Chapter 6: Metabolism – Energy and Enzymes (Outline) Forms of Energy Two Laws of Thermodynamics Cells and Entropy Metabolic Reactions ATP – Energy for Cells Metabolic Pathways and Enzymes Energy of Activation Enzyme-Substrate Complex Redox reactions

Forms of Energy The capability to do work or produce an effect and is divided into two types Kinetic- the energy of motion, such as waves, electrons, atoms, molecules, substances and objects Mechanical energy of motion Electrical (e.g. lightning) Thermal (i.e. geothermal energy) Radiant (e.g. visible light, x-rays, gamma rays and radio waves)

Forms of Energy Potential: Stored energy or energy that is "waiting to happen" Chemical - energy stored in the bonds of atoms and molecules Nuclear – energy stored in the nucleus of an atom (the energy that holds the nucleus together) Stored mechanical energy (e.g. compressed springs)

Laws of Thermodynamics First law: Law of conservation of energy Energy cannot be created or destroyed, but Energy CAN be changed from one form to another

Laws of Thermodynamics Second law: Energy cannot be changed from one form to another without a loss of usable energy Therefore, no process requiring a conversion of energy is ever 100% efficient Carbohydrate Metabolism Carbohydrate Synthesis

Cells and Entropy Law of entropy: The spontaneous movement from order to disorder (randomness) Waste energy goes to increase disorder Entropy – a measure of the degree of a system’s disorder which increases over time Cars rust, dead trees decay, buildings collapse; all these are examples of entropy in action

Cells and Entropy

Energy Flow in Living Things

Metabolic Reactions and Energy Transformations Metabolism: Sum of cellular chemical reactions in a cell Reactants participate in a reaction Products form as result of a reaction Free energy – the amount of energy available to perform work Concept of free energy was developed by Gibbs For a reaction to occur spontaneously free energy must decrease - the products must have less free energy than the reactants

Metabolic Reactions & Energy Transformations Energy Changes in Metabolic Reactions Exergonic Reactions - products have less free energy than reactants (i.e. spontaneous and releases energy) Example: ATP breakdown Endergonic Reactions - products have more free energy than reactants (i.e. not spontaneous and requires energy) Example: Muscle contraction

ATP: Energy for Cells Adenosine triphosphate (ATP) Composed of High energy compound used to drive metabolic reactions Constantly being generated from adenosine diphosphate (ADP) and a molecule of inorganic phosphate Composed of Adenine, ribose (C5 sugar), and 3 phosphate groups Biological advantages to the use of ATP It provides a common energy currency used in many types of reactions When ATP becomes ADP + ℗ the amount of energy released is sufficient for biological function with little waste of energy (~ 7.3 kcal per molecule) ATP breakdown can be coupled to endergonic reactions that prevents energy waste

The ATP Cycle

ATP and Coupled Reactions Energy released by an exergonic reaction is captured in ATP That ATP is used to drive an endergonic reaction Occur in the same place, at the same time, why? Because the energy released by the hydrolysis of ATP is higher than the energy consumed by the endergonic reaction

Coupled Reactions A cell has two ways to couple ATP hydrolysis ATP is used to energize a reactant ATP is used change the shape of a reactant Both can be achieved by transferring ℗ to the reactant so that the product is phosphorylated Example: Ion movement across the plasma membrane of a cell through carrier proteins Attachment of amino acids to a growing polypeptide chain

Coupled Reactions – Muscle Contraction

Enzymes A  B  C  D  E  F  G E1 E2 E3 E4 E5 E6 Protein molecules that function as catalysts, however ribozymes are made of RNA not proteins The reactants of an enzymatically accelerated reaction are called substrates Each enzyme accelerates a specific reaction Each reaction in a metabolic pathway requires a unique and specific enzyme End product will not appear unless ALL enzymes are present and functional E1 E2 E3 E4 E5 E6 A  B  C  D  E  F  G

“A” is Initial Reactant Metabolic Pathways Reactions usually occur in a sequence Products of an earlier reaction become reactants of a later reaction Such linked reactions form a metabolic pathway Begins with a particular reactant, Proceeds through several intermediates, and Terminates with a particular end product AB C D E FG “A” is Initial Reactant “G” is End Product Intermediates

Energy of Activation Reactants are often “reluctant” to participate in the reaction Energy must be added to at least one reactant to initiate the reaction Energy of activation - minimum amount of energy required to trigger a chemical reaction Enzyme Operation: Enzymes operate by lowering the energy of activation Accomplished by bringing the substrates into contact with one another under mild conditions Does not get consumed by the reaction nor does it alter the equilibrium of the reaction, thus it remains intact

Energy of activation (Ea)

Enzyme-Substrate Complex The “lock and key” model of enzyme activity The active site is made up of amino acids and has a very specific shape The enzyme and substrate slot together to form a complex, as a key slots into a lock This complex reduces the activation energy for the reaction Then the products no longer fit into the active site and are released allowing another substrate in

Enzyme-Substrate Complex Induced fit model The active site complexes with the substrates by weak interactions, such as hydrogen bonds, etc Causes active site to change shape Shape change forces substrates together, initiating bond Some enzymes participate in the reaction (e.g. Trypsin – active site contains 3 amino acids with R groups interacting with the peptide bond (to break the bond and introduce H2O

Synthesis vs. Degradation Enzyme complexes with two substrate molecules Substrates are joined together and released as single product molecule Degradation: Enzyme complexes with a single substrate molecule Substrate is broken apart into two product molecules

Enzymatic action

Naming enzymes Enzyme names usually end in ase Many enzyme names have 3 parts: (substrate name) (type of reaction) (ase) Substrate Enzyme Lipid Lipase Urea Urease Maltose Maltase Cellulose Cellulase Lactose Lactase Sucrose Sucrase

Factors Affecting Enzyme Activity Substrate concentration Enzyme activity increases with substrate concentration More collisions between substrate molecules and the enzyme When the active sites of the enzyme are filled, with increasing substrate, the enzyme’s rate of activity cannot increase any more Amount of active enzyme can also increase or limit the rate of an enzymatic reaction

Factors Affecting Enzyme Activity Temperature Enzyme activity increases with temperature Warmer temperatures cause more effective collisions between enzyme and substrate However, hot temperatures destroy enzymes, how? Denaturation – enzyme’s shape changes and it can no longer bind its substrate efficiently However, there are exceptions such as Some prokaryotes living in hot springs Coat color pattern in Siamese cats

Factors Affecting Enzyme Activity Optimal pH Most enzymes are optimized for a particular pH Changes in pH can make and break intra- and intermolecular bonds, and/or hydrogen bonds thus changing the globular shape of the enzyme pH change can alter the ionization of R side chains Under extreme conditions of pH, the enzyme becomes inactive (due to altered shape) Example: Pepsin (stomach) and trypsin (small intestine)

Effect of Temperature and pH

Enzyme Cofactors Many enzymes require additional help in catalyzing their reaction from a coenzyme or cofactor Cofactor is used to refer to inorganic metallic ions such as zinc, copper and iron, which is required by an enzyme Coenzyme is a non-protein organic molecule Many of the coenzymes are derived from vitamins Enzyme activity decreases if vitamin is not available (i.e. vitamin-deficient disorder) Niacin deficiency results in skin disease (pellagra), while riboflavin deficiency results in cracks at the corner of mouth

Enzyme Inhibition Competitive inhibition Substrate and the inhibitor are both able to bind to the active site If a similar molecule is present, it will compete with the real substrate for the active sites Product will form only when the substrate, not the inhibitor, is at the active site This will regulate the amount of product

Enzyme Inhibition Noncompetitive inhibition Noncompetitive inhibitors are considered to be substances which when added to the enzyme change the enzyme in a way that it cannot accept the substrate The inhibitor binds to another location (allosteric site) on the enzyme and inactivates the enzyme molecule Both competitive and noncompetitive inhibition are examples of feedback inhibition

Competitive and Noncompetitive Inhibitors

Feedback Inhibition The end product of a pathway inhibits the pathway’s first enzyme The metabolic pathway is shut down when the end product of the pathway is bound to an allosteric site on the first enzyme of the pathway Normally, enzyme inhibition is reversible and the enzyme is not damaged

Oxidation and Reduction The loss of one or more electrons Reduction The gain of one or more electrons Reduction-oxidation (redox) reactions Simultaneous reaction in which one molecule is oxidized and another is reduced Redox reactions occur during photosynthesis and cellular respiration

Electron Transfer in Redox Reactions