L13.Metabolic Flux, Energy Transformations, and Thermodynamic Coupling

Vocabulary practice flashcards covering cellular bioenergetics, metabolic flux regulation, thermodynamic coupling, oxidation-reduction potentials, and phosphoryl group transfer energetics.

Metabolism

The sum of all energy transformations and chemical reactions within a cell, encompassing both catabolic and anabolic pathways.

Catabolic Pathways

Metabolic pathways that break down foodstuffs and complex molecules into smaller cellular building blocks, releasing usable free energy and producing heat as a byproduct.

Anabolic Pathways

Biosynthetic metabolic pathways that consume energy to assemble small cellular building blocks into the macromolecules required for cellular function and survival.

Metabolic Flux

The rate of metabolite flow through a metabolic reaction or pathway, controlled by altering cellular conditions to allow specific pathways to run.

Four Mechanisms of Controlling Metabolic Flux

1) Adjusting enzyme quantity via transcription, translation, or degradation; 2) Regulating enzyme catalytic activity directly (such as by phosphorylation or inhibition); 3) Altering reactant and/or product concentrations; 4) Thermodynamic coupling to an exergonic reaction.

Thermodynamic Coupling

The linking of an endergonic (unfavorable) reaction with an exergonic (favorable) reaction through a shared chemical intermediate, driving the overall coupled reaction forward with a net negative $\Delta G$.

Standard Reduction Potential ($E^{\circ\prime}$)

A quantitative measure in volts ($\text{V}$) of a molecule's affinity to accept electrons under standard conditions ($\text{pH} = 7$), measured relative to the standard hydrogen reference ($H_2/H^+ = 0\,\text{V}$).

Electron Carriers

Coenzymes (such as $NAD^+$ and $FAD$) that accept, shuttle, and donate electrons between oxidation and reduction half-reactions in cellular metabolism.

Redox Free Energy Equation

The thermodynamic equation $\Delta G^{\circ\prime} = -nF\Delta E^{\circ\prime}$, which relates the change in reduction potential ($\Delta E^{\circ\prime}$) to standard free energy ($\Delta G^{\circ\prime}$), where $n$ is the number of transferred electrons and $F$ is Faraday's constant.

Faraday's Constant ($F$)

A proportionality constant used to calculate free energy changes in redox reactions, equal to $96.485\,\text{kJ}/(\text{V}\cdot\text{mol})$.

Spontaneity Rule for Coupled Redox Reactions

A coupled redox reaction is predicted to be thermodynamically spontaneous and favorable in a cell when it possesses a positive $\Delta E^{\circ\prime}$, yielding an overall negative $\Delta G^{\circ\prime}$.

Phosphoryl Group Transfer Potential

The tendency of a phosphorylated compound to transfer its phosphate group to another molecule or water, quantified by the magnitude of its standard free energy of hydrolysis ($\Delta G^{\circ\prime}$).

Phosphoanhydride Bonds of ATP

The two high-energy chemical bonds connecting the phosphate groups ($P-O-P$ linkages) of ATP, the cleavage of which releases substantial free energy.

Adenosine Triphosphate (ATP)

The primary and most versatile cellular energy intermediate carrying phosphate groups, positioned in the middle of the phosphoryl group transfer hierarchy ($\Delta G^{\circ\prime} = -30.5\,\text{kJ/mol}$ for outer bond hydrolysis) to balance both energy capture and donation.

Sequential Cleavage of ATP

A pathway of energy release where the terminal phosphoanhydride bond of ATP is hydrolyzed to produce ADP and $P_i$ ($\Delta G^{\circ\prime} = -30.5\,\text{kJ/mol}$), followed if needed by hydrolysis of ADP to AMP and $P_i$ ($\Delta G^{\circ\prime} = -32.8\,\text{kJ/mol}$).

Pyrophosphate Cleavage of ATP

A pathway of energy release where the inner phosphoanhydride bond of ATP is hydrolyzed to generate AMP and inorganic pyrophosphate ($PP_i$, $\Delta G^{\circ\prime} = -46.5\,\text{kJ/mol}$), with $PP_i$ subsequently cleaved into two $P_i$ ($\Delta G^{\circ\prime} = -19.2\,\text{kJ/mol}$).

Phosphocreatine (Creatine Phosphate)

A rapid-access energy reservoir in skeletal muscle with a high standard free energy of hydrolysis ($\Delta G^{\circ\prime} = -43.1\,\text{kJ/mol}$) that can phosphorylate ADP to generate ATP with an overall $\Delta G^{\circ\prime} = -12.6\,\text{kJ/mol}$.

Phosphoenolpyruvate (PEP)

A phosphorylated intermediate with the highest phosphoryl transfer potential listed in cellular metabolism, releasing $\Delta G^{\circ\prime} = -61.9\,\text{kJ/mol}$ upon hydrolysis to pyruvate and inorganic phosphate ($P_i$).