MCB3020 Week 4: Metabolism and Metabolic Diversity

Aerobic respiration

involves the complete oxidation of glucose using oxygen as the terminal electron acceptor, yielding the highest ATP per glucose (approximately 36 ATP). It includes glycolysis, the citric acid cycle, and the electron transport chain.

Fermentation

an anaerobic process where organic compounds serve as both electron donors and acceptors. It produces only 2 ATP per glucose and regenerates NAD+ for glycolysis to continue. Typical products include lactic acid or ethanol and CO₂.

Anaerobic respiration

uses inorganic molecules other than oxygen (like nitrate, sulfate, or ferric iron) as terminal electron acceptors. It proceeds via electron transport chains similar to aerobic respiration but yields less ATP due to less energetic electron acceptors.

Energy yield

highest in aerobic respiration, lowest in fermentation.

Electron acceptors

oxygen in aerobic, inorganic molecules in anaerobic, organic molecules in fermentation.

Pathway completeness

complete oxidation in aerobic and anaerobic respiration; partial in fermentation.

Glycolysis

occurs in the cytoplasm; converts glucose (C₆H₁₂O₆) into two pyruvate molecules, producing 2 ATP and reducing NAD+ to NADH. Enzymes involved include hexokinase, phosphofructokinase, and pyruvate kinase.

Pyruvate oxidation

pyruvate is transported into the mitochondria (or equivalent in prokaryotes) and converted into acetyl-CoA, releasing CO₂ and generating NADH.

Citric acid cycle (Krebs cycle)

acetyl-CoA combines with oxaloacetate to produce CO₂, NADH, FADH₂, and a small amount of ATP (via substrate-level phosphorylation). Key enzymes include citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase.

Electron transport chain (ETC)

NADH and FADH₂ donate electrons to membrane-bound carriers (e.g., NADH dehydrogenase, cytochromes). Electrons flow through the chain, ultimately reducing oxygen to water. The energy released pumps protons across the membrane, creating a proton motive force.

ATP synthesis

via ATP synthase, harnessing the proton motive force to produce ATP (oxidative phosphorylation).

Free energy (G)

the energy available to perform work.

Exergonic reactions

have a negative ΔG (free energy change), releasing energy, and are spontaneous.

Endergonic reactions

have a positive ΔG, require an input of energy, and are non-spontaneous.

Determining reaction type

based on the sign and magnitude of ΔG, which depends on reactant and product energies.

Activation energy (Ea)

the minimum energy needed to initiate a reaction by bringing reactants into a reactive state.

Enzymes

biological catalysts, mostly proteins, that increase reaction rates by lowering activation energy.

Active sites

specific regions where substrates bind via weak interactions (hydrogen bonds, van der Waals, hydrophobic).

Specificity

dictated by the enzyme's shape and binding site compatibility.

Prosthetic groups

tightly bound cofactors (e.g., heme in cytochromes).

Coenzymes

loosely bound, often derived from vitamins (e.g., NAD+/NADH, FAD/FADH₂), participate in redox reactions without being consumed.

Energy conservation

in microbes, hinges on redox reactions, where electrons are transferred from donors to acceptors. The energy released during these transfers is harnessed to generate ATP or store energy in high-energy compounds. For example, NADH oxidation releases electrons that drive proton pumping in the electron transport chain, creating a proton motive force.

Electron donor

molecule that is oxidized (loses electrons).

Electron acceptor

molecule that is reduced (gains electrons). Example

Redox couple

a pair of chemical species involved in electron transfer (e.g., NAD+/NADH).

Reduction potential (E0′)

measures a compound's tendency to gain electrons; more positive E0′ indicates a better electron acceptor.

Redox tower

compounds with higher E0′ tend to accept electrons from those with lower E0′, reactions proceed spontaneously from donors with lower E0′ to acceptors with higher E0′.

Redox tower

The redox tower arranges redox couples by their reduction potentials from most negative (good donors) at the top to most positive (good acceptors) at the bottom.

In oxidation

NADH donates electrons, becoming oxidized to NAD+.

In reduction

NAD+ gains electrons to form NADH. This cycling is central to redox reactions in metabolism, facilitating energy transfer.

Generation

during electron transport, complexes pump protons (H+) from the cytoplasm to the outside of the membrane, creating an electrochemical gradient.

Components

a pH gradient (more H+ outside) and an electrical potential (inside negative).

Functions of PMF

Drives ATP synthesis via ATP synthase, Powers flagellar rotation, Facilitates active transport of nutrients and waste products.

Fermentation

organic electron donors and acceptors, no external electron acceptor.

Aerobic respiration

organic or inorganic donors with oxygen as acceptor.

Anaerobic respiration

inorganic donors with acceptors like nitrate, sulfate, or ferric iron.

Chemolithotrophy

inorganic donors (H₂S, Fe²⁺) with oxygen or other acceptors.

Phototrophy

light as energy source, with or without inorganic electron donors. Each pathway varies in energy yield and environmental adaptation.

Gluconeogenesis

synthesizes glucose from non-carbohydrate precursors such as pyruvate, lactate, or amino acids. It requires energy input (ATP, GTP) and builds glucose molecules, thus classified as an anabolic pathway essential for maintaining blood glucose levels and biosynthesis.

Metabolism

total of all biochemical reactions in a cell.

Catabolism

energy-releasing breakdown of molecules to produce energy and precursors.

Anabolism

energy-consuming synthesis of complex molecules from simpler ones.

Exergonic reactions

release free energy (ΔG < 0), spontaneous.

Endergonic reactions

require energy input (ΔG > 0).

Terminal electron acceptor

molecule at the end of an electron transport chain that receives electrons (e.g., O₂ in aerobic respiration, nitrate in anaerobic respiration).