Bioenergetics and Enzymes

Bioenergetics and Enzymes

Introduction

• Metabolism encompasses all biochemical reactions organisms use to transform matter and energy.
• Catabolic pathways release energy by breaking down complex molecules into simpler ones.
• Anabolic pathways consume energy, using simple molecules to build more complex ones.

Free Energy

• Free energy is energy available to do work.
• Work is any physical motion or change, excluding the random motion of particles (thermal energy).
• Every chemical reaction involves a characteristic free energy change.
• Exergonic reactions release free energy and occur spontaneously.
• Endergonic reactions absorb free energy and need a coupled exergonic reaction to occur.

ATP

• ATP (adenosine triphosphate) is a ribonucleotide that serves as an energy carrier in cells and a monomer in RNA synthesis.
• ATP picks up energy from exergonic reactions and transfers some of it to endergonic reactions.
• ATP is present in all cells (eukaryotic and prokaryotic) and is thought to be an ancient molecule.
• ATP hydrolysis is an exergonic process, with a free energy change (∆G'º) of -30.5 kJ/mol under standard conditions. The equation is as follows: ATP + H2O -> ADP + Pi
• ATP hydrolysis needs to be catalyzed by enzymes (ATPases) to occur rapidly.
• ATP synthesis is an endergonic process.
• The ATP cycle links cellular exergonic and endergonic reactions through ATP synthesis and hydrolysis.

Enzymes

• Enzymes are catalysts that speed up biochemical reactions, with almost all being proteins.
• Enzymes lower the activation energy of a reaction to accelerate it.
• Enzymes do not alter the overall free energy change (∆G) of reactions.
• Cells control reactions by altering the quantity/identity of enzymes or enzyme activity regulation.
• Enzymes are highly specific to one or a few substrates.
• Active sites on enzymes bind substrates to enable catalysis to occur.
• Enzyme specificity arises from the interaction between the substrate(s) and the active site.
• Active sites and enzymes are regenerated without change during a reaction to enable catalytic cycles.
• Factors that affect enzyme activity include:
  • Substrate concentration: Enzymes are saturable, meaning there are finite active sites to process substrates at a maximum rate
  • Temperature
  • pH
• Cofactors are non-protein molecules or ions required by enzymes for catalytic activity.
• Enzyme activity is also affected by inhibitors and activators.
• Enzymes act sequentially in metabolic pathways.
• Feedback inhibition is a control circuit where the product of a pathway inhibits the first enzyme, regulating the pathway.

Oxidation – Reduction Reactions (Redox Reactions)

• Oxidation is the loss of electrons; reduction is the gain of electrons.
• Electron loss and gain may involve entire or shared electrons.
• Electrons are often transferred along with hydrogen atoms.
• Electron transfer can either absorb or release free energy.
• Common electron carriers are NADH, NADPH, and FADH2.