Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr Chapter 5 Ground Rules of Metabolism (Sections )

5.1 A Toast to Alcohol Dehydrogenase Metabolic processes build and break down organic molecules such as ethanol and other toxins Alcohol breakdown directly damages liver cells, and interferes with normal processes of metabolism Currently the most serious drug problem on college campuses is binge drinking

Alcohol Metabolism The enzyme alcohol dehydrogenase helps the liver break down toxic alcohols (ethanol)

5.2 Energy and the World of Life There are many forms of energy: Kinetic energy, potential energy Light, heat, electricity, motion Energy cannot be created or destroyed (first law of thermodynamics) Energy can be converted from one form to another and thus transferred between objects or systems

Energy Disperses Energy tends to disperse spontaneously (second law of thermodynamics) A bit disperses at each energy transfer, usually as heat Entropy is a measure of how dispersed the energy of a system has become

Key Terms energy The capacity to do work kinetic energy The energy of motion entropy Measure of how much the energy of a system is dispersed

Key Terms first law of thermodynamics Energy cannot be created or destroyed second law of thermodynamics Energy tends to disperse spontaneously

Kinetic Energy

Entropy Entropy tends to increase, but the total amount of energy in any system always stays the same

Fig. 5.3, p. 76 Entropy Time heat energy Stepped Art Entropy

Work Work occurs as a result of an energy transfer A plant converts light energy to chemical energy in photosynthesis Most other cellular work occurs by transfer of chemical energy from one molecule to another (such as transferring chemical energy from ATP to other molecules)

Energy’s One-Way Flow Living things maintain their organization only as long as they harvest energy from someplace else Energy flows in one direction through the biosphere, starting mainly from the sun, then into and out of ecosystems Producers and then consumers use energy to assemble, rearrange, and break down organic molecules that cycle among organisms throughout ecosystems

Energy Conversion It takes 10,000 pounds of feed to raise a 1,000- pound steer About 15% of energy in food builds body mass; the rest is lost as heat during energy conversions

Energy Flow Energy flows from the environment into living organisms, and back to the environment Materials cycle among producers and consumers

Fig. 5.5, p. 77 Consumers animals, most fungi, many protists, bacteria nutrient cycling Producers plants and other self-feeding organisms sunlight energy Energy Flow

Potential Energy Energy’s spontaneous dispersal is resisted by chemical bonds Energy in chemical bonds is a type of potential energy, because it can be stored potential energy Stored energy

Key Concepts Energy Flow Organisms maintain their organization only by continually harvesting energy from their environment ATP couples reactions that release usable energy with reactions that require it

5.3 Energy in the Molecules of Life Every chemical bond holds energy – the amount of energy depends on which elements are taking part in the bond Cells store and retrieve free energy by making and breaking chemical bonds in metabolic reactions, in which reactants are converted to products

Key Terms reaction Process of chemical change reactant Molecule that enters a reaction product A molecule that remains at the end of a reaction

Chemical Bookkeeping In equations that represent chemical reactions, reactants are written to the left of an arrow that points to the products A number before a formula indicates the number of molecules The same number of atoms that enter a reaction remain at the reaction’s end

Chemical Bookkeeping

2H 2 O (water) Fig. 5.6, p. 78 Stepped Art Reactants 4 hydrogen atoms + 2 oxygen atoms Products 4 hydrogen atoms + 2 oxygen atoms 2H 2 (hydrogen) O 2 (oxygen) Chemical Bookkeeping

Energy In, Energy Out In most reactions, free energy of reactants differs from free energy of products Reactions in which reactants have less free energy than products are endergonic – they will not proceed without a net energy input Reactions in which reactants have greater free energy than products are exergonic – they end with a net release of free energy

Key Terms endergonic “Energy in” Reaction that converts molecules with lower energy to molecules with higher energy Requires net input of free energy to proceed exergonic “Energy out” Reaction that converts molecules with higher energy to molecules with lower energy Ends with a net release of free energy

Energy In, Energy Out

Fig. 5.7, p. 78 Free energy energy out energy in 2H 2 O O2O2 2H H 2 O Energy In, Energy Out

Why Earth Does Not Go Up in Flames Earth is rich in oxygen—and in potential exergonic reactions; why doesn’t it burst into flames? Luckily, energy is required to break chemical bonds of reactants, even in an exergonic reaction activation energy Minimum amount of energy required to start a reaction Keeps exergonic reactions from starting spontaneously

Activation Energy

Fig. 5.8, p. 79 O2O2 Free energy 2H 2 Activation energy Products: 2H 2 O Difference between free energy of reactants and products Reactants: Activation Energy

ATP—The Cell’s Energy Currency ATP is the main currency in a cell’s energy economy ATP (Adenosine triphosphate) Nucleotide with three phosphate groups linked by high- energy bonds An energy carrier that couples endergonic with exergonic reactions in cells

ATP

Fig. 5.9a, p. 79 A Structure of ATP. ribose adenine three phosphate groups ATP

Phosphorylation When a phosphate group is transferred from ATP to another molecule, energy is transferred along with the phosphate Phosphate-group transfers (phosphorylations) to and from ATP couple exergonic reactions with endergonic ones phosphorylation Addition of a phosphate group to a molecule Occurs by the transfer of a phosphate group from a donor molecule such as ATP

ATP and ADP

Fig. 5.9b, p. 79 B After ATP loses one phosphate group, the nucleotide is ADP (adenosine diphosphate); after losing two phosphate groups, it is AMP (adenosine monophosphate) ribose adenine AMP ATP ADP ATP and ADP

ATP/ADP Cycle Cells constantly use up ATP to drive endergonic reactions, so they constantly replenish it by the ATP/ADP cycle ATP/ADP cycle (which is endergonic, which is exergonic?) Process by which cells regenerate ATP ADP forms when ATP loses a phosphate group, then ATP forms again as ADP gains a phosphate group

ATP/ADP Cycle

Fig. 5.9c, p. 79 energy out ADP + phosphate energy in C ATP forms by endergonic reactions. ADP forms again when ATP energy is transferred to another molecule along with a phosphate group. Energy from such transfers drives cellular work. ATP/ADP Cycle

5.4 How Enzymes Work Enzymes makes a reaction run much faster than it would on its own, without being changed by the reaction catalysis The acceleration of a reaction rate by a molecule that is unchanged by participating in the reaction Most enzymes are proteins, but some are RNAs

Substrates Each enzyme recognizes specific reactants, or substrates, and alters them in a specific way substrate A molecule that is specifically acted upon by an enzyme

Active Sites Enzyme specificity occurs because an enzyme’s polypeptide chains fold up into one or more active sites An active site is complementary in shape, size, polarity, and charge to the enzyme’s substrate active site Pocket in an enzyme where substrates bind and a reaction occurs

An Active Site

Fig. 5.10a, p. 80 An Active Site

Fig. 5.10a, p. 80 active site enzyme A Like other enzymes, hexokinase’s active sites bind and alter specific substrates. A model of the whole enzyme is shown to the left. An Active Site

Fig. 5.10b, p. 80 An Active Site

Fig. 5.10b, p. 80 reactant(s) B A close-up shows glucose and phosphate meeting inside the enzyme’s active site. The microenvironment of the site favors a reaction between the two substrate molecules. An Active Site

Fig. 5.10c, p. 80 An Active Site

Fig. 5.10c, p. 80 product(s) C Here, the glucose has bonded with the phosphate. The product of this reaction, glucose-6-phosphate, is shown leaving the active site. An Active Site

Lowering Activation Energy Enzymes lower activation energy in four ways: Bringing substrates closer together Orienting substrates in positions that favor reaction Inducing the fit between a substrate and the enzyme’s active site (induced-fit model) Shutting out water molecules induced-fit model As the substrate begins to bind to the enzyme, the active site actively changes shape to accommodate the incoming substrate. This improves the fit between the two.

Lowering Activation Energy

Fig. 5.11, p. 80 Free energy Reactants Products Transition state Activation energy with enzyme Activation energy without enzyme Time Lowering Activation Energy

Effects of Temperature, pH, and Salinity Each type of enzyme works best within a characteristic range of temperature, pH, and salt concentration: Adding heat energy boosts free energy, increasing reaction rate (within a given range) Most human enzymes have an optimal pH between 6 and 8 (e.g. pepsin functions only in stomach fluid, pH 2) Too much or too little salt disrupts hydrogen bonding that holds an enzyme in its three-dimensional shape

Enzymes and Temperature

Fig. 5.12, p. 81 Temperature Enzyme activity temperature- sensitive tyrosinase normal tyrosinase 40°C (104°F)30°C (86°F)20°C (68°F) Enzymes and Temperature

Enzymes and pH

Fig. 5.13, p. 81 pH trypsin glycogen phosphorylase pepsin Enzyme activity Enzymes and pH

Help From Cofactors Most enzymes require cofactors, which are metal ions or organic coenzymes in order to function cofactor A metal ion or a coenzyme that associates with an enzyme and is necessary for its function coenzyme An organic molecule that is a cofactor

Coenzymes and Cofactors Coenzymes may be modified by taking part in a reaction Example: NAD + becomes NADH by accepting electrons and a hydrogen atom in a reaction Cofactors are metal ions Example: The iron atom at the center of each heme In the enzyme catalase, iron pulls on the substrate’s electrons, which brings on the transition state

Antioxidants Cofactors in some antioxidants help them stop reactions with oxygen that produce free radicals (harmful atoms or molecules with unpaired electrons) Example: Catalase is an antioxidant antioxidant Substance that prevents molecules from reacting with oxygen

Key Concepts How Enzymes Work Enzymes tremendously increase the rate of metabolic reactions Cofactors assist enzymes, and environmental factors such as temperature, salt, and pH can influence enzyme function