Enzymes and biochemical Reactions Metabolism is all the chemical reactions that occur in a cell to keep an organism alive. Cells are challenged to make chemical reactions go quickly. The best way to speed up a reaction is to increase the temperature. But cells will die if they get heated. Instead, cells use enzymes to lower the energy required for a reaction to occur.

Laws of Thermodynamics First Law of Thermodynamics Energy can not be created or destroyed but can only be changed from one form to another. Cells for example, change chemical bond energy to kinetic or motion energy like in a muscle cell. Second Law of Thermodynamics When energy is changed from one form to another, some useful energy is always “lost” as heat. This is also a challenge for cells as they cannot be overheated. Biochemical reactions in cells must be as efficient as possible so the least amount of heat is lost.

Energy and Chemical Reactions The two main types of reactions are: 1. Exothermic or exergonic reactions release energy from bonds the product molecules are smaller than the reactant molecules For example, cellular respiration: Glucose + oxygen  carbon dioxide + water + energy (useful and heat energy)

LE 8-6a Reactants Energy Products Progress of the reaction Amount of energy released (  G < 0) Free energy Exergonic reaction: energy released

2. Endothermic or endergonic reactions - energy is stored in the bonds of molecules - the product molecules are larger than the reactant molecules For example, photosynthesis: carbon dioxide + water + sunlight  glucose + oxygen

LE 8-6b Reactants Energy Products Progress of the reaction Amount of energy required (  G > 0) Free energy Endergonic reaction: energy required

Free Energy The energy available for chemical reactions is called free energy. The symbol is  G. The hydrolysis of table sugar (sucrose) to glucose and fructose is exergonic so the free energy is negative.  G = −29 kJ/mol Forming sucrose is endergonic with  G = +29 kJ/mol

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Enzymes Enzymes are special proteins that speed up or catalyse a reaction. Enzymes are biological catalysts. Enzymes are not permanently changed by the reaction. All enzyme names end in “ase”. Enzymes can be reused and reused. Enzymes have specific shapes and work only for one exact reaction or step in a biochemical pathway of many steps.

LE 8-13 Sucrose C 12 H 22 O 11 Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Do Enzymes Work? Enzymes work by lowering the energy of activation. Every chemical reaction between molecules involves bond breaking and bond forming. The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA). Activation energy is often supplied in the form of heat from the surroundings.

LE 8-14 Transition state CD A B EAEA Products CD A B  G < O Progress of the reaction Reactants C D A B Free energy

LE 8-15 Course of reaction without enzyme E A without enzyme  G is unaffected by enzyme Progress of the reaction Free energy E A with enzyme is lower Course of reaction with enzyme Reactants Products

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induced Fit Model of Enzyme Action The reactant that an enzyme acts on is called the enzyme’s substrate. The enzyme binds to its substrate, forming an enzyme-substrate complex. The active site is the region on the enzyme where the substrate binds. Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The active site involves a small number of key functional groups that actually bind the substrates. The rest of the protein structure is needed to maintain these functional groups in position. How do active sites work?

LE 8-16 Substrate Active site Enzyme Enzyme-substrate complex

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How does an enzyme lower activation energy? In an enzymatic reaction, the substrate binds to the active site. The active site can lower an EA barrier by: 1.Orienting substrates correctly 2.Straining substrate bonds 3.Providing a favorable microenvironment 4.Covalently bonding to the substrate

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Substrates Enzyme Products Substrates enter active site; enzyme changes shape so its active site Holds the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Active site acts on substrates Substrates are converted into products. Products are released. Active site is available for two new substrate molecules. Quick Review of Induced Fit Model of Enzyme Action Enzyme-substrate complex

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings HOMEWORK Read p 36 – 44 Do p 40 # 2-5, 7,8 Do p 54 # 1-7