Metabolism: Transforming Matter and Energy

Overview of Metabolism

Metabolism encompasses all chemical reactions within a cell, including those that consume or generate energy. It is an emergent property of life, arising from the orderly interactions between molecules. Metabolic pathways consist of a series of steps, each catalyzed by a specific enzyme, transforming a starting molecule into a product.

• Catabolic pathways break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

Metabolic Pathways

Metabolic pathways are stepwise series of reactions, each catalyzed by a specific enzyme. This organization allows for regulation and efficiency in cellular processes.

Forms of Energy and Thermodynamics

Types of Energy

Energy is the capacity to cause change and can be used to do work. It exists in several forms:

• Kinetic energy: Energy associated with motion.

• Potential energy: Energy possessed due to location or structure.

• Chemical energy: Potential energy available for release in a chemical reaction.

The Laws of Thermodynamics

Biological processes are governed by the laws of thermodynamics:

• First Law: Energy can be transferred and transformed, but not created or destroyed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Free Energy and Spontaneity

Gibbs Free Energy (ΔG)

Free energy is the portion of a system’s energy that can do work at constant temperature and pressure. The change in free energy (ΔG) determines whether a reaction is spontaneous:

• ΔG = ΔH – TΔS

• ΔG: Change in free energy

• ΔH: Change in enthalpy (total energy)

• ΔS: Change in entropy

• T: Temperature in Kelvin

Spontaneous processes have negative ΔG and increase stability.

Exergonic vs. Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

• Exergonic reactions: Net release of free energy (ΔG < 0), spontaneous.

• Endergonic reactions: Absorb free energy (ΔG > 0), nonspontaneous.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) consists of ribose, adenine, and three phosphate groups. It is a nucleotide and serves as the primary energy carrier in cells.

ATP Hydrolysis and Energy Coupling

Energy is released when ATP is hydrolyzed (terminal phosphate removed), driving cellular work by coupling exergonic and endergonic reactions.

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, powering endergonic reactions.

ATP in Cellular Work

ATP powers chemical, transport, and mechanical work in cells. Hydrolysis of ATP changes protein shape and binding ability, facilitating these processes.

The ATP Cycle

ATP is regenerated by phosphorylation of ADP, using energy from catabolic reactions. This cycle couples energy-yielding and energy-consuming processes.

Enzymes: Biological Catalysts

Role and Function of Enzymes

Enzymes are proteins that act as catalysts, speeding up reactions by lowering activation energy barriers. They are not consumed in the reaction.

Activation Energy and Reaction Progress

Activation energy (EA) is the initial energy required to start a reaction. Enzymes lower EA, allowing reactions to proceed at moderate temperatures.

Enzyme Specificity and Induced Fit

Enzymes are highly specific, binding to substrates at their active sites. The induced-fit model describes how the enzyme changes shape to enhance catalysis.

Factors Affecting Enzyme Activity

• Substrate concentration: Higher concentration increases reaction rate until enzyme saturation.

• Temperature: Each enzyme has an optimal temperature; too high causes denaturation.

• pH: Each enzyme has an optimal pH; extremes can denature the enzyme.

• Cofactors: Nonprotein helpers (inorganic or organic) required for enzyme function.

• Enzyme inhibitors: Chemicals that reduce enzyme activity (competitive or noncompetitive).

Regulation of Enzyme Activity

Allosteric Regulation

Allosteric regulation involves binding of regulatory molecules at sites other than the active site, affecting enzyme activity. This can either inhibit or stimulate the enzyme.

Feedback Inhibition

Feedback inhibition occurs when the end product of a metabolic pathway inhibits an earlier step, preventing overproduction and conserving resources.

Example: The synthesis of isoleucine from threonine is regulated by feedback inhibition, where isoleucine inhibits the first enzyme in the pathway.

Additional info: These mechanisms are essential for maintaining homeostasis and efficient cellular function.