An Introduction to Metabolism

1. An Introduction to Metabolism Ch. 8 AP Biology Ms. Haut

2. Metabolic Pathways • Catabolic Pathways • Release energy by breaking down complex molecules into simpler ones • Cellular respiration provides energy for cellular work C6H12O6 + 6O2 6CO2 + 6H2O + energy • Energy released drives anabolic reactions

3. Metabolic Pathways • Anabolic Pathways • Consume energy by building molecules • Photosynthesis uses energy 6CO2 + 6H2O energyC6H12O6 + 6O2

4. Organisms Transform Energy Solar Energy (EK) Plants (glucose) Stored in chemical bonds (EP) Animals Break down Sugars; Some used (EK), some stored in chemical bonds (EP)

5. Energy • Kinetic energy is energy associated with motion • Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules • Potential energy is energy that matter possesses because of its location or structure • Chemical energy is potential energy available for release in a chemical reaction • Energy can be converted from one form to another

6. CO2 Heat Chemical energy H2O First law of thermodynamics Second law of thermodynamics Laws of Thermodynamics • First Law—Energy can be transferred, but never created or destroyed • Second Law—Every energy transfer results in increased entropy (randomness in the universe) • Some of the energy is converted to heat • Reactions occur spontaneously

7. Free Energy • Organisms live at the expense of free energy (portion of a system’s energy available for work) acquired from the surroundings • Free energy is needed for spontaneous changes to occur

8. Gibbs-Helmholtz Equation G = H - TS • Can be used to determine if a reaction is spontaneous • Spontaneous reactions occur in systems moving from instability to stability Total energy enthalpy Temp (K) Free energy entropy High energy Low energy

9. Gibbs-Helmholtz Equation G =  H - T  S • In chemical reactions, reactions absorb energy to break bonds • Energy is then released when bonds form between rearranged atoms of the product Measure of heat in the reaction

10. Key Importance of G • Indicates amount of energy available for work • Indicates whether a reaction will occur spontaneously (low G) • G decreases as reaction approaches equilibrium • G increases as reaction moves away equilibrium • G = 0 when a reaction is in equilibrium

11. Chemical Reactions In cellular metabolism, exergonic reactions drive endergonic reactions

12. Rate of Reactions • G indicates spontaneity not speed of reaction • Spontaneous reactions will occur if it releases free energy (- G ), but may occur too slowly to be effective in living cells • Can leave sucrose in sterile water for yrs. with hydrolysis occuring; add sucrase and reaction will hydrolyze in seconds • Biochemical reactions require enzymes to speed up and control reaction rates

13. ATP couples exergonic reactions to endergonic reactions • A cell does three main kinds of work: • Mechanical • Transport • Chemical • To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

14. ATP Powers Cellular Work Unstable Bonds—can release energy when broken Energy transferred to another molecule (phos-phorylated intermediate) with the phosphate More stable Less stable

15. P i P LE 8-11 Protein moved Motor protein Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP ATP + P i P P i Solute transported Solute Transport work: ATP phosphorylates transport proteins P NH2 NH3 P + + Glu i Glu Reactants: Glutamic acid and ammonia Product (glutamine) made Chemical work: ATP phosphorylates key reactants

16. The Regeneration of ATP • ATP is a renewable resource that is regenerated by addition of a phosphate group to ADP • The energy to phosphorylate ADP comes from catabolic reactions in the cell • The chemical potential energy temporarily stored in ATP drives most cellular work

17. Enzymes • Catalyst—chemical agent that speeds up a chemical reaction without being consumed by the reaction • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

18. The Activation Energy Barrier • 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

19. Enzymes • Catalytic proteins that speed up metabolic reactions by lowering energy barriers • Reactants must absorb energy to reach transition state (unstable) • Rxn occurs and energy is released as new bonds form to make products • G for overall rxn is difference b/w G of products and G of reactants

20. Substrate Specificity of Enzymes • Substrate—reactant that an enzyme acts • Substrate binds to the active site on the enzyme • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

21. Induced Fit Model of Enzymatic Reactions

22. How do Enzymes Work? • Active site holds 2 or more reactants in the proper position to react • Induced fit may distort chemical bonds so less thermal energy is needed to break them • Active site may provide micro-environment that aids a reaction (localized pH) • Side chains of amino acids in active site may participate in reaction

23. Enzyme Activity • A cell’s physical and chemical environment affects enzyme activity • Each enzyme has optimal environmental conditions that favor the most active enzyme conformation

24. Effects of Temperature • Optimal temp. allows greatest number of molecular collisions without denaturing the enzyme • Reaction rate  when temperature  • Kinetic energy increases and collisions increases • Beyond optimal temperature, reaction rate slows • Too low, collisions b/w substrate and active site don’t occur fast enough • Too high, agitation disrupts weak bonds of the tertiary structure of enzyme (enzyme unfolds)

25. Effects of pH • Optimal pH range for most enzymes is pH 6 – 8 • Beyond optimal pH, reaction rate slows • Too low (acidic) H+ ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme • Too high (basic) OH- ions interact with amino acid side-chains and disrupt weak bonds of the tertiary structure of enzyme

26. Cofactors • Small non-protein molecules that are required for proper enzyme catalysis • Inorganic—Zn, Fe, Cu • Coenzymes—vitamins

27. Effects of Substrate Concentration • The higher the [substrate], the faster the rate (up to a limit) • If [substrate] high enough, enzyme is saturated with substrate • Reaction rate depends on how fast the active site can convert substrate to product • When reaction is saturated with substrate, you can speed up reaction rate by adding more enzyme

28. Effects of Enzyme Inhibitors • Competitiveinhibitors—chemicals that resemble an enzyme’s normal substrate and compete with it for the active site • Blocks active site from substrate • If reversible, can be overcome by increasing substrate concentration

29. Competitive Inhibitor

30. Allosteric site Effects of Enzyme Inhibitors • Noncompetitiveinhibitors—chemicals that bind to another part (allosteric site)of an enzyme • Causes enzyme to change shape and prevents substrate from fitting in active site • Essential mechanism in cell’s regulating metabolic reactions

31. Negative Feedback

32. Metabolic Control often Depends on Allosteric Regulation • Allosteric enzymes have 2 conformations, catalytically active and inactive • Binding of an activator to the allosteric site stabilizes active conformation • Binding of an inhibitor (noncompetitive) to the allosteric site stabilizes inactive conformation

33. Control of Metabolism • In feedback inhibition, the end product of a metabolic pathway shuts down the pathway • Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed

34. Specific Localization of Enzymes Within the Cell • Structures within the cell help bring order to metabolic pathways • Some enzymes act as structural components of membranes • Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria