PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Chapter 13 Introduction to EnzymesChymotrypsin with bound substrateRead pages459 – 466 & 470•Enzymes are the agents of metabolic function•Enzymes endow cells with the remarkable capacity to exert kinetic control over thermodynamic potentiality EnzymesProperties of Enzymes(1) High reaction rates(2) Catalyze reactions at physiological conditions (milder reaction conditions)(3) Have a high degree of specificity (e.g. only A is converted to B)(4) Can be regulated (e.g., A is only converted to B under certain conditions)Catalytic Power•Enzymes can accelerate reactions as much as 1016 over uncatalyzed rates! •Urease is a good example: –Catalyzed rate: 3x104/sec –Uncatalyzed rate: 3x10 -10/sec –Ratio is 1x1014 !Some enzymes achieve remarkable catalytic efficiencyGlycolysisEnzyme catalyzed reactions are much faster than the corresponding uncatalyzed reactionE + S ES EP E + PS PBiochemical free energy change atpH 7.0Activation energiesSubstrate(s) Product(s)EnzymeFunction of DNA ligaseCoenzymesCofactors may be metal ions (such as Zn2+ required for the catalytic activity of carboxypeptidase A)Coenzymes are organic molecules (such as the NAD+ in YADH)Metal CofactorsOrganic Cofactors (also called coenzymes)Other cofactors• Known as prosthetic groups• Permanently attached with their protein• Often by covalent bonds• Example: Heme in hemoglobinSpecificity•Enzymes selectively recognize proper substrates over other molecules •Enzymes produce products in very high yields - often much greater than 95% •Specificity is controlled by structure - the unique fit of substrate with enzyme controls the selectivity for substrate and the product yieldAn enzyme-substrate complexIllustrates both the geometrical & physical complementarity between enzymes and substratesStereospecificityEnzymes are highly specific in both in binding chiral substrates & in catalyzing their reactions.Stereospecificity arises because enzymes virtue of their inherent chirality.Proteins consists of only L-amino acids.The enzyme binding siteE + S ES EP E + PInduced fit mechanism is most prevalent in enzymes. The enzyme active site adapts its structure to interact with the substrate & transition states.The same set of non-covalentinteractions that enable a proteinto fold are involved in stabilizingthe interaction between the substrate and enzyme.How do enzymes accelerate reactions?1. Chemical reactions between the substrate and functional groups on the enzyme can provide alternative, lower-energy reaction pathways.(Example:group transfer through an intermediate with the group transiently covalently attached to the enzyme)2. Binding energy, GB, is a major source of free energy used by enzymes to lower the activation energies of
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