1 / 35

Chapter 6

Chapter 6. Mechanisms of Enzyme Action. Enzymatic Catalysis. Activation Energy (AE) – The energy require to reach transition state from ground state. AE barrier must be exceeded for rxn to proceed. Lower AE barrier, the more stable the transition state (TS)

yardley
Télécharger la présentation

Chapter 6

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 6 Mechanisms of Enzyme Action

  2. Enzymatic Catalysis • Activation Energy (AE) – The energy require to reach transition state from ground state. • AE barrier must be exceeded for rxn to proceed. • Lower AE barrier, the more stable the transition state (TS) • The higher [TS], the move likely the rxn will proceed.

  3. Enzymatic Catalysis S  Ts  P

  4. Transition (TS) State Intermediate • Transition state = unstable high-energy intermediate • Rate of rxn depends on the frequency at which reactants collide and form the TS • Reactants must be in the correct orientation and collide with sufficient energy to form TS • Bonds are in the process of being formed and broken in TS • Short lived (10–14 to 10-13 secs)

  5. Intermediates • Intermediates are stable. • In rxns w/ intermediates, 2 TS’s are involved. • The slowest step (rate determining) has the highest AE barrier. • Formation of intermediate is the slowest step.

  6. Enzyme binding of substrates decrease activation energy by increasing the initial ground state (brings reactants into correct orientation, decrease entropy) • Need to stabilize TS to lower activation energy barrier.

  7. ES complex must not be too stable • Raising the energy of ES will increase the catalyzed rate • This is accomplished by loss of entropy due to formation of ES and destabilization of ES by • strain • distortion • desolvation

  8. Transition State Stabilization • Equilibrium between ES <-> TS, enzyme drives equilibrium towards TS • Enzyme binds more tightly to TS than substrate Transition state analog

  9. Mechanistic Strategies

  10. Polar AA Residues in Active Sites

  11. Common types of enzymatic mechanisms • Substitutions rxns • Bond cleavage rxns • Redox rxns • Acid base catalysis • Covalent catalysis

  12. Substitution Rxns • Nucleophillic Substitution– • Direct Substitution Nucleophillic = e- rich Electrophillic = e- poor transition state

  13. Oxidation reduction (Redox) Rxns • Loose e- = oxidation (LEO) • Gain e- = reduction (GER) • Central to energy production • If something oxidized something must be reduced (reducing agent donates e- to oxidizing agent) • Oxidations = removal of hydrogen or addition of oxygen or removal of e- • In biological systems reducing agent is usually a co-factor (NADH of NADPH)

  14. Cleavage Rxns • Heterolytic vs homolytic cleavage • Carbanion formation (retains both e-) R3-C-H  R3-C:- + H+ • Carbocation formation (lose both e-) R3-C-H  R3-C+ + H:- • Free radical formation (lose single e-) R1-O-O-R2 R1-O* + *O-R2 Hydride ion

  15. Acid-Base Catalysis • Accelerates rxn by catalytic transfer of a proton • Involves AA residues that can accept a proton • Can remove proton from –OH, -NH, -CH, or –XH • Creates a strong nucleophillic reactant (i.e. X:-)

  16. : : Acid-Base Catalysis carbanion intermediate

  17. Covalent Catalysis • 20% of all enzymes employ covalent catalysis A-X + B + E <-> BX + E + A • A group from a substrate binds covalently to enzyme (A-X + E <-> A + X-E) • The intermediate enzyme substrate complex (A-X) then donates the group (X) to a second substrate (B) (B + X-E <-> B-X + E)

  18. Covalent Catalysis Protein Kinases ATP + E + Protein <-> ADP + E + Protein-P • A-P-P-P(ATP) + E-OH <-> A-P-P (ADP) + E-O-PO4- • E-O-PO4- + Protein-OH <-> E + Protein-O- PO4-

  19. The Serine Proteases • Trypsin, chymotrypsin, elastase, thrombin, subtilisin, plasmin, TPA • All involve a serine in catalysis - thus the name • Ser is part of a "catalytic triad" of Ser, His, Asp (show over head) • Serine proteases are homologous, but locations of the three crucial residues differ somewhat • Substrate specificity determined by binding pocket

  20. Serine Proteases are structurally Similar Chymotrpsin Trypsin Elastase

  21. Substrate binding specificity

More Related