Metabolism: Energy and Enzymes

Chapter Outline I) Cells and the Flow of Energy II) Metabolic Reactions and Energy Transformations III) Metabolic Pathways and Enzymes

I) Cells and the Flow of Energy Energy is the ability to do work or bring about a change Cells use acquired energy to maintain their organization, grow, develop and reproduce There are two fundamental types of energy

Potential Energy Potential = stored energy waiting to do work  its capacity to do work is not being used at this moment (e.g. gravitational, elastic, chemical forms)

Kinetic Energy Kinetic = energy at work or the energy of movement (e.g. light, heat, mechanical forms)

1) Cells and the Flow of Energy Under the right conditions, energy can be transformed from one type to another (potential < kinetic) and from one form to another (e.g. chemical> thermal) These energy transformations follow the Laws of Thermodynamics

Energy Transformations

The First Law of Thermodynamics The First Law of Thermodynamics (sometimes called the Law of Conservation of Energy) states the amount of energy in any process is constant Energy can not be created nor destroyed by ordinary processes, only transformed from one form to another form, and between potential and kinetic types With each transformation within any system, some energy is released to the surroundings as “useless” heat energy (which can not do useful work)

“useless” Heat energy “useless” Heat energy

The Second Law of Thermodynamics The Second Law of Thermodynamics (sometimes called the Entropy Law) states that the amount of “useful” energy decreases when energy transformations occur (because “useless” heat energy is released) Therefore, there is a tendency for all systems to reach the lowest possible “useful” energy level Entropy is the term for the measure of the amount of disorder (loss of higher “useful” energy) in a system Therefore, there is a tendency for all systems to reach the highest possible entropy level

“Useful” Energy = Chemical Energy High Low “Useless” Energy = Heat Energy Low High Entropy = State of Disorder Low High

As an example, plants trap light energy from the sun in the process of photosynthesis (anabolism) to produce carbohydrates Plants use the carbohydrates to provide energy to maintain themselves (i.e. growth, reproduction, homeostasis, etc) High useful energy Low useful energy

High useful energy Low useful energy Plants lose their energy when heterotrophic organisms feed on them through digestion (catabolism) Plants become disordered (increase in entropy) as they pass through the digestive system of an organism as the latter processes the plants for nutrients in order to maintain the organization of its cells

What is the Source of all Energy on Earth? Energy must always be put into a system in order to sustain it, because all of the chemical reactions that occur in cells are decreasing the amount of “useful” energy and increasing the amount of “useless” energy Where does this input of energy come from?

The Sun The Sun constantly loses useful light energy and increases in entropy, in order to maintain life on Earth

II) Metabolic Reactions and Energy Transformations Metabolism = the sum of all of the chemical reactions in a cell Anabolism = reactions that synthesize complex molecules from simple molecules Catabolism = reactions that breakdown complex molecules into simple molecules

Chemical Reactions Reactants = Substrate = the substances that start a chemical reaction Products = the substances that form as a result of a chemical reaction Reactants  Products Energy is released or absorbed in chemical reactions

Chemical Reactions and Energy Reactants  Products Chemical reactions proceed spontaneously if there is an increase in entropy (= decrease in “useful” energy) as the reaction proceeds to products The “useful” energy of the products < the “useful” energy of the reactants

Chemical Reactions and Energy Gibb’s Free Energy (Δ G) = amount of energy available after a chemical reaction to do work = “useful” energy of products – “useful” energy of the reactants

Exergonic Reactions Chemical reactions that release energy The amount of “useful” energy in the products is less than that of the reactants Gibbs Free Energy (Δ G) is negative e.g. cellular respiration Δ G = -686 kcal

: Cellular Respiration (CH2O)n

Endergonic Reactions Chemical reactions that require an input of energy to proceed to products The amount of “useful” energy in the products is greater than that of the reactants Gibbs Free Energy (Δ G) is positive e.g. photosynthesis Δ G = +686 kcal

: Photosynthesis + O2

complex simple

Coupled Reactions Where do endergonic reactions get there input energy from ? From exergonic reactions that they are coupled to.

Coupled Reactions How is the energy transferred from the exergonic reaction to the endergonic reaction? By the energy carrier molecule ATP.

Adenosine Triphosphate (ATP) Cells do not directly use forms of energy : the energy of chemical fuel molecules (e.g. glucose) must be transformed into ATP in an exergonic chemical reaction ATP is then used to provide the energy to complete an endergonic chemical reaction. There are a number of additional energy carrier molecules involved in cell metabolic processes such as photosynthesis and cell respiration: NAD+, NADP+, FAD and the Cytochromes.

Adenosine Triphosphate (ATP) The second and third phosphate bonds of ATP are unstable. When this phosphate bond is broken by hydrolysis, energy is released (an exergonic reaction). This released energy is just perfect for the amount of energy needed for many cell reactions. This is why we call ATP an energy carrier. It "carries" the energy needed to do the cell work. The product of the hydrolysis of ATP is a molecule of ADP and a free phosphate molecule (Pi ). Much of the energy released when ATP is broken is in the form of less useful heat energy.

III) Metabolic Pathways and Enzymes Metabolic Pathway = series of linked chemical reactions that begins with a particular reactant and terminates with an end product

Controlling Metabolic Pathways Metabolic pathways must be tightly regulated for our cells to function: 1) Cells couple endergonic chemical reactions with exergonic chemical reactions 2) Cells have energy-carrier molecules that capture energy released in exergonic reactions and transport it to endergonic reactions 3) Cells regulate chemical reactions using enzymes, which are protein catalysts

Chemical Reactions, Activation Energy and Enzymes All reactant molecules are in constant motion For any chemical reaction to get proceed, the reactants must come together at the right bonding place at the right time No matter how "spontaneous" a chemical reaction is, some energy is needed to get the reaction started This energy is called the activation energy

: Cellular Respiration (CH2O)n

Chemical Reactions, Activation Energy and Enzymes Reactants molecules will come together randomly, but far too slowly at normal earth temperatures Catalysts speed up the rate of chemical reactions by lowering the activation energy The catalyst is not part of the chemical reaction. It is neither a reactant nor a product A catalyst facilitates the reaction and is never consumed in the reaction In living organisms, we use a special class of catalysts, called enzymes

Enzymes Enzymes are globular proteins with tertiary structure There is a place on the surface of the enzyme where reactant molecules can bind this "notch" is the active site The active site has a precise size, shape, and electrical charge that exactly complements the reactant molecules enzymes are highly specific to their reactant molecules Therefore, each chemical reaction that occurs in cells has its own enzyme

Mechanism of Enzyme Action When a reactant molecule binds to the enzyme, it "fits" into the active site of the enzyme  induced fit This binding temporarily distorts the reactant molecules  transition state In the transition state, the bonds of the reactant molecules are more easily broken (lowered activation energy), thereby promoting the reaction Once the reaction occurs, the active site is altered, releasing the product The enzyme is unaffected by the reaction.

Factors Affecting Reaction Rate 1) Substrate Concentration 2) Enzyme Concentration 3) Temperature 4) pH 5) Presence of Competitive Inhibitor 6) Presence of Allosteric Inhibitor 7) Presence of Enzyme Cofactors 8) Presence of Metal Ions

1) Effect of Substrate Concentration on Reaction Rate 1) At lower substrate concentrations, the active sites on most of the enzyme molecules are not filled 2) At higher substrate concentrations, there are more collisions between the substrate and enzyme molecules  faster reaction rate 3) The maximum reaction rate is reached when the active sites are almost continuously filled 3 2 1

2) Effect of Enzyme Concentration on Reaction Rate 1) If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because there is not enough enzyme for all of the reactant molecules 2) As the amount of enzyme is increased, the rate of reaction increases 3) If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate 3 2 1

3) Effect of Temperature on Enzyme Activity

4) Effect of pH on Enzyme Activity

5) Effect of Competitive Inhibitor on Reaction Rate

Types of Competitive Inhibitors e.g. cyanide competes with oxygen for the active site of the enzyme cytochrome oxidase e.g. penicillin competes for the active site of an enzyme unique to bacteria

6) Effect of Allosteric Inhibitor on Reaction Rate

Allosteric Inhibition

7) Effect of Enzyme Cofactors on Reaction Rate Cofactors = inorganic molecule required by enzyme for proper functioning of enzyme e.g. copper (Cu+), zinc (Zn++), iron (Fe++), magnesium (Mg++), potassium (K+), and calcium (Ca++) ions The cofactors bind to the enzyme and participate in the reaction by removing electrons, protons , or chemical groups from the substrate.

7) Effect of Enzyme Coenzymes on Reaction Rate Coenzymes = organic (non- protein) molecule required for proper functioning of enzyme e.g. NAD, FAD, vitamin complexes Coenzymes often remove electrons from the substrate and pass them to other molecules Often the electron is added to a proton to form a hydrogen atom before it is passed In this way, coenzymes serve to carry energy in the form of electrons (or hydrogen atoms) from one compound to another

8)Presence of Metal Ions The addition of heavy metal ions such as mercury, lead and arsenic to an enzyme-mediated reaction decreases the reaction rate The heavy metal ions denature the enzyme by destroying the 3-dimensional shape of the active site