Chapter 6 Metabolism: Energy and Enzymes

Metabolism The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways Catabolic pathway: A metabolic pathway that releases energy by breaking down complex molecules to simpler compounds. (process?) Anabolic pathway: A metabolic pathway that synthesizes a complex molecule from simpler compounds. (process?)

Energy The capacity to do work; Cells must continually use energy to do biological work. Types of energy Kinetic Energy: The energy of motion, which is directly related to the speed of that motion. Moving matter does work by imparting motion to other matter. Potential Energy: The energy stored by matter as a result of its location or spatial arrangement.

Transformations between kinetic and potential energy.

Energy as explained by the Laws of Thermodynamics First Law: (conservation of energy) –Energy cannot be created nor destroyed; it can only change from one from to another. –Energy is found in covalent bonds b/t atoms Second Law: Every energy transfer or transformation increases the entropy of the universe. –Entropy: A quantitative measure of disorder or randomness. –Energy cannot be changed from one form to another with out a loss of usable energy (heat = the most random form of energy)

Chemical energy: Energy stored in the chemical bonds of molecules; a form of potential energy. Energy is trapped inside molecules; the more complex the molecule the more energy it has, visa versa.

QOD How does the second law of thermodynamics help explain the diffusion of a substance across a membrane?

QOD answer The second law is the trend toward randomness. Equal concentrations of a substance on both sides of a membrane is a more random distribution than unequal concentrations. Diffusion of a substance to a region where it is initially less concentrated increases entropy, as mandated by the second law.

Energy Transformations In a reaction: A + B  C + D A and B are the reactants C and D are the products Free Energy (G): The amount of energy that is free to do work after a chemical reaction takes place. Change in Free Energy (  G)

Free Energy Exergonic Reaction: the free energy of reactants are greater than free energy of products (-  G) happens spontaneously. Endergonic Reaction: the free energy of the reactants are less than the free energy of products (+  G) requires input of energy.

- Because systems at equilibrium are at a minimum of G and can do no work, a cell that has reached metabolic equilibrium is dead! The fact that metabolism as a whole is never at equilibrium is one of the defining features of life. - Cells use the products from one reaction as the reactants in a second reaction, which pulls the first reaction in one direction. - Energy is utilized more efficiently in small increments

QOD Cellular respiration uses glucose, which has a high level of free energy, and releases CO 2 and water, which have low levels of free energy. Is respiration spontaneous or not? Is it exergonic or endergonic? What happens to the energy released from glucose?

QOD Answer Cellular respiration is a spontaneous and exergonic process. The energy released from glucose is used to do work in the cell, or is lost as heat.

Coupling Reactions When the energy released by exergonic reactions is used to drive an endergonic reaction. ATP  ADP + P –Highly Exergonic –The breakdown of ATP is coupled to all of a cells endergonic reactions

ATP: The Energy Currency of Cells When cells require energy they “spend” (breakdown) ATP. Cells are constantly producing ATP because it is in high demand.

Function of ATP

ATP is constantly recycled in cells (more efficiency) RespirationPhotosynthesis

6.3 Metabolic Pathways and Enzymes

Reactions in cells are orderly Metabolic Pathways: orderly sequence of chemical reactions, where each one is catalyzed by a specific enzyme. Enzymes: Proteins that act as catalysts that speed up chemical reactions. –Not part of the reaction, they only aid in the process. –Enzyme does not change itself only the reactants of the reaction. –Specific to reaction (only catalyze one reaction) Substrate: a reactant in an enzymatic reaction

Energy of Activation (E A ) The amount of free energy that must be added to cause molecules to react –Heating Example: AB + CD  AC + BD

Enzymes speed up reactions by lowering the energy of activation (E A )

Enzyme-Substrate Complexes Enzymes lower the energy of activation by forming a complex with their substrate. Active Site: small region on surface of enzyme where substrate binds. Induced-fit model: when substrate binds to enzyme, the active site changes shape to facilitate the reaction.

Enzyme-Substrate Complexes Enzymes do not get used up in a reaction (substrate does), so only a small amount of enzyme is required. Every cell reaction requires a specific enzyme; therefore enzymes are named after their substrates by adding the ending “ase”.

Speed of Enzyme Activity Four factors that affect enzyme activity. 1.Substrate Concentration 2.Temperature 3.pH 4.Enzyme Inhibition

Substrate Concentration Enzyme activity increases as substrate concentration increases. Due to higher probability of collisions between substrate molecules and enzyme. As the enzyme’s active sites are filled the enzyme activity levels off (reaches a max rate).

Temperature and pH As temperature rises, enzyme activity increases b/c there are more molecular collisions. If temperature rises beyond a certain point, the activity of the enzyme declines rapidly ( enzyme [protein] is denatured ). Each enzyme has an optimal pH that maintains its normal shape. A change in pH causes denaturation of enzyme which decreases activity.

Enzyme Inhibition When an enzyme is prevented from binding with its substrate. 2 types 1.Competitive Inhibition: another similar molecule competes with the substrate for the enzyme’s active site. Decreases product formation 2.Noncompetitive Inhibition: a molecule binds to the Allosteric site which changes the shape of the active site and thus it’s ability to bind to substrate.

QOD Malonate is a competitive inhibitor of the enzyme succinate dehydrogenase. Describe how malonate would prevent the enzyme succinate dehydrogenase from acting on its normal substrate succinate.

QOD Answer As a competitive inhibitor, malonate binds to the active site of succinate dehydrogenase and so prevents the normal substrate, succinate, from binding.

Enzyme Inhibition Feedback Inhibition: A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway. Inhibitor can be either Competitive or Noncompetitive When product is abundant, there is more inhibition and enzyme activity drops. When product is used up, there is less inhibition and enzyme activity increases.

Fig 6.8

An example of feed back inhibition that directs different metabolic pathways. What would happen if there was high levels of Q in the cytoplasm? What would happen if there was high levels of O in the cytoplasm?

Metabolic Pathways & Redox reactions Oxidation: The loss of electrons from a substance involved in a redox reaction. Reduction: The addition of electrons to a substance involved in a redox reaction. In oxidation-reduction reactions, electrons pass from one molecule to another –Happens at the same time –Photosynthesis and respiration are examples In living things H+ ions accompany e-, so oxidation is a loss of H atoms and reduction is a gain of H atoms

Photosynthesis 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 H atoms are transferred from water to carbon dioxide. (water is oxidized, carbon dioxide is reduced) The coenzyme NADP + is needed as an electron acceptor to remove hydrogen from water: NADP + + 2e - + H +  NADPH

Respiration C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O Opposite of photosynthesis (glucose is oxidized and oxygen is reduced) Respiration requires the coenzyme NAD + as the electron acceptor to remove Hydrogen atoms from glucose: NAD + + 2e - + H +  NADH