An Introduction to Metabolism Chapter 8

Metabolism What is metabolism? The sum total of all chemical reactions in an organism How do catabolic and anabolic pathways differ? Catabolism - breaks down complex molecules Anabolism - builds complex molecules

Energy Energy is the capacity to cause change or do work Kinetic Energy Associated with the relative motion of objects Heat or thermal energy is kinetic energy associated with the random movement of atoms Potential Energy Chemical energy is potential energy available for release in a chemical reaction

The Laws of Thermodynamics The first Law = Energy can be transformed, but is never lost, or created The second law = With every energy transfer, entropy is increased Entropy is a measure of disorder

How does a cell maintain its high level of order if with every energy transfer entropy is increased? Life demands a constant input of energy

What is free energy? Free energy is the portion of a system’s energy that is available to perform work The term free does not mean that there is no cost to using this energy It means that the energy is available

How Do Reactions Occur? Spontaneous reactions may occur very slowly All reactions require free energy of activation (EA) Uphill portion represents the EA required to start the reaction Downhill portion represents the loss of free energy by the molecules in the reaction DG is the difference in free energy of products and reactants

Exergonic and Endergonic Reactions Less free energy in products than reactants Energetically downhill DG is negative More free energy in products than reactants Energetically uphill DG is positive Give an example of the coupling of exergonic and endergonic reactions in living organisms

Is this reaction exergonic or endergonic?

How can the EA barrier be overcome? Temperature Temperatures that are too high denature organic molecules, so what else is there? ENZYMES Enzymes lower the EA barrier so that reactions can occur at lower temperatures

What are Enzymes? Catalysts change the rate of the reaction without being altered themselves Enzymes are biological catalysts Enzymes are proteins, whose three dimensional shape allows for their ability to react specifically

Enzyme / Substrate Relationship What is the substrate? It is the reactant upon which an enzyme reacts Enzymes are substrate specific Only the active site of the enzyme actually binds the substrate

The Active Site The active site is where the enzyme-substrate interaction occurs Most enzyme-substrate interactions are the result of weak bonds The active site may cause the enzyme to hold onto the substrate in a very specific way The active site may provide a micro-environment (e.g. low pH) which enhances a reaction

Enzyme Activity Temperature pH Enzyme Concentration Substrate Concentration

Cofactors Non-protein molecules that help enzymes function Bind to active site to enhance enzymatic reactions Cofactors may be inorganic metals such as zinc, iron, or copper Coenzymes are organic cofactors (e.g. vitamins)

Enzyme Inhibition Competitive inhibitors - mimic the substrate and compete for the active site Non-competitive inhibitors bind to the enzyme away from the active site, and indirectly cause a change in the active site

Competitive and Noncompetitive Inhibition

Allosteric Regulation Regulatory molecules that bind to the enzyme’s allosteric site changing the shape of the enzyme Allosterically regulated enzymes have a quaternary protein structure Each subunit of the enzyme has an active site and an allosteric site Allosteric activators stabilizes the active site Allosteric inhibitors deactivates the active site

Feedback Inhibition

Cooperativity