The higher the free energy change for the transition barrier, the slower the reaction rate.
Enzymes lower energy barrier by forcing the reacting molecules through a different transition state.
Eg., C 12 H 22 O 11 + 12O 2 12CO 2 + 11H 2 O Reaction Intermediates Transient chemical species formed during a rxn Rate limiting step When many steps are involved the overall rate of the Reaction is determined by the step which has the Highest activation-energy
Relationship between reaction equilibrium and free energy Equilibrium constant is directly related to free energy Large negative free energy favors reaction
Rate of a reaction Unimolecular or first order reaction; V=k[S] Units are s -1 Rate for second order reaction; V= k[S 1 ][S 2 ] for transition state k = k T / h e - Δ G/RT Lower activation energy means faster rxn rates
How enzymes do it? Binding energy ( Δ G B ) Energy derived from enzyme substrate interaction. -transient covalent bond in the active site -non-covalent interactions to form ES
Interpreting V max and K m Steady state kinetics is the means by which biochemists can compare and characterize The catalytic efficiencies of enzymes.
K cat it is the limiting rate of any enzyme catalyzed reaction. Eg. , K cat is first order rate constant has units s -1 . It is also called Turnover number ( the number of substrate molecules converted to product) .
Michaelis constant Km Also known as turnover number
Initial velocity The enzyme substrate complex
Maximal velocity The velocity when the enzyme is saturated with substrate
Catalytic velocity The conc. of substrate @ which the velocity is half maximal
rxns. are enzyme reactions with two substrates.
In Lineweaver-Burk plot of a simple enzymatic rxn. , what is the value of the y-intercept @ 1/Vo axis?
Regulatory enzymes Exhibit increased or decreased catalytic activity in response to certain signals. -Reversible non-covalent modulation - allosteric modulators -small metabolites or cofactors -Reversible covalent modification -inhibition with separate regulatory protein -proteolytic cleavage
Heterotropic Allosteric Modulation In multienzyme pathways the regulatory enzyme is inhibited by the end product. Eg., L-Threonine to L-Isoleucine in bacteria.
Kinetics of Allosteric enzymes -sigmoid curve is observed -substrate concentration is represented by [S] 0.5 or K 0.5 homotropic allosteric enzymes
-reflects cooperative interaction between protein molecules -small changes in conc. of modulator can be associated with large changes in activity
Heterotropic Allosteric enzymes -Activation may cause increased velocity for fixed K 0.5 -Negative modulator cause decreased velocity for fixed K 0.5
-increased V max with little change in substrate
Learning check! Match the following Competitive Inhibiton decreases Km and Vmax Uncompetitive inhibition decreases [E]T Mixed inhibition increases Km Inactivation decreases Vmax, while Km may increase or decrease Mixed inhibition is characterized by two dissociation constants for the inhibition. True False
Enzyme inhibitors Interfere with catalysis, slowing or halting enzymatic reactions. -Reversible Inhibition -competitive Since it is reversible so increasing the concentration of substrate remove inhibition.
Uncompetitive inhibition Lower the measured V max Apparent K m also decreases