Life Science Uge 3: Carbohydrates, Energy, and Enzyme Regulation

Comprehensive practice flashcards reviewing carbohydrate structure, bioenergetics, thermodynamics, and enzyme function from Key Concepts 3.3 and 8.1–8.5.

Anabolism (anabolisme)

Metabolic reactions (metaboliske reaktioner) that construct complex molecules from simpler ones, requiring an input of free energy ($\Delta G > 0$).

Catabolism (katabolisme)

Metabolic pathways (metaboliske veje) that break down complex molecules into simpler compounds, releasing stored chemical energy ($\Delta G < 0$).

Exergonic Reaction (exergonisk reaktion)

A chemical reaction that proceeds spontaneously with a net release of usable free energy ($\Delta G < 0$).

Endergonic Reaction (endergonisk reaktion)

A non-spontaneous chemical reaction that requires an input or absorption of free energy ($\Delta G > 0$) to proceed.

Enzymes (enzymer)

Biological catalysts (biologiske katalysatorer), typically proteins or RNA molecules, that accelerate biochemical transformation rates without being permanently altered.

Active Site (aktivt site)

The specific three-dimensional catalytic pocket or region of an enzyme (enzym) where substrate (substrat) molecules bind.

Substrate (substrat)

The specific reactant molecule upon which an enzyme (enzym) binds and acts during a biochemical reaction.

Activation Energy (aktiveringsenergi)

The initial input of energy ($E_a$) required to push reactant molecules into an unstable transition state (transitionstilstand).

Allosteric Regulation (allosterisk regulering)

The regulatory process where a effector molecule binds to an enzyme (enzym) at a distinct non-active site (allosterisk site), inducing a shape change that alters substrate affinity.

Competitive Inhibition (kompetitiv hæmning)

Reversible enzyme inhibition where an inhibitor molecule directly competes with the substrate (substrat) for active site binding.

Non-competitive Inhibition (non-kompetitiv hæmning)

Reversible enzyme inhibition where an inhibitor binds to a site distinct from the active site (aktivt site), altering enzyme structure regardless of substrate presence.

Feedback Inhibition (feedback-hæmning)

A regulatory mechanism where the final end-product of a metabolic pathway (metabolisk vej) allosterically inhibits the enzyme catalyzing the initial commitment step (etableringstrin).

Metabolic Pathways (metaboliske veje)

Interconnected series of enzyme-catalyzed chemical reactions where the product of one step serves as the reactant for the next.

Enzyme-Substrate Complex (enzym-substrat kompleks)

A temporary molecular intermediate formed when substrate molecules bind non-covalently to the active site (aktivt site) of an enzyme.

Transition State (transitionstilstand)

An unstable, high-energy reactive state that reactants must reach before chemical bonds can break and products form.

Uncompetitive Inhibition (ukompetitiv hæmning)

Reversible inhibition where an inhibitor binds exclusively to the pre-formed enzyme-substrate complex (enzym-substrat kompleks), preventing product release.

First Law of Thermodynamics (termodynamikkens første lov)

The fundamental principle stating that energy cannot be created or destroyed, keeping total system energy constant before and after transformations.

Second Law of Thermodynamics (termodynamikkens anden lov)

The principle stating that energy transformations increase entropy ($S$) and disorder, rendering a portion of energy unavailable to perform work.

Free Energy (fri energi)

The portion of total system energy ($G$) available to perform work at constant temperature and pressure, defined by $G = H - TS$.

Coenzyme (koenzym)

A relatively small non-protein organic molecule that temporarily binds to an enzyme's active site (aktivt site) to transfer chemical groups or electrons.

How do enzymes (enzymer) accelerate biochemical reaction rates without altering overall chemical equilibrium (kemisk ligevægt) or free energy change ($\Delta G$)?

Enzymes lower the activation energy ($E_a$) barrier for both forward and reverse reactions equally. Because free energy change ($\Delta G$) depends exclusively on initial reactant and final product states, equilibrium position and net energy changes remain unchanged.

Why can competitive inhibition (kompetitiv hæmning) be overcome by increasing substrate (substrat) concentration, whereas non-competitive inhibition (non-kompetitiv hæmning) cannot?

Competitive inhibitors compete directly for the active site (aktivt site); elevated substrate concentration outcompetes inhibitor binding. Non-competitive inhibitors bind to a distinct regulatory site, altering enzyme conformation and catalytic capacity regardless of substrate saturation.

What is the energetic advantage of targetting the commitment step (etableringstrin) during feedback inhibition (feedback-hæmning) of a metabolic pathway (metabolisk vej)?

Inhibiting the initial irreversible step prevents the cell from expending free energy ($G$) and precursor molecules on downstream intermediate synthesis when final metabolic products are already abundant.

How does elevated temperature impact enzyme activity (enzymaktivitet) before and after surpassing an enzyme's optimal temperature (optimal temperatur)?

Below optimum, increasing temperature adds kinetic energy, raising collision rates. Beyond optimum, excessive thermal energy breaks weak non-covalent interactions (hydrogen bonds), leading to denaturation (denaturering) and loss of active site structure.

How do shifts in environmental pH alter substrate binding at an enzyme's active site (aktivt site)?

pH changes alter the ionization state of acidic and basic amino acid side chains ($\text{—COOH}$ vs $\text{—COO}^-$ and $\text{—NH}_2$ vs $\text{—NH}_3^+$). This disrupts ionic attractions and hydrogen bonds critical for tertiary structure and substrate binding.

How does energy coupling (energikobling) enable endergonic reactions (endergoniske reaktioner) to take place in biological systems?

Cells pair an endergonic process ($\Delta G > 0$) with a strongly exergonic process ($\Delta G < 0$), typically ATP hydrolysis (ATP-hydrolyse). If the net sum of free energy changes is negative ($\Delta G_{\text{net}} < 0$), the combined reaction proceeds spontaneously.

What structural feature accounts for the high free energy released during ATP hydrolysis (ATP-hydrolyse)?

The three adjacent negatively charged phosphate groups in ATP exert mutual electrostatic repulsion. Hydrolysis of the phosphoanhydride bond (phosphoanhydridbinding) relieves repulsion, yielding ADP and inorganic phosphate ($\text{P}_i$) in a much lower free energy state.

What three physical and chemical catalytic mechanisms do enzymes (enzymer) utilize at the active site (aktivt site) to lower activation energy ($E_a$)?

1. Substrate orientation (substratorientering): positioning reactants precisely for bonding. 2. Physical strain (fysisk spænding): stretching bonds toward their transition state (transitionstilstand). 3. Chemical group addition (kemisk gruppetilsætning): utilizing acid-base or covalent side-chain interactions.

How does the induced fit (induceret pasform) model refine the traditional lock-and-key concept of enzyme catalysis?

Induced fit describes a dynamic structural shift where substrate binding causes conformational changes in the enzyme. This aligns catalytic amino acid side chains around the substrate while excluding water from the active site.

Why does an enzyme-catalyzed reaction reach a maximum velocity ($V_{\max}$) at high substrate (substrat) concentrations?

Substrate saturation (substratmætning) occurs when every active site (aktivt site) is continuously occupied. At this point, free enzyme concentration becomes the rate-limiting factor, preventing further rate increases.

How do allosteric activators (allosteriske aktivatorer) increase the catalytic rate of multisubunit enzymes?

Allosteric activators bind to regulatory sites (allosteriske siter) and stabilize the enzyme's active form (aktiv konformation), enhancing substrate binding affinity across catalytic subunits.

Why do reaction velocity plots against substrate concentration show a sigmoidal curve for allosteric enzymes (allosteriske enzymer) rather than a hyperbolic curve?

Allosteric enzymes display subunit cooperativity (kooperativitet): initial substrate binding to one subunit induces conformational shifts in adjacent subunits, rapidly increasing substrate affinity in a non-linear sigmoidal manner.

What defines the unique binding dynamics of uncompetitive inhibition (ukompetitiv hæmning)?

Uncompetitive inhibitors bind exclusively to the pre-formed enzyme-substrate complex (enzym-substrat kompleks). Because binding requires prior substrate attachment, adding more substrate cannot overcome the inhibition.

By what mechanism does aspirin act as an irreversible inhibitor (irreversibel hæmmer) of cyclooxygenase (COX)?

Aspirin covalently transfers an acetyl group to a serine amino acid side chain near the active site of cyclooxygenase. This permanent modification physically blocks substrate entry, preventing prostaglandin synthesis.

How do cells dynamically regulate enzyme activity via reversible phosphorylation (reversibel phosphorylering)?

Protein kinases (proteinkinaser) transfer phosphate groups from ATP to specific amino acid side chains, inducing shape changes that activate or deactivate enzymes. Protein phosphatases (proteinphosphataser) reverse this by removing the phosphate.

What condition defines chemical equilibrium (kemisk ligevægt) regarding reaction rates and free energy change ($\Delta G$)?

Chemical equilibrium is attained when forward and reverse reaction rates are identical. Net product and reactant concentrations remain constant, resulting in zero free energy change ($\Delta G = 0$).

How do highly organized cells maintain internal structural order without violating the second law of thermodynamics (termodynamikkens anden lov)?

Cells remain open systems that consume free energy ($G$) and catabolize complex nutrients, releasing heat and simple waste molecules ($\text{CO}_2$, $\text{H}_2\text{O}$) that increase the net entropy ($S$) of the universe.

What occurs during acid-base catalysis (syre-base katalyse) within an active site (aktivt site)?

Acidic or basic amino acid side chains in the active site transfer protons ($\text{H}^+$) to or from the substrate, destabilizing covalent bonds to facilitate bond cleavage or formation.

What function do metal ion cofactors (metalionkoenzymer) perform in enzyme-catalyzed oxidation-reduction reactions (oxidations-reduktionsreaktioner)?

Bound metal ions (e.g., $\text{Fe}^{2+}$, $\text{Cu}^{2+}$) reversibly gain or lose electrons without detaching from the enzyme matrix, stabilizing charge development and facilitating electron transfer.

Why does the discovery of ribozymes (ribozymer) support the 'RNA world' hypothesis for the origin of life?

Ribozymes demonstrate that RNA molecules can act as self-catalytic structural scaffolds that catalyze vital biochemical reactions (such as peptide bond formation) without requiring protein enzymes.

What key functional distinction separates prosthetic groups (prostetiske grupper) from coenzymes (koenzymer)?

Prosthetic groups are permanently bound non-amino acid molecular partners (e.g., heme, FAD) attached to enzymes, whereas coenzymes are transient organic carriers (e.g., NAD, ATP) that bind and release dynamically.

Given an endergonic conversion with $\Delta G_1 = +4.0\,\text{kcal\,mol}^{-1}$ coupled to ATP hydrolysis with $\Delta G_2 = -7.3\,\text{kcal\,mol}^{-1}$, what is the overall net free energy change ($\Delta G_{\text{net}}$)?

Summing individual free energy changes gives $\Delta G_{\text{net}} = \Delta G_1 + \Delta G_2 = +4.0 + (-7.3) = -3.3\,\text{kcal\,mol}^{-1}$. Because $\Delta G_{\text{net}} < 0$, the overall coupled reaction proceeds exergonically.

How does standard free energy change ($\Delta G^\circ$) differ from actual cellular free energy change ($\Delta G$)?

Standard free energy change ($\Delta G^\circ$) is measured under fixed lab conditions ($25\,^\circ\text{C}$, $1\,\text{atm}$, $1\,\text{M}$ solutes, $\text{pH } 7$). Actual free energy change ($\Delta G$) depends on real-time cellular concentrations of reactants and products.

How does systems biology (systembiologi) model complex metabolic pathways (metaboliske veje) within cells?

Systems biology uses computational algorithms to map intersecting enzymatic reaction pathways as interdependent networks, predicting system behavior when enzyme activity or metabolite concentration changes.

Why do animal cells store glucose (glukose) as the polysaccharide glycogen (glykogen) rather than free monosaccharide molecules?

Storing $1000$ free glucose molecules creates $1000$ times the osmotic pressure (osmotisk tryk) of a single glycogen macromolecule. Glycogen storage prevents excessive water entry and osmotic swelling while maintaining stored energy.