01:694:301 Lecture Notes - Lecture 8: Enzyme Inhibitor, Zymogen, Activation Energy
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14 Nov 2018
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Chapter 8 – ‘Enzymes: Basic Concepts and Kinetics’
Enzymes: Highly Specific Catalysts
• Understand the general statements about enzymes on 219-229
o They are very specific in both the reactions they catalyze and their choice of
reactants or ‘substrates’
! This specificity is due to enzyme’s precise interaction with substrate
• This precision is result of 3-D structure of enzyme protein
o Nearly all known enzymes are proteins
o The catalytic activity of many enzyme’s can depend on cofactors
! These cofactors are able to execute chemical reactions that can’t be done by aa
• Metal cofactors – Mg2+, Zn2+, etc.
• Coenzymes – tend to be derived from vitamins; tightly or loosely bounded to
enzyme
o Tightly bounded – ‘prosthetic groups’; loose – ‘cosubstrates’; they get
released’
• Note that if enzymes uses same coenzymes then usually perform similar
catalysis
o They can be regulated several ways (feedback/allosteric inhibition, regulatory
proteins, covalent modification, and zymogen cleavage)
o They can transform energy
! Ex: ATP – enzyme myosin in muscle; membrane pumps and channels
o They can’t move the equilibrium point
! Enzymes accelerate the attainment of eq. but do not shift their positions.
! The equilibrium position is a function only of the free-energy difference
between reactants and products.
o They catalyze reactions by stabilizing the transition state (‡)
! The transition state molecular structure in which substrate in
no longer a reactant but not yet a product
! Enzymes accelerate rxn by lowering activation energy
! It is the least stable and rarely occupied because it has highest
free energy
o They have an active site cleft; and they often change shape in
response to ‘correct’ substrate
! Enzyme catalyzed reaction reached maximum velocity
! Active site formed by aa residues; takes up small volume of
enzyme; non-polar micro-env.;
• Enzymes bring together substrates " enzyme-substrate
complex [ES]
o Substrates are bound by multiple weak forces (H-bonds, vdW, etc.)
• Interaction of the enzyme and substrate at the active site promotes the
formation of the transition state
! Koshland’s Induced Fit (pg. 228)
• Active site changes shape for substrate to fit only after substrate is bound
o Binding energy is what is used by enzyme to lower activation energy
! Max binding energy released with enzyme facilitates formation of ‡
• Note – 6 Enzymes classes ‘OTHLILI’
Enzymes & Free Energy
• To understand how enzyme’s work need to consider 2 thermodynamic properties:
o 1) Free energy difference b/w reactants and products
! Determines whether reaction take place spontaneously
o 2) Energy required to initiate conversion of R " P
! Determines rate of reaction " (affected by enzymes)
• An important point in the chapter is the discussion of Gibb’s Free Energy or !G
o At equilibrium, the enthalpy change for a reaction will equal the entropy change
times the absolute temp. " !H = T!S
• For reactions not at equilibrium, these two quantities differ, and the difference shows
the available reaction energy, thus: !G = !H – T!S
o Defined this way – reactions at eq. have !G = 0
o Reactions which will move FWD spontaneously have !G < 0 (negative)
o Reactions that will REV (back up) have !G > 0 (positive)
o When STD conditions are applied, that is, all [] = [1M] then the value of free
energy change will be !G°’, STD free energy change *at pH 7
o Note: units of energy " kcal
o !G is dependent on [P] and [R]
• KNOW the 5 statements listed on pg. 222 and Equations (1) and (5) on pg. 223
o (1) Rxn can only take place spontaneously if !G is negative (exergonic)
o (2) Sys is at eq. and no net change can take place if !G = 0
o (3) Rxn does not occur spontaneously if !G is positive
(endergonic)
! Free energy is needed to drive this rxn
o (4) The !G of rxn only depends on !G of products – !G of
reactants (final-initial)
! The !G of rxn is independent of path (mechanisms) of the
transformation
o (5) The !G provides no information about the rate of a reaction.