LESSON 4: ENERGY AND ENZYMES

Biological systems must obey the basic laws of thermodynamics:

1. energy is neither created nor destroyed, but not all energy can be used
2. disorder tends to increase

Much of evolution is driven by the need to capture and use energy more and more efficiently. Organisms that can do more with less energy generally have a competitive advantage over other organisms. In this lesson we will review the principles of thermodynamics, free energy, and equilibrium. We will look at the importance of catalysis in biological systems and look at how certain proteins, called enzymes, work to regulate processes within living organisms. We will also learn about the basic tools used over and over in biological reactions. These tools include ATP, reduced cofactors, redox reactions, and substrate-level phosphorylation.

Learning Objectives

• Understand the first and second laws of thermodynamics and how they impact evolutionary trends.

• Understand the properties of enzymes and why catalysis is important with respect to the efficient use of energy.

• Understand the roles of ATP and reduced cofactors in shuttling energy and electrons around within cells.

• Understand how oxidation and reduction reactions (redox) always occur together.

• Know how substrate level phosphorylation transfers chemical bond energy by coupling chemical reactions.

Topics covered in this Lesson

• Thermodynamics

• Enzymes I

• Enzymes II

• Conservation of Energy

Thermodynamics

Energy is a somewhat difficult concept to grasp. Most simply, energy is defined as the capacity to do work. Organisms must be able to capture and effectively use energy in order to survive and reproduce. Because of this, life is restricted by the fundamental laws that govern energy, called the laws of thermodynamics.

Learning Objectives

• Energy is the capacity to do work.

• Understand the difference between potential energy and kinetic energy, and be able to describe several examples of each.

• Know the first and second laws of thermodynamics, and why they are important to living systems.

• Understand that reactions with a negative change in free energy (G) are exergonic and proceed spontaneously.

• Understand that reactions with a positive change in G are endergonic and require an input of energy to proceed.

• Know that chemical reactions are reversible.

Enzymes I

Life depends on chemical reactions that occur within cells and organisms. Without enzymes to catalyze these reactions, most would proceed at a rate far too slow for life to exist. Most enzymes are proteins that catalyze very specific reactions – therefore there are literally millions of different types of enzymes in the biological world.

Learning Objectives

• Enzymes are biological catalysts.

• Most enzymes are proteins.

• Understand the importance of the active sites of enzymes.

• Understand the effect of enzymes on the activation energy of reactions, and why this makes chemical reactions proceed more quickly.

• Know that enzymes do not affect the change in free energy in a reaction.

• Know that most enzymes only catalyze very specific reactions.

• Be able to describe the lock and key and induced fit mechanisms of enzyme action.

Enzymes II

All enzymes contain active sites, regions of the molecule that bind substrate and catalyze reactions. There are several main types of reactions that occur at active sites which are discussed in this module. In order for these reactions to occur, the shape of enzymes and their active sites must be maintained. Enzyme shape can be affected by a number of different factors, including temperature and pH. Many enzymes must also have other types of molecules bound to them in order to be active.

Learning Objectives

• Be able to describe three different types of reactions that occur in the active sites of enzymes.

• Understand how temperature, pH and substrate and enzyme concentrations affect the activity of enzymes.

• Understand how allosteric enzymes work, and why they are often important in metabolic pathways.

• Understand the difference between competitive and non-competitive inhibitors.

• Understand the differences between coenzymes, cofactors and prosthetic groups, and why these molecules are important for many enzymes.

Conservation of Energy

While energy is essential for life, cells must use energy in a controlled manner. In biological systems, energy is usually held in chemical bonds, and is transferred in a stepwise fashion, through the breaking and formation of those chemical bonds. In this way, small, manageable amounts of energy are transferred at each step in energy pathways. There are several common types of chemical reactions that are used in this way to transfer energy in cells. Cells also use several specific types of molecules to carry, or shuttle, energy to different locations in the cell.

Learning Objectives

• Understand why it is important that energy transfers in metabolic pathways occur in small steps.

• Know that ATP is a nucleotide that is used to shuttle energy to different places in the cell. Its high-energy phosphate bonds store potential energy.

• Hydrolysis of ATP releases energy that can be used to fuel endergonic reactions.

• Substrate-level phosphorylation transfers energy by coupling chemical reactions.

• Understand the basic principles of oxidation and reduction (redox) reactions that involve the transfer of energy-containing electrons and are always coupled together.

• Know that NAD is an important electron shuttle in the cell.