Biochemical Energetics, Enzymes, and Metabolism
Energy, Reactions, and Catalysis
In many reactions you don’t need to apply energy: some chemicals are very stable (e.g., potassium permanganate) and won’t react on their own. In other cases, energy must be supplied to start the reaction (e.g., a match being struck).
Graphically, consider two kinds of reactions: spontaneous (doesn't need input energy to proceed) versus reactions that require energy input to proceed (endergonic) or to proceed faster (energy barriers).
Activation energy concept: you typically must overcome a barrier (activate bonds) to get a reaction going. Heating can supply energy to destabilize bonds and speed up reactions.
Heat as energy and temperature: heat increases molecular motion, leading to more collisions and faster reactions. Temperature is the average kinetic energy of molecules.
Lowering activation energy via catalysts: a catalyst provides an alternative pathway with a lower energy barrier, so the reaction can proceed more rapidly at a given temperature.
Uncatalyzed vs catalyzed barriers can be visualized as two hills (activation barriers); the catalyzed path has a lower hump.
Major biochemical energy systems
Photosynthesis and cellular respiration are two key energy-related pathways.
Photosynthesis (capture light energy to drive synthesis of glucose):
Involves photosystems I and II to capture extra energy for chemical synthesis.
Cellular respiration (release chemical energy by oxidizing glucose):
Outputs ATP used to power cellular processes.
Photosystems and energy capture are part of the light-dependent reactions; later lectures will cover details.
ATP: the cellular energy currency
Adenosine triphosphate (ATP) is the primary energy currency in cells.
Structure: a ribose sugar (a five-carbon sugar with five oxygens, as noted in the transcript), adenine, and three phosphate groups.
When ATP is cleaved, energy is released to drive nearby reactions:
The phosphates are relatively unstable; breaking the bonds releases usable energy for endergonic reactions.
The ATP/ADP cycle:
ADP + (P_i) can be recharged back to ATP in mitochondria via cellular respiration.
ATP acts as a carrier of energy, not a long-term storage molecule like sugars or fats.
Energy storage and usage:
Partially charged battery analogy: ATP with some phosphate groups ready to transfer energy; fully charged ATP stores usable energy for immediate needs.
Energy from sugars is ultimately used to restore ATP from ADP and Pi.
Role of water and hydration:
Water is involved in ATP replenishment and cellular respiration, underscoring why good hydration supports energy metabolism.
Lifespan considerations (depicted as ideas in the talk):
Lifespan without food is variable in the notes: commonly quoted ranges include a few days up to a month-and-a-half, with ~50 days mentioned as a rough estimate in the transcript.
Without water, energy production becomes critical very quickly: the brain is an energy-hungry organ.
Enzymes: biological catalysts
Enzymes are biological catalysts that speed up chemical reactions without being consumed.
They often act on one specific substrate and convert it to products, remaining after the reaction to catalyze more reactions.
Hydrogen peroxide example:
Hydrogen peroxide is toxic if accumulated; the enzyme catalase breaks it down rapidly to produce water and oxygen:
Carbonic anhydrase example:
Catalyzes the hydration of CO₂ to carbonic acid:
This enzyme is extremely efficient (hundreds of thousands of molecules processed per second in beaker experiments described).
Enzyme specificity and active sites:
The active site is the region where the substrate binds.
Lock-and-key analogy: substrate fits precisely into the active site like a key fits a lock.
Enzyme–substrate complex formation distorts certain bonds, placing tension that makes bond breakage easier.
Example: lactose digestion by lactase; lactose intolerance occurs when the specific enzyme is lacking or reduced.
Multi-enzyme complexes and channeling:
In many pathways, enzymes work in series, where the product of one reaction is the substrate for the next.
Some pathways involve multi-enzyme complexes acting like mini-factories to streamline conversions.
RNA as a catalyst (ribozymes):
In 1981, it was demonstrated that RNA can catalyze certain reactions, changing our view of catalysis.
Intramolecular catalysis: RNA catalyzing reactions within itself.
Intermolecular catalysis: RNA acting on another molecule, not just its own sequence.
Enzyme kinetics and regulation
Dependence on concentrations:
Higher substrate concentration generally speeds up the reaction (up to saturation limits).
Higher enzyme concentration generally increases reaction rate as well.
Temperature and pH effects:
Enzymes have an optimal temperature (around ~37°C for many human enzymes).
At very low temperatures, activity is minimal; at higher temperatures (e.g., ~45°C), enzymes can denature and lose function.
Denaturation is the loss of native 3D structure due to extreme conditions.
Inhibition and regulation:
Competitive inhibition: inhibitor competes with substrate for the active site (e.g., oxygen vs. carbon monoxide as a theoretical example).
Non-competitive (allosteric) inhibition: inhibitor binds to a site other than the active site, changing enzyme shape so the active site cannot catalyze efficiently.
Allosteric activators can turn on enzyme activity; allosteric inhibitors turn it off.
Deactivation and reactivation can be dynamic and regulated as needed (e.g., insulin lowering blood sugar via promoting uptake of glucose; glucagon promoting glycogen breakdown to raise blood sugar).
Cofactors and coenzymes in regulation:
Cofactors: inorganic helpers bound to enzymes (e.g., iron in hemoglobin at the active site for oxygen binding; copper in some organisms).
Coenzymes: organic molecules (often vitamins) that assist enzyme function.
Hemoglobin color changes reflect different metals in cofactors (iron makes red blood; copper-containing systems can be bluish; cobalt-containing compounds can be purplish).
Allosteric regulation in metabolism:
Enzymes can be switched on/off by allosteric effectors, enabling rapid responses to metabolic needs.
Cofactors, coenzymes, and metal roles
Cofactors:
Inorganic cofactors often essential for catalysis or structural stabilization.
Examples include iron in hemoglobin (oxygen transport) and copper in certain biological contexts (blue color in some arthropod blood).
Coenzymes:
Organic molecules (often vitamins) that participate in the chemical reaction by transferring electrons, atoms, or functional groups.
Terminology and implications:
Enzymes with non-protein components depend on these helpers for activity.
Some diseases or metabolic issues arise from deficiencies in cofactors or coenzymes.
Pathways and metabolic contexts
Multistep processes:
Many biosynthetic pathways are not single-step; they proceed through multiple enzymes in sequence (e.g., the Calvin cycle in photosynthesis building glucose through stepwise carbon fixation).
Calvin cycle (overview):
A multistep process that assembles a six-carbon sugar through successive carbon additions, mediated by multiple enzymes.
Electron transport chain (ETC):
A sequence of membrane-bound proteins that transfer electrons from one carrier to the next, creating a proton gradient used to synthesize ATP.
Practical lab and study notes:
In lab settings, you may vary substrate concentration or enzyme amount to observe effects on reaction rates.
Enzyme structure (primary, secondary, tertiary, quaternary) can be altered by extreme temperatures or pH, affecting catalytic activity.
Digestion and human physiology context
Digestive enzymes operate most effectively in the right pH environments and temperatures within the body.
Stomach digestion:
The stomach provides acidic conditions to begin digestion of proteins; chyme moves to the small intestine where pH changes and most digestion occurs.
Small intestine:
The majority of digestion and absorption occurs here; enzymes act to break down carbohydrates and proteins into absorbable units.
Enzyme activity and safety:
Considerations about enzyme inhibitors or denaturation apply to the digestive system as well as lab experiments.
Examples, cautions, and practical takeaways
Practical chemical hazards from the transcript:
Heterogeneous reactions such as potassium perchlorate + catalyst can be dangerous if performed improperly.
Hydrogen peroxide, while useful as a lab reagent, is toxic at high concentrations; cells have enzymes (e.g., catalase) to prevent buildup.
Theoretical biology notes:
Enzymes are highly specific; many catalyze a single reaction with a single substrate.
Some organisms rely on specialized enzymes to digest dietary fibers (cellulose) that humans otherwise cannot break down.
The broader message:
Energy capture and release in biology hinges on chemical bonds, catalysts, and tightly regulated pathways that ensure life-sustaining chemistry proceeds efficiently and safely.
Quick recap of key terms
Activation energy: the energy barrier that must be overcome for a reaction to proceed.
Catalyst: a substance that lowers the activation energy, increasing the rate of a reaction without being consumed.
Endergonic: a reaction that requires input of energy (ΔG > 0).
ATP (adenosine triphosphate): the primary energy currency of the cell; ATP hydrolysis yields usable energy.
ADP (adenosine diphosphate) and Pi (inorganic phosphate): products of ATP hydrolysis; recycled to ATP.
Active site: the enzyme region where the substrate binds.
Enzyme–substrate complex: the temporary complex formed during catalysis, often with bond distortion to facilitate reaction.
Lock-and-key model: classic metaphor for substrate specificity to the active site.
Competitive inhibition: inhibitor competes with substrate at the active site.
Allosteric inhibition/activation: regulators bind at sites other than the active site to modulate activity.
Cofactor: inorganic helper required for enzyme activity.
Coenzyme: organic molecule (often a vitamin) that assists enzyme function.
Ribozymes: RNA molecules with catalytic activity (intramolecular and intermolecular catalysis).
Calvin cycle: multi-enzyme process for glucose production in photosynthesis.
Electron transport chain (ETC): series of proteins transferring electrons to generate ATP.
Denaturation: loss of enzyme structure and function due to extreme conditions.
Digestion: enzymatic breakdown of food components into absorbable units.