Intro to metabolism

• Glycolysis

  My first metabolic pathway

  
• Metabolic Pathway Necessities

  • What organism is it taking place in?
  • What organ/tissue is/are involved?
  • What is the subcellular location of the pathway?
  • What type of pathway is it?
  • What enzymes/cofactors are involved?
  • What are the control points?
  • What are the starting substrates/end products?
  • What is the overall effect/ "purpose" of the pathway?

  
• Where in the cell is energy generated?

  • Animal Cell - Mitochondria, Cytosol
  • Plant Cell - Chloroplasts, Mitochondria, Cytosol
• Mature RBCs lack organelles, so without mitochondria...
• Working muscle – oxygen cannot be supplied to vigorously working muscles as fast as ATP is utilized. Consequently, the muscles must generate some of the required energy in an anaerobic manner.
• Glucose Metabolism – major players

  • Insulin
  • Glucagon
  • Muscle
  • Red blood cells (RBCs)
  • Liver
  • Brain
  • Adipose tissue

  
• Glycolysis


  Also called Embden-Meyerhof pathway (E.M-Pathway), it occurs in cytosol, 10 step catabolic pathway, objective is to produce energy (ATP) and the reducing equivalent (NADH) from catabolizing hexose sugars, can occur aerobically or anaerobically

  
• Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria or are oxygen starved - erythrocytes, cornea, lens etc.

  
• Glycolysis is very essential for brain tissues which is dependent on glucose for energy.

  
• Glycolysis can catabolize carbs anaerobically unlike many other energy yielding substrates

  
• Glycolysis is a central metabolic pathway, many of its intermediates are primary substrates or shared intermediates for other pathways e.g. glycogen synthesis, the pentose phosphate pathway, fatty acid synthesis, amino acid synthesis etc.

  
• Phases of Glycolysis

  Divided into two distinct phases: 1. Energy investment (and lysis) phase, 2. Payoff phase

  
• Hexokinase Reaction

  Phosphorylated sugar molecules do not readily penetrate cell membranes without specific carriers, this commits glucose to further metabolism in the cell, one of 3 key regulatory enzymes of glycolysis, allosterically inhibited by glucose-6-PO4, allosterically stimulated by Pi, requires Mg2+ for activity, step is irreversible

  
• Hexokinase vs Glucokinase

  Hexokinase has low Km, Glucokinase has high Km

  
• Energy Investment Phase & Lysis

  2 ATP used
• Phosphofructokinase Reaction

  Fructose 6-phosphate is phosphorylated to Fructose-1, 6-bisphosphate by Phosphofructokinase (PFK-1), the PFK reaction is the rate-limiting step, enzyme allosterically activated by ADP and AMP conc, inhibited by ATP and citrate (high energy), irreversible reaction

  
• The Role of Hormones in Glycolysis Regulation - Insulin

  High glucose conc in blood (fed state) causes the release of insulin from the b-cells of the pancreas, insulin binds to its receptors on liver, muscle and adipose tissue which causes a cascade of reactions inside those cells, protein phosphatase is one of these enzymes activated during this cascade of reactions, many enzymes in carbohydrate metabolism are less active when phosphorylated, protein phosphatase dephosphorylates them causing them to become more active – eg PFK-2, glycogen synthase, pyruvate kinase

  
• The Payoff Phase

  NAD+ in the cytosol is limited and must be regenerated, 4 ATP is synthesized in the payoff phase but because 2 ATP is used in the Investment Phase the NET production is 2 ATP, ATP is synthesized by substrate level phosphorylation from high energy precursors in this phase

  
• The Role of Hormones in Glycolysis Regulation - Glucagon

  The effects of insulin are reversed (the starved state) by the hormone glucagon which is made in the a-cells of the pancreas, binding of glucagon to its receptors on liver, muscle and adipose tissues causes a cascade of cellular events intracellularly to activate protein kinase, protein kinase phosphorylates PFK-2, pyruvate kinase and glycogen synthase inactivating them, as such, glycolysis and glycogen synthesis stops and reactions that provide sugars for the body can ensue

  
• Aerobic Fate of Pyruvate – Acetyl CoA


  In the presence of O2, pyruvate enters the mitochondria (outer – aquaporin, inner – pyruvate translocase in symport with H+), where it is oxidatively decarboxylated to acetyl CoA and enters the TCA

  
• Anaerobic Fates of Pyruvate - Lactate

  During glycolysis NAD+ is reduced NADH, NAD+ in the cytosol is limited, problem – without oxygen, NADH cannot be re-oxidized back to NAD+ and glycolysis will stop, solution: pyruvate is reduced to lactate to regenerate NAD+

  
• Anaerobic Fates of Pyruvate - Ethanol

  Alcoholic Fermentation – the production of ethanol and CO2 from the anaerobic metabolism of pyruvate in microbes such as yeasts, the purpose of this reaction is to regenerate NAD+, ethanol is synthesized as a waste product and is excreted from the cell, responsible for the alcoholic content in fermented drinks such as wine, rum, vodka, beer etc

  
• Metabolite

  A substance involved in metabolism
• Precursor

  A substance from which another substance is formed
• Intermediate

  A substance formed during a metabolic process
• End product

  The final substance produced in a metabolic process
• Metabolism

  • Sum of all the biochemical reactions which fuel and maintain cellular life
  • Metabolic reactions occur in specific cellular locations eg cytosol, mitochondria, ribosome, cell membrane etc

  
• Importance of Metabolism

  • Allows cells to extract energy from complex food substances
  • Digest complex food substances into their simple monomers
  • Use these monomers to synthesize cellular components necessary for cellular/organismal growth, repair, homeostasis, reproduction, movement, communication, defence etc
  • Storage of fuel molecules and usage when necessary
  • Degrade toxins
• Catabolism

  • Complex molecules broken down into simpler molecules (eg polysaccharides to monosaccharides or decreasing number of carbons)
  • Exergonic – release energy usually by oxidation or hydrolysis
  • Generates ATP and/or reducing equivalents (NADH, NADPH and or FADH2)
• Anabolism

  Synthetic, energy utilizing, ATP using pathways, endothermic
• Anabolic reactions

  • Condensation
  • Carboxylation
  • Methylation
  • Dehydrogenation
  • Dehydration
  • Amination
  • Phosphorylation
• Catabolic reactions

  • Hydrolysis
  • Decarboxylation
  • Demethylation
  • Hydrogenation
  • Hydration
  • Deamination
  • Dephosphorylation
• Metabolic pathway


  • A series of chemical reactions where the product of one reaction serves as the reactant for the next reaction
  • The purpose is usually to synthesize biologically useful compounds (usually uses ATP/NADH/NADPH) or degrade and/or excrete used compounds (usually makes ATP/NADH/NADPH)
  • Pathways often produce important intermediates which can be used in other pathways
  • Multiple pathways often work together to produce a physiological effect e.g. weight gain

  
• Metabolic reaction

  A biochemical reaction where a metabolite is specifically reacted by an Enzyme and Coenzyme to give a product
• Reversible reactions


  • Uses the same enzyme to perform the forward and reverse reaction
  • Equilibrium reactions
  • Usually NOT regulatory steps
• Irreversible reactions


  • Uses different enzymes to perform forward and reverse reactions
  • Non-equilibrium reactions
  • Usually regulatory steps
• Organization of metabolic pathways

  • Pathways consist of sequential steps
  • The enzymes may be separate
  • May form a multienzyme complex
  • May be a membrane-bound system
  • New research indicates that multienzyme complexes are more common than once thought

  
• Integration of metabolic pathways

  • Individual metabolic pathways interact with each other in a highly regulated manner to produce physiological effects
  • Pathways interact with each other to form metabolic networks
  • Metabolic networks orchestrate physiological changes, impacting overall health and well-being throughout an organism's life

  
• Compartmentalization of metabolic pathways

  • Compartmentalization of pathways permits integration and regulation of metabolism
  • Increases metabolic efficiency as enzymes and their substrates are concentrated in a unique space
  • Provides ideal microenvironment for specific reactions eg low pH in lysosome for hydrolytic reactions

  
• Types of metabolic pathways

  • Catabolic/Degradative /Energy Generating/ATP producing Pathways/Exothermic
  • Anabolic/Synthetic/Energy Utilizing/ATP Using Pathways/Endothermic
  • Catabolic pathways involve oxidative reactions producing reducing equivalents- NADH+H+ and FADH2
  • Catabolic pathways converge to few end products
  • Anabolic pathways diverge to synthesize many biomolecules
  • Amphibolic pathways - serve both in catabolism and anabolism, occur at the crossroads of metabolism, link between Anabolic and Catabolic pathways