Catabolism of Carbohydrates

• All organisms metabolize substances to sustain cellular processes by: breaking it down and synthesizing macromolecules from its precursors
• Catabolic pathways are exergonic (release energy) and endergonic (require energy).
• ATP provides the energy needed for chemical reactions that require energy input.
• The catabolic process is the breakdown of complex molecules into simpler ones.
• Organisms differ in the pathway that they undergo and very dependent on: functions or cells and requirements of the pathway
• Carbon fixation is the conversion of carbon dioxide into organic compounds, which occurs through photosynthesis.
• Anaerobes: does not require oxygen gas for all pathways they undergo
• Aerobes: requires oxygen gas for all its major catabolic pathways
• Respiratory Processes: maximize energy yield by oxidizing metabolites and electron transfer
• Catabolism: process of breaking down macromolecules to small precursors with a concominant release of energy
• Most cells obtain energy from one two major catabolic reactions such as cellular respiration and beta-oxidation
• Cellular Respiration: central catabolic pathway for all organisms
• Anaerobic Stage: Glycolysis

  A series of 10 enzyme catalyzed reactions that breaks down a glucose molecule into two pyruvate molecules
• Glycolysis is the only stage of cellular respiration that does not require O2
• Oxidative or Mitochondrial Stage

  The main energy producing pathways of the body

  Includes intermediate phase and krebs cycle
• Intermediate Phase: Pyruvate oxidized by NAD+ to form Acetyl CoA, which enters Krebs Cycle
• Krebs Cycle (Citric acid cycle): A series of chemical reactions that occur within mitochondria
• Beta Oxidation: The breakdown of fatty acids to produce acetyl coenzyme A
• Electron Transport Chain: The final step of aerobic metabolism where electron carriers pass their electrons through a chain of proteins embedded in the inner mitochondrial membrane
• Fatty acids are broken down through beta-oxidation to release their stored energy.
• Carbohydrate Catabolism: Glycolysis (Embden-Meyerhof Pathway)

  Breaks down glucose into smaller molecules that can pass through the mitochondrial membrane to allow for its complete oxidation or provide precursors for other pathway or processes

  Also it provides energy for completely anaerobic processes
• Glycolysis: a series of 10 enzyme catalyzed reactions by which glucose (6C) is oxidized to two molecules of pyruvate (3C)

  Occurs in the cytosol of the cell

  Energy Invesment Phase (Step 1 to 5): wherein two ATP are used to convert glucose into fructose-1,6-bisphosphate and sub-sequentially cleaved into

  dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G-3-P).

  Energy generation phase (Step 6 - 10): wherein two glyceraldehyde-3-phosphate (G-3-P) molecules are converted to pyruvate in a series of steps, with the formation of 4 ATPs and 2 NADHs.
• Gylcolysis Glucose-6-phosphate is the activated form of glucose for metabolism in the cell. Without such reaction, glucose cannot be metabolized. In the liver and adipose tissues, this reaction is mediated by the hormone insulin which mediates the action of kinase (the enzyme catalyzing this reaction). The conversion of glucose-6-phosphate drives the absorption of glucose into

  the liver.
• 
  

  The formation of fructose-1,6-bisphosphate is the committed step in glycolysis, such that once this is formed, there in no

  turning back in the reaction.
• Pyruvate is the end product of glycolysis, the anaerobic phase of cellular respiration.
• undergo several alternative routes which include

  1. the conversion of pyruvate to lactate in muscle fibers when oxygen level is very low, happens during very high muscular activity;
  2. conversion of pyruvate to acetaldehyde then to ethanol (or conversion to acids) during fermentation reactions in anaerobic organisms
  3. conversion to acetyl CoA in cells containing mitochondria, or when oxygen level is high
• Lactate Fermentation

  • anaerobic process, the NADH produced is reconverted to NAD+ by converting pyruvate to lactate, or to ethanol
  • this reaction ensures the continuity of the glycolytic process because NAD+ is regenerated for the reaction which oxidizes glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.
• Cori Cycle: liver is its main maetabolic organ
• Intermediate Step

  • In cells containing mitochondria, or when oxygen level is high in aerobic organisms, pyruvate enters the mitochondria and is converted to acetyl CoA
  • First oxidation reaction, releasing a carbon in the form of CO2 from pyruvate, the product (an acetyl group) is condensed with a carrier molecule, coenzyme A, producing acetyl CoA.
• This reaction is the intermediate phase in cellular respiration and commits pyruvate to the complete oxidation in the aerobic phase of cellular respiration (Krebs cycle).

  NADH will be oxidized back to NAD+ in the electron transport chain, with the production of 2.5 ATP molecules.

  There are 2 NADH molecules produced from 2 pyruvate molecules, thus the net yield from one glucose in the intermediate phase is 5 ATP molecules.
• Citric Acid Cylce: tricarboxylic acid cycle ehich takes place in the mitochondria

  • The aerobic phase of cellular respiration, which oxidizes the remaining two carbons of pyruvate (in the form of acetyl CoA) and thus completes the oxidation of glucose
  • The fuel is the two-carbon acetyl group of acetyl CoA, with each turn of the cycle two carbons are released as CO2
• • The first step is the condensation of acetyl CoA with oxaloacetate to form citrate (or citric acid), after a series of 8 steps in the cycle, oxaloacetate is regenerated in the process (step 8).
  • This reaction ensures the continuity of the aerobic phase of cellular respiration.
  • Reduced cofactors will be re-oxidized in the ETC.
• Oxidative Phosphorylation: Electron Transport Chain

  • The electron transport chain (ETC) facilitates the passage of energy trapped in FADH2 and NADH during oxidation steps in glycolysis and citric acid cycle
  • ETC is a series of biochemical reactions in which intermediate carriers (protein and non-protein) aid the transfer of electrons and hydrogen ions from NADH and FADH2 (oxidized) releasing additional energy
  • The energy is stored in a proton gradient pumped at the intermembrane space which is then used to synthesize ATP
  • The ultimately receiver of electrons is oxygen, O2
• Peter Mitchell proposed the chemiosmotic theory
• Energy-releasing oxidations lead to proton pumping and create a pH gradient across the inner mitochondrial membrane
• There is a higher concentration of H+ in the intermembrane space than inside the mitochondria
• The proton gradient provides the driving force for protons to move back into the mitochondrion through a transport protein enzyme called ATP synthase
• The stored energy in the H+ gradient is used to chemically synthesize ATP, known as oxidative phosphorylation since the ATP generated is coupled to oxidation
• Approximately 4 H+ ions are needed to produce 1 ATP molecule
• Cytoplasmic NADH from Glycolysis

  • Not converted to NAD+ because there is no receiver of electrons in the cytoplasm
  • To regenerate NAD+, requires that the electrons of NADH must be transported into the mitochondria through electron transport chain
  • Mitochondrial membrane is impermeable to NADH, NAD+ not electrons thus, electrons must be passed on to the receivers and transported via shuttle systems