Microbial Metabolism: Catabolism and Anabolism

Microbial Metabolism

• Sum of all chemical reactions within an organism
• Divided into two main classes: catabolism and anabolism

Catabolism

• Reactions that generate energy
• Involves breaking down complex organic compounds into simpler ones
• Proteins broken down into amino acids, releasing energy in the process

Anabolism

• Reactions that require energy
• The biosynthesis or building of complex organic molecules from simpler ones
• Amino acids are built into proteins, consuming energy

ATP (Adenosine Tri-Phosphate)

• Form in which energy is stored
• Bonds between the three phosphate groups are high energy
• Breaking these bonds releases energy, driving chemical reactions
• Catabolic reactions generate it
• Anabolic reactions consume it
• ATP converts to ADP + Pi
• ADP converts to AMP + Pi
• ATP converts to AMP + PPi

ATP Coupling

• ATP couples anabolism and catabolism
• Catabolic reactions transfer energy from complex molecules to ATP
• Anabolic reactions transfer energy from ATP to complex molecules

Oxidation-Reduction Reactions

• Oxidation is the loss of an electron from a molecule
• Reduction is the gain of an electron by a molecule
• LEO says GER (Loss of Electrons is Oxidation, Gain of Electrons is Reduction)
• Oxidation and reduction occur simultaneously
• One molecule is oxidized, providing electrons
• Another molecule is reduced, receiving electrons

Nicotinamide Adenine Dinucleotide (NAD+)

• NAD+ is the oxidized form of this molecule
• NADH + H+ is the reduced form
• Acts as an electron carrier
• Electrons originate from donor molecules like glucose
• Two electrons are removed from a donor molecule and added to NAD+
• Two protons (H+) are also transferred

The Collision Theory

• Atoms and molecules are in continuous motion
• Random motion leads to collisions between atoms and molecules
• Collisions can break and form chemical bonds in reactants and products
• A minimum amount of energy, called the activation energy, is needed for a reaction to occur
• Reactions cannot occur if the activation energy is not achieved

Increasing Reaction Rate

• Increased by:
  • Increasing the temperature
  • Increasing the pressure
  • Adding an enzyme

Enzymes

• Catalysts that lower the activation energy of a particular reaction
• Not altered during the reaction and can be reused
• Acts on a specific substrate
• Each enzyme catalyzes only one reaction
• Lower activation energy by positioning the substrate, increasing the probability of a reaction
• Reactions can occur up to 10 billion times faster with an enzyme

Carbohydrate Catabolism

• Breakdown of carbohydrates to release energy
• Glucose is the most common carbohydrate used by cells for energy
• Two main processes to catabolize glucose: cellular respiration and fermentation

Cellular Respiration

• Occurs when oxygen is present
• More efficient than fermentation
• Releases approximately 32 ATP
• Occurs in three main stages:
  • Glycolysis
  • Intermediate step
  • Krebs Cycle (Citric Acid Cycle or TCA Cycle)
  • Electron Transport Chain

Glycolysis

• Splitting of sugar (glucose) into smaller molecules
• Occurs in a series of 10 reactions, each requiring a unique enzyme
• One glucose molecule is converted into two molecules of pyruvic acid (pyruvate)
• Can occur in the presence or absence of oxygen
• Releases a small amount of energy
  • 2 ATP
  • 2 NADH + H+

Intermediate Step

• Prepares pyruvate for entry into Kreb's Cycle
• One carbon dioxide molecule is removed from each pyruvate, releasing a total of 2 CO2
• Coenzyme A (CoA) is added to each of the two newly formed two-carbon sugars
• Addition of CoA forms 2 molecules of acetyl CoA
• A total of 2 NADH + H+ are formed

Krebs Cycle

• Two molecules of acetyl CoA are broken down into 4 CO2
• Produces more NADH + H+ and ATP
  • 2 ATP
  • 6 NADH + H+
  • 2 FADH2
• Occurs in the cytoplasm of prokaryotic cells
• Occurs in the mitochondria of eukaryotic cells
• Requires oxygen to be present

Electron Transport Chain

• Last step of aerobic respiration occurs in the bacterial plasma membrane
• Occurs in the inner mitochondrial membrane of eukaryotes
• Electrons from NADH are transported through a series of electron carriers
• Electrons are deposited on oxygen, forming water
• Chemical formula: 1/2O2 + 2 e- + 2 H+ -> H2O
• Energy is released and used to make ATP during electron transfer
• 2.5 ATP are produced per NADH oxidized
• 1.5 ATP are produced per FADH2
• The entire process of cellular respiration produces 32 ATP per glucose molecule

Fermentation

• Occurs to produce energy when oxygen is absent
• Begins with glycolysis
• No Krebs cycle
• No electron transport chain
• Produces only 2 ATP per glucose molecule
• Two molecules of pyruvate are converted into fermentation products after glycolysis
• Two main types: alcohol fermentation and lactic acid fermentation

Alcohol Fermentation

• Carried out by yeast, fungi, and some bacteria
• Pyruvate is converted into acetaldehyde
• Acetaldehyde is converted into ethanol
• Produces 2 ATP

Lactic Acid Fermentation

• Pyruvate is converted into lactic acid
• Performed by many bacteria, e.g., Lactobacillus acidophilus
• Can cause food spoilage
• Used in food production, e.g., yogurt from lactic acid fermentation of milk sugars
• Produces 2 ATP

Food Catabolism

• Bacteria use glucose as the primary energy source
• They can also use carbohydrates, lipids, and proteins

Carbohydrate Catabolism by Bacteria

• Polysaccharides like starch are broken down into component sugars
• Sugars are converted into glucose
• Glucose enters the metabolic cycle beginning with glycolysis

Lipid Catabolism by Bacteria

• Lipids are broken down into fatty acids and glycerol by the enzyme lipase
• These components enter the metabolic cycle via glycolysis or Kreb's cycle

Protein Catabolism by Bacteria

• Proteins are broken into component amino acids by proteases
• Amino acids enter the metabolic cycle via glycolysis or Kreb's cycle

Anabolic Reactions

• Anabolism is equal to biosynthesis
• Glycolysis and Kreb’s cycle components serve as the starting point for amino acids, lipids, and nucleotides
• These components are precursor metabolites

Metabolic Diversity

• Organisms classified based on nutritional needs
  • Their source of energy
  • Their source of carbon

Energy Source Classification

• Organisms are either chemotrophs or phototrophs

Carbon Source Classification

• Organisms are either heterotrophs or autotrophs

Photoautotrophs

• Use light energy to produce ATP
• Use CO2 as a carbon source
• A.k.a. photosynthesis
• Includes some bacteria, algae, and plants

Photoheterotrophs

• Use light energy to produce ATP
• Use organic carbon from food to satisfy their carbon requirements
• Includes some bacteria

Chemoautotrophs

• Use chemical reactions to obtain energy
• Use CO2 as a carbon source
• Obtain energy from reduced inorganic molecules such as H2, H2S, and CO
• Only done by some bacteria

Chemoheterotrophs

• Use chemical reactions to obtain energy
• Use organic molecules as a source of carbon
• Obtain energy reactions involving reduced organic molecules
• Includes all animals, fungi, protozoa, and most bacteria
• Includes medically relevant bacteria