Lecture 15 Metabolism

What type of system is the human body regarding thermodynamics, and what does this imply?

The human body is an isothermal system operating at a constant temperature and pressure, meaning heat flow is not a source of energy; cells instead rely entirely on free energy ($\Delta G$).

How do our bodies handle total ATP storage limitations?

Because the body has a very small limit on total ATP storage and cannot simply synthesize large amounts out of nothing, it rapidly converts ADP or AMP back into ATP to maintain cellular energy balance.

What is the definition of metabolism?

The entire network of interconnected, highly regulated chemical reactions that take place in the body to allow cells to acquire, alter, store, and utilize energy.

What are the two major divisions of metabolism, and what are their overall roles?

Anabolism (the synthesis of large organic molecules from smaller units, requiring energy/ATP) and catabolism (the degradation of complex biomolecules into simpler products, releasing energy/ATP).

How do anabolism and catabolism differ in terms of REDOX reactions?

Catabolic pathways are oxidative and lead to the reduction of cofactors (producing NADH, NADPH, $FADH_2$), whereas anabolic pathways are reductive and lead to the oxidation of cofactors ($NAD^+$, $NADP^+$, FAD).

What biochemical changes define an oxidation reaction versus a reduction reaction?

Oxidation occurs when a carbon loses a hydride ion ($H:$) or gains/increases bonds to oxygen; reduction is the exact opposite (gaining a hydride ion or losing/decreasing bonds to oxygen).

What are the main functions of ATP in cellular processes?

ATP acts as the energy currency of life by donating phosphate groups or AMP, and by driving otherwise thermodynamically unfavorable reactions.

What role do thioesters like Acetyl-CoA play in cellular energy?

Thioesters have a high standard free energy of hydrolysis ($\Delta G^{\circ\prime} = -31.4~kJ/mol$), making them rich sources of energy for driving metabolic acyl transfer reactions.

What are the three structural patterns of metabolic pathways?

Metabolic pathways can be linear, circular, or branched (which can be converging or diverging).

Why does metabolism proceed through discrete, multi-step pathways rather than uncontrolled combustion?

Multistep pathways control energy release, prevent large destructive energy bursts or losses, and create essential control points for regulation in response to changing cellular conditions.

What is the difference between metabolic equilibrium and steady state?

At equilibrium, the forward and reverse reaction rates are equal with no net flow; at steady state, reactants are continuously supplied and removed at matching rates (flux), keeping intermediate concentrations constant while allowing continuous work.

Why is a steady state required in metabolism?

It ensures that cells have continuous, immediate access to products without waiting for synthesis, allows for rapid pathway regulation, and prevents energy loss.

What is metabolic compartmentalization, and why is it beneficial?

The segregation of metabolic processes into distinct cellular locations (e.g., cytosol vs. mitochondria), which allows separate metabolite pools, simultaneous opposing pathways, high local concentrations, and coordinated enzyme regulation.

What are the two primary types of cofactors required by enzymes?

Metal ions and coenzymes (many of which are derived from vitamins that must be obtained from the diet).

What are activator ions versus metalloenzymes?

Activator ions are loosely associated mono- and divalent cations (like $K^+$, $Ca^{2+}$, $Mg^{2+}$) that help facilitate reactions without direct participation (e.g., shielding charges), whereas metalloenzymes have tightly bound transition metals (like Fe, Zn, Cu) that often act directly as electrophiles or in REDOX steps.

What is the structural and functional difference between cosubstrates and prosthetic groups?

Cosubstrates bind at the active site, are chemically altered during the reaction, dissociate, and are later regenerated by another enzyme; prosthetic groups remain permanently bound (covalently or noncovalently) to the enzyme active site and return to their original state after each catalytic cycle.