated

Isomerization of Dihydroxyacetone Phosphate (DHAP)

• Triose phosphate isomerase interconverts DHAP and glyceraldehyde 3-phosphate.

Oxidation of Glyceraldehyde 3-phosphate

• First oxidation-reduction reaction in glycolysis, catalyzed by glyceraldehyde-3-phosphate dehydrogenase (GPDH).
• NAD+ is regenerated in the cell for glycolysis through either lactate production (anaerobic glycolysis) or by respiration (aerobic glycolysis).

Synthesis of 3-phosphoglycerate, ATP Production, and Shift of Phosphate Group

• Steps in glycolysis producing ATP & phosphorylated intermediates

Dehydration of 2-phosphoglycerate

• Processes to form phosphoenolpyruvate

Formation of Pyruvate

• Catalyzed by pyruvate kinase (PK), the third irreversible step in glycolysis.
• Feedforward regulation by fructose-1,6-bisphosphate.
• Regulation by phosphorylation - the protein kinase leads to inactivation. Dephosphorylation reactivates it.
• PK deficiency relates to RBC maturation and lack of mitochondrial function.

Reduction of Pyruvate to Lactate

• Lactate dehydrogenase (LDH) catalzyes the conversion of pyruvate to lactate by reducing NAD+ to NADH

Energy Yield from Glycolysis

• Anaerobic glycolysis: 2 molecules of ATP/glucose; no net change in NADH.
• Aerobic glycolysis: 2 molecules of ATP/glucose; 2 molecules of NADH.

Hormonal Regulation of Glycolysis

• Insulin and glucagon reciprocally regulate glycolysis at 3 major control points.
• The regulatory enzymes involved are transcriptionally upregulated (insulin) or downregulated (glucagon).
• Hormonal regulation is coupled with the quick allosteric inhibition and activation, and covalent (phosphorylation/dephosphorylation) regulation.

Summary

• Key elements of glycolysis: 3 irreversible reaction steps, importance of ATP and NAD+.

Why is CHO Metabolism of Interest to You?

• Changes in cancer cells:
  • Cancer cells often preferentially use anaerobic glycolysis (Warburg effect)
  • Changes in glucose metabolism are implicated.

Changes in Glucose Metabolism in Cancer Cells

• Detailed molecular mechanisms involved in the change in glucose metabolism in cancer cells.

Aldolase B-Mediated Fructose Metabolism Drives Metabolic Reprogramming of Colon Cancer Liver Metastasis

• Summary of article about fructose metabolism in colon cancer.

What You Should Know at the End of This Lecture

• Summary of key points for glycolysis in metabolism and related topics.