Lecture 6 Enzymes

Lecture 6 Enzymes - Enzymes Proteins RNA Enzymes can...

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Unformatted text preview: Enzymes Proteins RNA Enzymes can require no other chemical group for activity. Some enzymes require additional components called cofactors or coenzymes. A coenzyme or cofactor tightly bound to the enzyme called prosthetic group. A complete, catalytically active enzyme with its bound cofactors is called holoenzyme. The protein part of the enzyme without the cofactors called apoenzyme. How enzymes work Enzyme provides an environment in which the reaction is more favorable. The weakbonding interactions between an enzyme and its substrate might be used to distort the substrate and catalyze the reaction. Enzymes affect reaction rates, not equilibria. E + S ES EP E + P G'O = RT ln K'eq K'eq = [P]/[S] Induced Fit Change enzyme in conformation upon substrate binding, induced by multiple weak interactions with the substrate. Induced fit serves to bring specific functional groups on the enzyme into position for catalysis. Enzyme Catalysis Equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature. The position and direction of the equilibrium are not affected by any catalyst. The function of the catalyst is to increase the rate of the reaction. The starting point for either forward or reverse reaction is called the ground state. Free energy, G There are set parameters to describe free energy changes: (T=298 K; partial pressure of the gas 1 atm or 101.3 kPa; solute concentration 1 M) Go standard free energy G'o biochemical standard free energy (Standard free energy at pH 7.0) Catalyzed Reaction cont. At the top of the activation energy hill is a point at which decay to S or P is equally probable. This is transition state. The difference in energy levels of the ground and transition states is called activation energy. ES and EP reaction intermediates. Reaction with many steps has a rate determined by the rate with the highest activation energy: ratelimiting step. Catalytic Power and Specificity of Enzyme Binding energy, GB, is a major source of free energy used by enzymes to lower the activation energies of reactions. Most of the catalytic power is derived from the free energy release by forming multiple weak bonds and interactions between enzyme and substrate. Weak interactions are optimized in the transition state. Active site groups contribute to catalysis Specific acidbase catalysis (water) General acidbase catalysis Covalent catalysis Metal ion catalysis Rate Enhancement by Entropy Reduction Rate enhancement by Entropy reduction General acidbase catalysis Catalysis that uses H+ and OH ions present in water is known as: Specific acidbase catalysis. Proton transfer mediated by amino acid side chains in the active site: General acid base catalysis Covalent and General acid-base catalysis Enzyme Kinetics Enzyme Kinetics E + S ES E + P Initial rate or initial velocity, Vo Maximum velocity, Vmax MichaelisMenten Equation Michaelis Constant, Km k1 k2 E + S ES E + P k1 k2 Assumption: k2 is small and can be ignored. Km = (k2 + k1)/k1 Kcat- rate constant for rate limiting step Dependence of Initial Velocity on Substrate Concentration LineweaverBurke Equation is the reciprocal of both sides of Michaelis Menten equation. ...
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