overview of metabolism
Overview of Metabolism
Preparation to discuss metabolism extensively in class.
Focus will start with catabolism and then proceed to anabolism.
General Metabolic Pathways
All living organisms generally share common metabolic pathways.
Major differences exist among different organisms but core processes are similar.
Metabolic pathways in bacteria often mirror those occurring in human cells.
Metabolism involves breaking down larger molecules into smaller ones to harvest energy for cellular function.
The energy produced is primarily in the form of ATP (Adenosine Triphosphate).
Byproducts of Metabolism
Breakdown of carbohydrates and fatty acids produces energy-depleted byproducts such as carbon dioxide (CO₂).
ATP has a crucial role in biosynthesis processes, e.g., using ATP to convert amino acids into proteins.
Steps of Metabolism
Metabolic processes utilize small steps instead of one large step to optimize energy capture.
This method allows for incremental energy release, which facilitates energy storage in molecules with comparable energy levels.
Metabolism Analogy: Fuel Cell vs. Fire
Metabolism is likened to a fuel cell instead of a fire due to distinct operational parameters:
Fuel cells operate at near ambient temperatures, while fires work at higher temperatures.
In fuel cells, oxygen does not intervene directly with fuel until later in the process, resembling cellular respiration.
Fuel Cell Example: Methanol Fuel Cell
Components: An anode and a cathode.
Anode Reaction: Methanol + Water → 1 CO₂ + 6 H⁺ + 6 e⁻
Electrons can perform useful work, e.g., power a vehicle or charge a battery.
Protons transferred to the cathode, where they and the electrons combine with oxygen to form water.
Oxygen is the ultimate electron acceptor, akin to its role in cellular metabolism.
Contrast with Wood Fire
Wood fire involves direct interaction of oxygen with wood carbohydrates, leading to immediate electron transfer and heat production without useful work.
Metabolic Anode as Citric Acid Cycle
The citric acid cycle utilizes acetyl CoA, a key metabolite from fats, carbohydrates, and amino acids.
For every acetyl CoA entering the cycle, eight electrons are harvested.
Electrons then enter the electron transport chain leading to oxidative phosphorylation and ATP production.
In the electron transport chain, oxygen acts as the terminal electron acceptor, forming water.
The Role of Electron Carriers
Electrons do not pass directly from the citric acid cycle to the electron transport chain.
They first engage with carriers, mainly NADH and FADH₂, which shuttle electrons to the chain.
The electron transport chain consists of series of redox reactions allowing stepwise energy extraction.
Redox Reactions
Fundamental to energy release in metabolism, particularly through oxidation/reduction of carbon.
Energy is primarily acquired through two-electron oxidation processes.
Example of Oxidation Reaction
Methane (CH₄) to Carbon Dioxide (CO₂) involves a series of two-electron steps:
CH₄ → Methanol
Methanol → Formaldehyde
Formaldehyde → Formic Acid
Formic Acid → CO₂
This process necessitates a specific ratio of oxygen per carbon throughout combustive changes.
Ratios of Oxygen to Carbon in Combustion:
Methane: 3 O₂ per 1 C
Methanol: 1.5 O₂ per 1 C
Formaldehyde: 1 O₂ per 1 C
Formic Acid: 0.5 O₂ per 1 C
CO₂ is fully oxidized and requires no further oxidation.
Characteristics of Oxidation in Biochemistry
Biochemical oxidations occur at physiological pH (around 7) and temperature (37°C).
Specific Redox Reaction Example: Ethanol to Acetaldehyde
Initial process involves removing a proton and two electrons from ethanol, forming a carbon-oxygen double bond in acetaldehyde.
Carbon becomes more oxidized, evidenced by increased carbon-oxygen bonds and decreased carbon-hydrogen bonds.
Role of Electron Transfer Agents
NAD⁺ (Nicotinamide adenine dinucleotide) functions primarily as an oxidizing agent in catabolic pathways.
NADP⁺ is differentiated with a phosphate group, serving in anabolic pathways.
The reduction of NAD⁺ to NADH involves adding hydride (H⁻) which reduces its positive charge.
Overall Reaction of NAD⁺ Reduction
Reaction: NAD⁺ + 2 e⁻ + H⁺ → NADH
Redox Pairing in Reactions
Ethanol oxidation is coupled with reduction of NAD⁺ to NADH and produces hydrogen ions, contributing to acidity in active muscle tissue.
Combating Potential and Gibbs Free Energy (ΔG)
Redox reactions can be coupled, similarly to combining exergonic and endergonic reactions.
Each reaction has an associated potential (E) governing spontaneity:
Example: NAD⁺ + 2 e⁻ → NADH potential: -0.32 V.
Acetaldehyde to ethanol potential: -0.2 V.
When combining reactions, reverse the reduction reaction to align electron transfer.
Example Reaction Combining:
Ethanol + NAD⁺ → Acetaldehyde + NADH + H⁺
Calculation of Overall Potential
Use combined potentials to determine spontaneity of reaction (if E is positive, reaction is spontaneous).
Apply Nernst equation to derive Gibbs Free Energy:
where:n = number of moles of electrons transferred,
F = Faraday's constant (96.5 kJ/V·mol),
E = free energy potential for the reaction.
Example Calculation for the Ethanol Reaction
E combined results in:
Conclusion: a positive ΔG indicates non-spontaneity, hence a negative E becomes a catalyst for spontaneous reactions.