Chemical Reactions in Cells

1. Chemical Reactions in Cells Energetics, Enzymes and Metabolic Reactions

2. Energy • Energy is the capacity for work or change. • Kinetic Energy = energy of movement • Potential Energy = stored energy • 1st Law of Thermodynamics • Energy can be transferred and transformed from one form to another but it cannot be created or destroyed.

3. usable usable usable usable Energy • 2nd Law of Thermodynamics • Energy transfer or transformation increases the entropy of the universe • Increase in entropy = randomness • Energy conversions result in a loss of useful energy

4. Spontaneity of a reaction depends on free • energy change G reaction = Gproducts – Greactants G G • If is negative, free energy is released and the reaction is spontaneous G • If is positive, free energy is consumed Free Energy = Energy Useful for Change

5. Free Energy change depends on changes in • total energy (enthalpy) • entropy (unusable energy, disorder) H S • In living systems, entropy changes have substantial influence • when is positive, and the term is large, a negative value predicts a spontaneous reaction S T S G H – G = T S Free Energy = Energy Useful for Change

6. Chemical Reactions • Involve the breaking and formation of chemical bonds • Reactants are converted to products. • Two types of reactions based on energy use: • Exergonic– free energy released • Endergonic – free energy consumed

7. Reactants changed to transition-state species high Energycontentofmolecules G low Progress of reaction Exergonic Reactions Burning glucose (sugar):an exergonic reaction Activation energy neededto ignite glucose Glucose + O2 Energy released byburning glucose C O2 + H2O

8. high Energycontentofmolecules G low Progress of reaction Endergonic Reactions Photosynthesis:an endergonic reaction Glucose Net energycaptured bysynthesizingglucose Activationenergy fromlight capturedby photosynthesis CO2 + H2O

9. Endergonic Reaction Exergonic Reaction D. Which type of reaction would have a positive value for G? Applying Your Knowledge A. Which type of reaction would be spontaneous? B. For which type of reaction will the products have a higher energy than the reactants? C. Which type of reaction releases energy?

10. + H2O + Pi+ free energy ATP Provides Energy for Cellular Reactions

11. Short-Term Energy Storage • Chemical Energy is stored in the bonds of ATP • ATP = adenosine triphosphate • ADP = adenosine diphosphate • to store energy • ADP + Phosphate + Energy ATP • to release energy • ATP  ADP + Phosphate + Energy

12. Coupled Reactions Pairing of an Exergonic reaction, often involving ATP, with an Endergonic reaction Note that overall free energy change is negative

13. Metabolic Reactions • Anabolic • link simple molecules to produce complex molecules (eg. dehydration synthesis of starch) • require energy • Catabolic • break down complex molecules to release simple ones (eg. hydrolysis of starch sugars) • release energy stored in chemical bonds

14. D E Metabolic Pathways InitialReactants Intermediates FinalProducts B C A Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Pathway 1 F G Pathway 2 Enzyme 5 Enzyme 6

15. Enzymes Assist in Biological Reactions Enzymes are biological catalysts. biological: composed of protein or, rarely, RNA catalyst: speeds up a reaction without being changed by the reaction

16. G –The value of and the ratio of reactants and products at equilibrium is the same as for an uncatalyzed reaction Properties of Enzymes • Enzymes speed up biological reactions by lowering the activation energy for the reaction. • Enzymes provide a surface where the catalysis takes place • The reaction reaches equilibrium more rapidly

17. high Energycontentofmolecules G low Progress of reaction Activation Energy: Controls Rate of Reaction Amount of energy required for reaction to occur transition state Activationenergy withoutcatalyst Activationenergy withcatalyst

18. Properties of Enzymes • Enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze. • Only substrates that fit the active site of the enzyme can bind and complete the reaction • active site: region on enzyme where substrates bind

19. induced fit Enzyme-Substrate Interactions Substrate Substrate 1 Substrates enter active site ActiveSite 2 Shape change promotes reaction Enzyme Product released;enzyme ready again

20. Chemical Events at Active Sites • Enzymes hold substrates in the proper orientation for the reaction to take place

21. Chemical Events at Active Sites • Enzymes induce strain in the substrate to produce a transition state favorable to reaction • Active site provides a microenvironment that favors the chemical reaction

22. Chemical Events at Active Sites • Active site directly participates in the reaction • covalent bonding can occur between enzyme and substrate • R groups of the enzyme’s amino acids can temporarily add chemical groups to the substrates

23. Molecules that Assist Enzymes • Cofactors: inorganic ions that bind to enzymes, eg. zinc • Coenzymes: small organic factors that temporarily bind to enzymes, eg. biotin, NAD, ATP • Prosthetic groups: non-protein factors that are permanently bound an enzyme, eg. heme

24. Factors Influencing Reaction Rate Rate no longer increases since the active sites of all enzymes are saturated with substrate • Substrate Concentration Rate is more rapid Rate is proportional to substrate concentration

25. DIPF Factors Influencing Reaction Rate • Competitive Inhibitors: Bind at the active site, compete for binding with substrate • Irreversible: form covalent bond with amino acids in the active site

26. Factors Influencing Reaction Rate • Competitive Inhibitors: Bind at the active site, compete for binding with substrate • Reversible: molecule similar to substrate occupies active site but does not undergo reaction

27. Factors Influencing Reaction Rate • Non-Competitive Inhibitors: Bind to a different site, cause a conformational change in the enzyme that alters the active site • Reversible

28. Factors Influencing Reaction Rate • Allosteric Regulation • Conversion between active and inactive forms of an enzyme due to binding of regulatory molecules at an allosteric site • Activators stabilize the active form • Allosteric inhibitors stabilize the inactive form

29. Factors Influencing Reaction Rate • Allosteric Regulation • Cooperativity: a substrate causing induced fit in one enzyme subunit can cause a change to the active form in all the other subunits

30. CH3 CH3 CH2 OH H C CH3 H C NH3 H C NH3 H C COOH COOH Feedback InhibitionIsoleucine allosterically inhibits enzyme 1 Enzyme Regulation: Feedback Inhibition Commitment step A B C D Enz. 1 Enz. 2 Enz. 3 Enz. 4 Enz. 5 Threonine(substrate) Isoleucine(end product) Feedback Inhibition: The product of a pathway inhibits an initial step in the pathway to decrease its own production

31. Properties of Enzymes • Three dimensional structure of an enzyme preserves its ACTIVE SITE • Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions)

32. fewer collisions between enzyme and substrate enzyme unfolds(denatures) enzyme unfolds(denatures) Effects of Temperature and pH on Enzymatic Activity

33. Where can an inhibitor bind to stabilize the inactive form of an enzyme? Where do the substrates bind? Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of a reaction. What is the model for a shape change caused by substrate binding to the enzyme? Active Site Activation Energy Allosteric Site Commitment step Induced fit Applying Your Knowledge