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

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

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

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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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)

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