Energy Flow and Enzyme Catalysis in the Cell: Study Notes

Chapter 5: Bioenergetics – The Flow of Energy in the Cell

Energy Flow in the Cell

Bioenergetics is the study of energy transformations in living cells. Cells require energy to perform essential functions, and this energy is primarily derived from the breakdown of molecules such as glucose.

• Energy Supply: Glucose releases 686 kcal/mole when its bonds are broken. Aerobic respiration breaks down glucose in a series of small reactions, each releasing only a fraction of the total energy stored in glucose at a time.

• Energy Storage: Most cells store energy in the bonds of ATP. ATP hydrolysis produces ADP and Pi and releases 7.3 kcal/mole.

Gibbs Free Energy (ΔG)

Gibbs free energy is a measure of the energy available to do work in a chemical reaction. It determines whether a reaction is spontaneous or requires energy input.

• Exergonic Reactions: These reactions release energy and have a negative ΔG (ΔG < 0). They occur spontaneously and are thermodynamically favorable.

• Endergonic Reactions: These reactions require energy input and have a positive ΔG (ΔG > 0). They are not spontaneous and require coupling to exergonic reactions to proceed.

• Example: Photosynthesis is an endergonic process, while cellular respiration is exergonic.

Reaction Type	ΔG Value	Spontaneity	Example
Exergonic	ΔG < 0	Spontaneous	Cellular Respiration
Endergonic	ΔG > 0	Non-spontaneous	Photosynthesis

Equation:

$\Delta G = \Delta H - T\Delta S$

Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (heat content) $T$ = temperature in Kelvin $\Delta S$ = change in entropy (disorder)

Activation Energy

Even exergonic reactions require an initial input of energy, called activation energy, to proceed. This energy is needed to overcome the energy barrier for the reaction.

• Enzymes lower the activation energy, increasing the rate of reaction without being consumed.

• Example: ATP hydrolysis is catalyzed by enzymes, allowing cellular processes to occur rapidly at physiological temperatures.

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

Chapter 6: Enzymes – The Catalysts of Life

Enzymes

Enzymes are biological catalysts, mostly proteins, that accelerate chemical reactions by lowering activation energy. They are highly specific for their substrates.

• Substrate Binding: Enzymes bind specific reactants (substrates) at their active site, which is a cluster of amino acids forming a unique three-dimensional structure.

• Induced Fit Model: The enzyme changes shape to better fit the substrate upon binding, enhancing catalytic efficiency.

• Optimal Conditions: Enzymes have optimal temperature and pH ranges. For example, human enzymes function best at body temperature and neutral pH, while thermophilic enzymes from extremophiles have different optima.

Enzyme Regulation

Cells regulate enzyme activity to control metabolic pathways and respond to changing conditions.

• Enzyme Inhibition:

  • Competitive Inhibition: Inhibitors resemble the substrate and bind to the active site, blocking substrate access.

  • Noncompetitive Inhibition: Inhibitors bind elsewhere on the enzyme, altering its shape and reducing activity.

• Allosteric Regulation: Enzymes are regulated by molecules binding to sites other than the active site (allosteric sites), causing conformational changes that increase or decrease activity. Allosteric enzymes often control key steps in metabolic pathways.

• Feedback Inhibition: The end product of a pathway inhibits an earlier enzyme, preventing overproduction of the product.

• Covalent Modification: Enzyme activity can be regulated by the addition or removal of chemical groups (e.g., phosphorylation of serine, threonine, or tyrosine residues).

Regulation Type	Mechanism	Example
Competitive Inhibition	Inhibitor binds active site	Sulfa drugs inhibiting bacterial enzymes
Noncompetitive Inhibition	Inhibitor binds allosteric site	Heavy metals inhibiting enzymes
Allosteric Regulation	Effector binds allosteric site	Regulation of phosphofructokinase in glycolysis
Feedback Inhibition	End product inhibits pathway	Threonine biosynthesis pathway
Covalent Modification	Phosphorylation/dephosphorylation	Protein kinases/phosphatases

Additional info: Enzyme regulation is essential for maintaining homeostasis and allowing cells to adapt to environmental changes. Many drugs and toxins act by inhibiting specific enzymes.