enzyme_mechanism

enzyme_mechanism - Enzymes • An enzyme is a protein or...

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Unformatted text preview: Enzymes • An enzyme is a protein or RNA based catalyst for biochemical reactions • Most, if not all metabolic pathways are regulated by regulating enzyme function. • Enzymes catalyze reactions under mild conditions. • Not much happens in cells without them • Enzymes work because they bind specific substrates with high fidelity. • A single amino acid chain folds up into a specific structure that then regulates the function of a cell. Pretty cool! There are many types of enzymes What is a catalyst? • increased rate of reaction • activation energy is reduced • ΔG o is NOT changed • • catalyst is not consumed by the reaction ∆ G 0’ ∆ G cat ‡ ’ • preferential binding of the transition state What does a catalyst do? Comparison of reaction rates with and without catalysis How does an enzyme do that? • Enzymes bind one or more substrate molecules with high specificity • Stereo isomer and geometric isomers are selected • Localize required cofactors in reactions • Change enthalpy of activation • Change entropy of activation • reaction rates up to 10 8 x the uncatalyzed reaction • enzymes are protein structures with “active sites” for binding “substrates” binding of substrate to active site initiates catalysis binding site is complementary to substrate structure high specificity for substrates enzymes recognize stereoisomers and geometric isomers • use of amino acid side chains for acid-base catalysis, and cofactors • binding energy from the bonds made between substrate and enzyme in the active site • entropy reduction: bringing substrates together in the correct orientation • desolvation of substrates: water solvation is replaced by amino acid side chains Models of Enzyme substrate interactions • Lock and key: DHFR • Substrates bind to preformed active site Models of Enzyme substrate interactions • Induced fit: Hexose kinase • Binding of hexose causes conformational change that allows active site to form. This prevents the transfer of P to H2O at active site. Xylose binds and cause conformational change but P is transferred to water not the sugar. Transition State Theory • Slow reactions face significant activation barriers that must be surmounted during the reaction k = k B T h exp −∆ G ≠ RT –transition state theory is applicable for catalysis –rate constants and free energies can be related Rate Acceleration • The enzyme lowers the activation barrier compared to the uncatalyzed aqueous reaction • In theory, the enzyme may also facilitate the tunneling through the barrier. This may be important for electrons. How to Lower Activation Energy?...
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enzyme_mechanism - Enzymes • An enzyme is a protein or...

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