Chapter8 - Chapter 8 Enzymes are catalysts Free energy and...

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Chapter 8 Enzymes are catalysts Free energy and enzymes Enzymes facilitate the transition state The Michaelis-Menten model Enzyme inhibitors Multisubstrate reactions
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Known as the “vital force” of life in cells: 1600s Later, as “ferments” due to the role in fermentation 1752: bird gastric juices and solvation (Reamur) 1836: Pepsin isolated from gastric juice (Schwann), and Berzelius coins “catalysis” (from Greek, meaning “dissolution”). 1897: Yeast “juice” can ferment. “Enzyme” is from Greek, “in yeast”. Early 1900s: Fisher and “Induced fit” 1913: Michaelis-Menten model 2005: BBMB 404 A brief history of enzymes
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Carbonic acid The power of enzymes: Carbonic Anhydrase 1 million per second!
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Proteolytic enzymes
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Enzymes can be very specific in their selection of substrates, or not Trypsin Trombin Subtilisin will cleave any peptide bond
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Thermodynamics and Enzymes
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A + B C + D G = D G 0 + RT ln [ C ][ D ] [ A ][ B ] For any reaction… We can write an equilibrium constant… K eq = [ C ][ D ] [ A ][ B ] And calculate free energy change… Remember:
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G 0' = - RT ln [ C ][ D ] [ A ][ B ] If we look at equilibrium and pH 7, and examine the biochemical standard state free energy change… Or we can substitute K eq and write… G 0' = - RT ln K eq For example… Free energy and equilibrium
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G = D G 0 + RT ln [ C ][ D ] [ A ][ B ] Remember, even if G 0' is positive, G can be negative due to concentrations. For example: G 0' = +1.8 kcal/mol G = -0.7 kcal/mol But if [DHAP]=2x10 -4 M , and [GAP]= 3x10 -6 M
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The role of kinetics
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A B Reactions can also be described kinetically We can write an equilibrium constant… K eq = k forward k backward =100 Enzymes can change reaction rates…. Enzymes speed up reactions, but do not change equilibrium constants 10 s -1 0.1 s -1 k forward = k backward = A B 1000 s -1 10 s -1 k forward = k backward = But can NOT change the overall equilibrium constant. K eq = k forward k backward =100 Remember:
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Examples of the power of enzymes Enzymes DO NOT change G
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Enzymes speed up reactions by lowering the free energy change needed to reach the “transition state”. Rate [ S t e - D G t Thus, enzymes catalyze both the forward and reverse reaction…
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In general, how do enzymes do these amazing things? (This is still an important question in biochemistry)
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The “lock and key” model: (Emil Fisher 1890s) Purines, hydrazines, sugars, optical activity, natural products, and later in his career, proteins and enzymes. “His keen understanding of scientific problems, his intuition and love of truth and his insistence on experimental proof of hypotheses marked him as one of the truly great scientists of all time.”
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The “induced fit” model You will see many specific examples….
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Why do we believe this? 1. Enzyme kinetic model
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Chapter8 - Chapter 8 Enzymes are catalysts Free energy and...

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